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Ortega, Lori A.
Movement and distribution of juvenile bull sharks, Carcharhinus leucas, in response to water quality and quantity modifications in a Florida nursery
h [electronic resource] /
by Lori A. Ortega.
[Tampa, Fla] :
b University of South Florida,
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Thesis (M.S.)--University of South Florida, 2008.
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ABSTRACT: Movement, distribution, and habitat use of juvenile bull sharks were examined in two studies using manual and passive acoustic telemetry. Research was conducted in the Caloosahatchee River, which serves as nursery habitat for this species, and is highly impacted due to anthropogenic alterations in water quality and quantity via dams and locks. Manual tracking yielded fine-scale results for eight individuals on home range size, rate of movement, swimming depth, linearity, direction of travel, tidal influence, diel pattern, as well as correlation with environmental variables. Changes in salinity, temperature, dissolved oxygen, turbidity, and pH played a role on the distribution of bull sharks. Passive monitoring of twelve individuals allowed for examination of trends in residency, home range, depth, and distribution in response to water quality alterations. Both studies documented a shift in the distribution of animals in response to significant modifications in salinity and flow levels. Sharks were distributed throughout the river at low flow rates, but were located only near the river mouth, or exited the river at discharges rates above 75 ms. Current water management policies are examined and recommendations are made which include the physiological preferences of this top-level predator.
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Water management policy
t USF Electronic Theses and Dissertations.
Movement and Distribution of Juvenile Bull Sharks, Carcharhinus leucas in Response to Water Quality and Quantity M odifications in a Florida Nursery by Lori A Ortega A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Environmental Science and Policy College of Arts and Sciences University of South Florida Major Professor: Philip van Beynen, Ph.D. Michelle R. Heupel, Ph.D. Philip J. Motta, Ph.D. Date of Approval: April 8, 2008 Keywords: acoustic telemetry, manual tracking, environmental quality, habitat use, water management policy Copyright 2008, Lori A Ortega
ACKNOWLEDGEMENTS I would like to express my sincere appreci ation to Dr. Michelle Heupel for giving me the opportunity to conduct this research. Thank you also to my committee, Dr. Philip van Beynen, Dr. Michelle Heupel, and Dr. Ph ilip Motta for their patience in reviewing many drafts of this manuscript. I would also like to thank Mote Ma rine Laborat ory staff C. Simpfendorfer, B. Yeiser and A. Ubeda for their help with field efforts and data collection, as well as student vol unteer interns M. Espinoza, K. Reiss, J. Price and A. Andyshak for their assistance in the field. For assistance with analysis I would also like to express appreciation to C. Simpfendorfer, C. Wells, B. Blackwell, and M. Dachsteiner. Finally, thank you to thank my family and friends for their su pport throughout this process. This research was funded in pa rt by the South Florida Water Management District and the National Shark Research C onsortium (NOAA Fisheries). L.A.O. was the recipient of the Mote Marine Laboratory and University of South Florida Graduate Fellowship in Elasmobranch Biology during the course of this work.
i TABLE OF CONTENTS LIST OF TABLES ............................................................................................................. ii i LIST OF FIGURES ........................................................................................................... iv ABSTRACT ...................................................................................................................... vi CHAPTER ONE: INTRODUCTION ..................................................................................1 CHAPTER TWO: HOME RANGE, MOVE MENT PATTERNS, AND WATER QUALITY PREFERENCES OF JUVENILE BULL SHARKS, Carcharhinus leucas IN A FLORIDA NURESRY .............................................................................6 Abstract ....................................................................................................................6 Introduction ..............................................................................................................7 Materials and Methods .............................................................................................9 Study Site .....................................................................................................9 Field Methods ............................................................................................11 Data Analysis .............................................................................................11 Home Range...................................................................................11 Movement ......................................................................................12 Water Quality .................................................................................14 Results ....................................................................................................................15 Home Range...............................................................................................17 Movement Patterns ....................................................................................21
ii Water Quality .............................................................................................23 Discussion ..............................................................................................................26 CHAPTER THREE: HABITAT US E OF JUVENILE BULL SHARKS, Carcharhinus lecuas IN A FLORIDA NURSERY AND THE INFLUENCE OF WATER QUALITY AND QUANTITY MODIFICATION .................................36 Abstract ..................................................................................................................36 Introduction ............................................................................................................36 Materials and Methods ...........................................................................................39 Study Site ...................................................................................................39 Field Methods ............................................................................................40 Data Analysis .............................................................................................42 Residency .......................................................................................42 Home Range and Depth .................................................................43 Water Quality and Quantity ...........................................................44 Results ....................................................................................................................45 Residency ...................................................................................................45 Home Range and Depth .............................................................................50 Water Quality and Quality .........................................................................56 Discussion ..............................................................................................................58 REFERENCES ..................................................................................................................67
iii LIST OF TABLES Table 1. Summary data for eight juvenile C. leucas tracked using acoustic telemetry within the Caloosah atchee River, FL in 2006 ..................................16 Table 2. Summary of movement variab les separated by diel period for same sharks tracked within the Caloosahatchee River .............................................20 Table 3. Results from univariate genera l linear models testing whether there was a significant difference in surface and bottom water quality variables between each track and for diel differences in surface and bottom water quality variables within each track ............................................24 Table 4. Linear regression model summary of actively tracked bull sharks 1 Â– 2 .........25 Table 5. Linear regression model summary of actively tracked bull sharks 4 Â– 7 .........25 Table 6. Biological data for bull sharks monitored within the Caloosahatchee estuary ..............................................................................................................46
iv LIST OF FIGURES Figure 1. The Caloosahatchee estuary ..........................................................................10 Figure 2. Movement of eight activel y tracked bull sharks within the Caloosahatchee River ...................................................................................17 Figure 3. Estimates of spatial usage of six actively tracked individuals as measured by minimum convex polygon .......................................................19 Figure 4. Diel rate of movement for the six actively tracked bull sharks ....................21 Figure 5. The location of the Caloos ahatchee River study site, with inset showing location of study site in Florida ......................................................42 Figure 6. Presence and absence of neonate bull sharks monitored within the Caloosahatchee River during 2006 and 2007 ...............................................47 Figure 7. Proportion of neonate bull sharks present, shown in grey, within the Caloosahatchee River relative to a) freshwater inflow as indicated by rates measured at Franklin Locks and c) salinity as indicated by values measured at th e Cape Coral Bridge ...................................................48 Figure 8. Daily home range of animals, shown in grey, present from June 2006 to November 2006 relative to a) fres hwater flow and b) salinity .................51 Figure 9. Relationship between distribu tion of acoustically tagged bull sharks within the Caloosahatchee River rela tive to a) freshwater influx and c) daily average salinity. Scatterp lots show daily mean river location relative to b) freshwat er and d) salinity ........................................................53
v Figure 10. Scatterplot of daily salinity versus river flow ...............................................57
vi Movement and Distribution of Juvenile Bull Sharks, Carcharhinus leucas in Response to Water Quality and Quantity Modifications in a Florida Nursery Lori A Ortega ABSTRACT Movement, distribution, and ha bitat use of juvenile bull sharks were examined in two studies using manual and passive acoustic telemetry. Research was conducted in the Caloosahatchee River, which serves as nurse ry habitat for this species, and is highly impacted due to anthropogenic alterations in water quality and quantity via dams and locks. Manual tracking yielded fine-scale results for eight indi viduals on home range size, rate of movement, swimming depth, linear ity, direction of travel, tidal influence, diel pattern, as well as corre lation with environmental variables. Changes in salinity, temperature, dissolved oxygen, turbidity, and pH played a role on the distribution of bull sharks. Passive monitoring of twelve individu als allowed for examination of trends in residency, home range, depth, a nd distribution in response to water quality alterations. Both studies documented a shift in the distribu tion of animals in res ponse to significant modifications in salinity and flow levels. Sh arks were distributed throughout the river at low flow rates, but were located only near the river mouth, or exited the river at discharges rates above 75 m3s-1. Current water management policies are examined and recommendations are made which include the ph ysiological preference s of this top-level predator.
1 CHAPTER ONE: INTRODUCTION An expansion of commercial shark fisheries has resulte d in the drastic population decline of many coastal shark species of th e Atlantic over the pa st 30 years (NMFS 1993; Burgess et al. 2005). The Fisheries Management Plan for Sharks of the Atlantic Ocean (FMP) stated that abundance has potentially declined as much as 75 % from the 1970s to the 1980s (NMFS 1993). Most shark species are not capable of rebounding quickly to population reduction since they generally have slow growth, late maturity, and low fecundity compared to bony fishes (Camhi et al. 1998). Signif icant elasmobranch population reduction affects their prey species and also may have second and third degree affects through trophic linkages (Stevens et al 2000; Schindler et al 2002). Bull sharks are managed as part of the large coastal shar k fishery complex in the Gulf of Mexico and the Atlantic Ocean. They are taken both commercially and r ecreationally and the complex is regarded as overfished (Corts et al. 2002). The FMP recognized a disturbing lack of information regarding shark fisherie s and biological data re quired for appropriate fisheries management. The FMP identified estuarine nurseries as areas of great concern for coastal sharks due to the direct exposure of these locations to anthropogenic alteration (NFMS 1993). Although the effect is unknown, th e degradation of water quality in estuaries has been identified as a potential th reat to coastal shark populat ions (NMFS 1993). Only a few estuarine systems exist globally that remain unaffected by the upstream alteration of their
2 freshwater inflow (Alber 2002), and approximately 60% of the worldwide storage of freshwater is held behind re gistered dams (Vrsmarty and Sahagian 2000). These areas are at risk from alteration in natural flow ra tes, which may have disastrous consequences downstream (Alber 2002). High le vels of variation in river flow discharge and salinity have been found to affect the distribution of aquatic species (Bain and Finn 1988; Moser and Gerry 1989; Paperno and Brodie 2004; Harrison and Whitfield 2006), and specifically on bull sharks (Heupel a nd Simpfendorfer 2008, Curtis 2008). Bull sharks are cosmopolitan in tropical a nd subtropical coastal, estuarine, and riverine waters (Garrick 1982, Compagno 1984). This elasmobranch species is able to reside in both freshwater a nd saltwater for extended periods of time due to unique physiology and osmoregulatory capabilities (Thorson 1971; Thorson et al. 1973; Montoya and Thorson 1982). This enables them to travel long distances in freshwater systems, which has been documented thr oughout the world (Sadowsky 1968, Branstetter 1981, Jensen 1976, Pillans et al. 2006). Females are thought to give birth in estuaries or in proximity to river mouths and juvenile s tend to move upstream after parturition (Thorson 1972; Last and Stevens 1994). Juveniles are rarely found in marine environments following birth (Branstetter a nd Stiles 1987; Thorson et al. 1973; Thorson 1976), possibly due to an inability to up-re gulate urea (Pillans and Franklin 2004), and have exhibited a distinct preference for estuarin e water. The bull shark is one of the most common large shark species in FloridaÂ’s coasta l and estuarine areas, many of which are believed to provide important nursery ha bitat (Snelson et al. 1984; Michel 2002; Simpfendorfer et al. 2005). Habitat utilization by this species has received limited
3 scientific attention despite it s broad distribution and known us e of freshwater and coastal systems. The Caloosahatchee River in southwest Florid a served as the site of this study and is a nursery for bull sharks in their first ye ar of life (Simpfendorfer et al. 2005). This system is highly impacted with major change s in its historic hydrology due to significant modifications in land and canal developmen t (Barnes 2005). The South Florida Water Management District (SFWMD) manages wate r flow into the river and water quality parameters can change rapidly, on a scale from hours to days (Simpf endorfer et al. 2005), depending on precipitation levels and disc harge regimes. Regul atory releases of freshwater from Lake Okeechobee have crea ted large changes in the natural quantity, timing, and quality of flow to the estuary (B arnes 2005). Alteration in flow rate has an influence on salinity, which is a critical dete rminant of estuarine habitat characteristics and can affect distribution of rooted vege tation, and sessile and motile biota (Alber 2002). Increased river discharge has been lin ked to behavioral changes in several bony fish species by altering their ha bitat selection (Bre nden et al. 2006; Alba nese et al. 2004) and abundance (Flannery et al 2002). Modifications in the Caloosahatchee River are often made without considering the biological integrity of the syst em (Haunert et al. 2000). It is believed that th e river has declined in the abundance, distribution, and species richness of juvenile fish due to changes in the natural salinity regime and freshwater discharge, although current data do es not yet exist to substantiate (Barnes 2005). In heavily regulated systems, such as the Caloosahatchee River, it is essential to understand the effect that modifications to environmental parameters have on resident species. A population decline of many coas tal shark species necessitates research to
4 understand both habitat use and response to ha bitat modification in order to formulate appropriate management policy. This study utilized both manual and passi ve acoustic telemetry to aid in the understanding of juvenile bull shark habitat use within nur sery grounds. Manual tracking provided data on short-term, fine-scale m ovement patterns while passive tracking allowed for a broader view of how animals we re distributed throughout the estuary. The purpose of study one was to gain an understa nding of detailed daily movement patterns and habitat use as well as examine the relatio nship between river habitat modification and movement. The purpose of study two was to examine in closer detail the relationship between distribution and abundance of juve nile bull sharks and physical factors, specifically flow rate and salinity, over a l onger period of time. With these goals, I addressed the following research questions for the short term, ma nual tracking study: 1. How are juvenile bull sharks util izing the estuarine nursery habitat? 2. Do juvenile bull sharks exhibit diel habitat use patterns in regards to home range size, rate of movement, depth, lin earity, direction, or tidal influence? 3. Do changes in salinity, temperature, turbidity, dissolved oxygen, or pH influence juvenile bull shark movement and habitat use? The following research questions were addressed for the long term, passive monitoring study: 1. How are juvenile bull shar ks utilizing the nursery ove r a longer time period? 2. What is the impact of high discharge rates on the residency and distribution of juvenile bulls sharks? 3. What would be an appropriate management regime for this top-level predator?
5 A comprehensive examination of movement on both short and long term scales provides a more complete understanding of how juvenile bull sharks utilize habitat as well as how water management practices influe nce shark behavior. An understanding of how juvenile bull sharks utili ze nursery habitat in response to significant artificial water quality modification is necessary for both biological and management purposes. Bull sharks are a commercially-important species a nd are currently believe d to be overfished. In order to maintain a sustainable populati on and healthy ecosystem, it will be necessary to protect immature stocks a nd essential habitats (Cortes et al. 2002). This study aims to contribute to the knowledge of how juvenile bull sharks utilize nursery ha bitat as well as how they are affected by water management decisions.
6 CHAPTER TWO: HOME RANGE, MOVEMENT PATTER NS, AND WATER QUALITY PREFERENCES OF JUVENILE BULL SHARKS, Carcharhinus leucas, IN A FLORIDA NURSERY Abstract Acoustic telemetry was used to examine home range size, small-scale movement patterns, and water quality pref erences of juvenile bull sh arks in the Caloosahatchee River, Florida. Movement pa ttern analysis included home ra nge size, rate of movement, swimming depth, linearity, direc tion, tidal influence, diel pa ttern, and correlation with environmental variables. Manual tacking occu rred before and after a large freshwater influx which divided the sharks into two gr oups based on movement patterns. The first group displayed increased rate of movement, distance traveled, and space utilization at night, and movements correlated with salinit y, temperature, and dissolved oxygen. The second group had an increased rate of move ment, distance traveled, and space utilization during the day, and movements correlated with temperature, dissolved oxygen, turbidity and pH. These juvenile bull sharks displayed distinct diel movement patterns that were influenced by physical factors, which may acc ount for the distribution of this top-level predator in the Caloosahatchee River.
7 Introduction The life history and ecology of euryhaline elasmobranchs is poorly understood, as is the extent of their ecol ogical role in freshwater a nd brackish systems (Martin 2005; Curtis 2008). Understanding behavior, especi ally movement patterns, will help define the ecological role of species within these sy stems. Movement is an essential process that enables fishes to fulfill their resour ce requirements in spatially and temporally changing environments (Schlosser and Ange rmeier 1995). Through movement, fish are able to choose the most suitable habitats in order to optimize surviv al and growth (Gowan and Fausch 2002). Although physical barriers such as dams or other habitat features have an obvious impact on movement patterns, the role of environmental factors as drivers of movement patterns has received less attenti on. Since many aquatic systems are being increasingly modified, it is especially important to study the relationship between movement and ecological charact eristics of mobile residents in order to predict how animals will respond to environmental change. Due to the close proximity of freshwater systems to human development, elasmobranchs that utilize reduced salinity environments may be especially vulnerable to anthropogeni c habitat modification, making it essential to gain a better understanding of their habitat utilization and environmental preferences. Bull sharks, Carcharhinus leucas are one of the few elasmobranch species known to be physiologically capable of toleratin g freshwater for extended periods of time (Thorson et al. 1973), and are found throughout the world in warm subtropical and tropical coastal, estuarine and riverine wa ters (Bass et al. 1973; Compagno 1984; Curtis 2008). The bull shark is one of the most co mmon large shark species in FloridaÂ’s nearshore coastal waters (Snelson et al. 1984; Wiley and Simpfendorfer 2007), and is well
8 known for its ability to travel long distances in freshwater systems. In South Africa, C. leucas has been reported to tr avel up to 1,120 km from the sea in the Zambezi River system (Bass et al. 1973). Bull sharks have also been reported 2,800 km up the Mississippi River (Thomerson et al. 1977). Movement of bull sharks into freshwater systems has also been reported in Brazil (Sadowsky 1968; 1971), the Gulf of Mexico (Springer 1940; Clark and Von Schmidt 1965; Branstette r 1981), Lake Nicaragua (Thorson et al. 1966; Jensen 1976; Tuma 1976), Australia (Thorson et al. 1973; Pillans et al. 2006) and the Indian River Lagoon system in Florida (Curtis 2008). Despite its broad distribution and known use of fres hwater systems, habitat utilization by this species has received limited scientific attention. The movement and behavior patterns of el asmobranchs have been the subject of study for several decades. With the use of acoustic telemetry, these studies have examined many aspects of movement incl uding short-term movement patterns, home range size, rate of movement, and dept h distribution (e.g. Mc Kibben and Nelson 1986; Gruber et al. 1988; Carey and Scharold 1990; Morrissey and Gruber 1993b). However, most of these studies focused on pelagic or coastal species with little telemetry data available for species utilizi ng freshwater or estuarine systems (Heupel et al. 2006; Simpfendorfer 2006; Collins et al. 2007; Cu rtis 2008). Telemetry research has highlighted the variety of temporal and spat ial patterns displayed by elasmobranchs in movement characteristics such as rate of movement and horizontal migrations (Sundstrm et al. 2001). Seve ral physical factors may inter act to define elasmobranch movement patterns, including water temperature (Morrissey and Gruber 1993a; Matern et al. 2000), oxygen levels (Parsons and Carlson 199 8), diel periodicity (Tricas et al. 1981;
9 McKibben and Nelson 1986; Klimley et al. 1988; Holland et al. 1992), tides (Ackerman et al. 2000; Medved and Marshall 1983) and salinity (Curtis 2008; Heupel and Simpfendorfer 2008). Furthermore, few species have been tracked for a full diel period, making it difficult to understand the relations hip between environmental characteristics and elasmobranch movement patterns. The Caloosahatchee River and San Carlos Bay in southwest Florida is a nursery area for bull sharks during their first year of life until reaching approximately 95 cm standard total length (STL) (Simpfendorfer et al. 2005). Although it is known that young bull sharks utilize this system, there is a l ack of information regarding the relationship between environmental factors a nd habitat utilization. The goal of this research was to investigate short-term detail ed space utilization, movement patterns, and to determine whether environmental variables influence s hort-term movement patterns of juvenile C. leucas With a better understand ing of habitat requirements for this euryhaline species, we make recommendations for water management in this environmentally sensitive river system. Materials and Methods Study Site The Caloosahatchee River extends 105 km and links Lake Okeechobee to San Carlos Bay on FloridaÂ’s southwest coast (Bar nes 2005). The river is the primary provider of freshwater to southern Charlotte Harbor (Figure 1). Sharks were tracked in the Caloosahatchee Estuary which consists of appr oximately 32 km of river habitat. Due to the long and narrow configuration of the river, the estuary experiences large water quality fluctuations generated by wind, tide, runo ff, and precipitation. These changes are
10 compounded by the artificial release of freshw ater from Lake Okeechobee, with variable discharge rates that have reached as high as 1278 m3s-1 (South Florida Water Management District 2008). The unnatural, ra pid flow of freshwater may cause severe damage to estuarine organisms and communitie s, especially during the wet season when freshwater release is at its highest levels (Barnes 2005). Conditions within the system can alter abruptly, on a scale from hours to da ys (Simpfendorfer et al 2005), providing an ideal location to examine C. leucas movement patterns in relation to environmental fluctuations. Figure 1: The Caloosahatchee estuary. Inset: Location of the study site in Florida and showing connections to Lake Ok eechobee and the Gulf of Mexico.
11 Field Methods Eight juvenile sharks were collected from June to August of 2006 via rod and reel fishing using circle hooks and frozen mullet, Mugil cephalis or fresh catfish, Arius felis and Bagre marinus Captured individuals were weighed, measured (s tretch total length Â– STL), sexed, and tagged with a single-barb pl astic dart tag inserted into the dorsal musculature adjacent to the first dorsal fin. In addition, a V13P (Vemco Ltd) acoustic depth sensing transmitter was attached to the dorsal fin via a rototag. Transmitters (13 x 84 mm) pulsed continuously on one of four ac oustic frequencies (75, 78, 81, or 84 kHz). Two transmitters of each frequency were used. One shark was tracked at a time and transmitters on the same frequency were spaced out during the course of the research to avoid signal overlap. A Vemco VR100 acoustic receiver and di rectional hydrophone mounted on the boat were used to manually track shark move ments. In order to eliminate potential influence on shark movement, an estima ted minimum distance of 100 m between the shark and the boat was maintained. Shark lo cation was recorded every 15 minutes for up to 24-hours using a global positioning system. Water quality samples were collected at the surface and bottom every 15 minutes using a Niskin bottle and tested for several parameters including salinity, temperature, dissolved oxygen, turbidity, and pH using a water quality meter, pH meter and turbidimeter. Data Analysis Home Range Positional fixes derived from active tracki ng were plotted over a digital orthoquad of the Caloosahatchee River and analy zed using ESRI ArcView 3.3 geographic
12 information systems software. The maximum size of activity space used by each animal was determined for day, night, and 24 hour (total) periods using minimum convex polygon analysis in the Animal Moveme nt Extension for ArcView (Hooge and Eichenlaub 2000). Movements between positions recorded from 0700 to 1900 were categorized as daytime and movements between 1900 and 0700 were categorized as nighttime in order to coincide with local sunrise and suns et times. A Brainerd-Robinson Similarity Coefficient Analysis, which measures the similarity of assemblages by comparing the proportional representation of each category within the assemblage, was used to determine whether there was a di fference in day and night home range size between tracks. Hierarchical cluster analysis using th e average linkage method and squared euclidean distance measure was conduc ted to support those results. A Wilcoxon signed rank test based on groupings from the cl uster analysis was us ed because the data were paired and non-normal. The test was performed to determine if there was a significant diel difference in home range size, however, only one cluster (n = 4) was used because the second cluster was too sma ll (n = 2) to perform any tests. Movement In order to describe movement patterns, six variables were used: swimming depth, rate of movement (ROM), linear ity of movement, direction of travel (upriver, downriver, shoreline), tidal stage, and di el period. The rate of move ment was calculated using the distance traveled between successive positional fixes divided by the sampling interval. In order to achieve normality, ROM data were normalized using a log transformation. A linearity index was calculated to determine if there was a linear or random trend to shark
13 movement. The linearity index values were determined using the formula from Bell and Kramer (1979): LI = (Fn Â– F1)/D where Fn was the last fixed location of the animal, F1 was the first fixed location, and D was the total distance traveled by the shark. Va lues of linearity ranged from 0 to 1, with values near zero representi ng random movements and values approaching 1 indicating linear travel. Direction of travel in degrees ( ai), or the angle of movement between fixes, was calculated between successive fixes using the formula described in Kernohan et al. (2001), with ( xi, yi) representing the first fix and ( xi+1, yi+1) representing the following fix. The degree of travel was calcu lated by the following criteria: ai = arctan ( Yi /Xi)(180 / ) if Xi > 0 180 + arctan ( Yi /Xi)(180 / ) if Xi < 0 90 if Xi = 0 and Yi > 0 270 if Xi = 0 and Yi < 0 The distance of the X vector was calculated as: Xi = xi+1 Â– xi The distance of the Y vector was calculated as: Yi = yi+1 yi The formula computed angles calculated in radi ans that were converted to degrees. This angle was then converted to a bearing from true north ( bi) with the equation: Bi = 90 ai If bi was negative, a value of 360 was added to the result to make the value positive. These results were used to determine if an individual was moving upriver (1), downriver
14 (-1), or towards the shoreline (0). If m ovement was between 330 and 120, movement was categorized as upriver, between 120 a nd 150 or 300 and 330 was considered towards the shoreline, and between 150 and 300 was c onsidered downriver. These angles were chosen because they most cl osely reflected the northeast to southwest trajectory of the river where the tracks occurred. A multivariate analysis of variance (M ANOVA) was conducted to assess whether there was a difference in shark depth, ROM, or linearity in relation to directional travel. A TukeyÂ’s post-hoc test was us ed to identify if ROM changed upriver, downriver, or perpendicular to the shoreline. A univariat e general linear model (GLM) determined if there were significant changes in ROM, shar k depth, and linearity among the tracks. A univariate GLM was also conducted to elucidat e if swimming depth, ROM, linearity, or directional travel displayed di el differences within tracks. Spearman correlation analysis was used to determine relationships between depth and ROM, linearity, or direction of travel. Spearman analysis was also used to determine if there was a tidal influence on ROM, depth, linear movement, or direction of travel. Water Quality To understand how water quality changed over time, a univariate GLM was used to determine if top and bottom values for salinity, temperature, dissolved oxygen (DO), turbidity, and pH were different among tr acks (i.e. over time) and if there were significant diel differences within each trac k for each variable. A correlation analysis was conducted to ensure that water quality va riables were independe nt. Variables were considered correlated if values fell betw een 0.3 and 0.8, however th e largest correlation was minimal at 0.32, supporting the independent analysis of each variable. Preference
15 for each of the water quality variables was analyzed using multiple linear regression comparing average habitat condition with th e latitudinal shark position. Latitude was chosen as the position variable due to the north-south orientation of the river and the higher degree of latitudinal he terogeneity in habitat character istics. Based on an analysis of the residual error, multiple regressions were again performed with the tracks separated into two groups, as determined via cluster anal ysis. Both groups of data were analyzed using CookÂ’s and Mahalanobis distance a nd two outliers were removed per group because they fell outside two standard deviatio ns from the mean. Statistical tests were performed with Statistica (1999) and SPSS (15.0), and a rejection level of 0.05 was employed. Results From June to August of 2006, 509 positional fixes were obtained for eight juvenile C. leucas that were actively tracked for peri ods of up to 24 hours. Six of these tracks were considered to be full tracks (i.e >21 hours) and were us ed in all statistical analyses (individuals 3 and 8 were omitted) data are shown in Table 1 and Figure 2. Animals 1 through 3 were caught in the same section in the northern portion of the estuary in salinities of 7.6 Â– 11.1 Â‰. Following a large freshwater influx, the salinity in that location dropped to approximately 2.6 Â‰, after which no additional sharks were captured despite extensive fishing efforts. Af ter moving closer to th e mouth of the river, an individual was caught in 6.5 Â‰ within 45 mi nutes. All subsequent individuals were captured in this lower portion of the estu ary in salinities rang ing between 6.5 and 12.5 Â‰ at time of capture. Results were reported as pooled data for all tests where a uniform trend was determined. However, in e ach case where uniformity between the six
16 individuals was not the result, animal beha vior patterns clustere d by tracks 1-2 and 4-7, and results were reported by cluster. Table 1: Summary data for eight juvenile C. leucas tracked using acoustic telemetry within the Caloosahatchee River, FL in 2006 Size is indicated as stretch total length (STL) in centimeters. TrackSexSizeSTL DateLatitudeLongitudeDuration (h)Total positional of captureof capturefixes 1M8014-Jun-0726.64392-81.892852475 2F7728-Jun-0726.64474-81.889802462 3F7806-Jul-0726.64983-81.88493713 4M8018-Jul-0726.64942-81.864732174 5M8401-Aug-0726.55795-81.925232479 6M7703-Aug-0726.55847-81.924762482 7F8208-Aug-0726.55818-81.924372474 8M10423-Aug-0726.52864-81.96098620
17 Figure 2: Movement of eight actively tracked bull sharks within the Caloosahatchee River. Inset maps provide closer detail of the movements within the two clusters. Home Range Activity space for the six complete tracks varied from 1.2 to 4.3 km2 (mean = 2.5 km2, median = 2.4 km2). When calculated as a distance measurement, sharks utilized a 1.9 to 4.8 km linear stretch of river. With all six tracks pooled, a larger space was used during the night (mean = 1.3 km2, median = 0.7 km2) than during the day (mean = 0.9 km2, median = 0.9 km2). However, results from the Brainerd-Robinson Similarity
18 Coefficient Analysis showed tracks 1 and 2 we re highly related in regards to home range size with a correlation value of 0.92, and tracks 4 though 7 were highly related, with all correlation values above 0.90. Hierarchical cl uster analysis supporte d a cluster of two groups, with the upriver tracks completed in June (n = 2) grouping and the downriver tracks conducted in July and August (n = 4) forming the second cluster. With data separated according to cluster membership, it was shown that track s 1 and 2 had larger nighttime home range sizes and tracks 4 through 7 had significantly (Wilcoxon, p < .0001) larger daytime home ranges (Figure 3).
19 Figure 3: Estimates of diel spatial usage of six actively tracked individuals as measured by minimum convex polygon. Total distance traveled by all indivi duals per 24 hour period ranged from 9.7 to 20.6 km, with a mean of 14.9 km. Sharks 1 and 2 traveled 4.32 and 7.22 km farther at night than during the day. Shark 4 had a sl ightly higher total di stance traveled during the night, but the difference was small (1.65 km). Sharks 5 through 7 had small diel variation (range 1.28 Â– 1.69 km) but generally displayed increased daytime travel distances, corresponding with diel spatial usage pattern s, as shown in Table 2.
20 Table 2: Summary of movement variables separated by diel period for same sharks tracked within the Caloosahatchee River. TrackDielTotal dist (km)ROM (m/min)Shark Depth (m) Linearity 1Day8.13184.108.40.206 Night12.45220.127.116.11 Total / Mean20.6015.11.40.2 2Day5.4910.21.00.4 Night12.718.104.22.168 Total / Mean18.2015.10.70.3 3Day7.2021.41.20.4 NightN/AN/AN/AN/A Total / Mean7.2021.41.20.4 4Day6.3322.214.171.124 Night7.9811.00.50.3 Total / Mean14.3011.20.70.2 5Day126.96.36.199.4 Night4.7188.8.131.52 Total / Mean10.908.11.10.3 6Day5.4184.108.40.206 Night220.127.116.11.3 Total / Mean9.706.31.00.3 7Day8.5618.104.22.168 Night6.822.214.171.124 Total / Mean15.4012.11.10.3 8Day12.0031.00.90.6 NightN/AN/AN/AN/A Total / Mean12.0031.00.90.6
21 Movement Patterns There was a significant difference in the ra te of movement (ROM) between all six tracks (GLM, df = 5, F = 12.991, p < .0001). Th e first two tracks showed an overall higher ROM (mean = 15.1 m/min) than the se cond cluster of individuals (mean = 9.3 m/min). These two individuals also m oved faster during nighttime hours (mean = 18.8 m/min) than during the day (mean = 10.9 m/min) In contrast, individuals in tracks 4 through 7 moved faster during the day (mean = 10.8 m/min) than at night (mean = 8.2 m/min) (Figure 4). Figure 4: Diel rate of movement fo r the six actively tracked bull sharks.
22 Significantly different swimming dept h was observed among the six tracks (GLM, df = 5, F = 8.8128, p < .0001). Mean bottom depth in tracking locations was 2.4 m and mean shark depth was 1 m from the su rface. All six indi viduals displayed the same trend regarding depth and were theref ore analyzed together. Each shark swam significantly closer to the surface during the ni ght (mean = 0.6 m) and were deeper in the water column during the day (mean = 1.5 m) (GLM, df = 6, F = 29.2176, p < .0001). There was no significant difference in lin earity either among tracks or between night and day within tracks for any indivi dual (Table 2). No significant relationship existed between either linearity or shark de pth with direction of travel (MANOVA, p > .05). However, all sharks moved at a differe nt rate relative to direction of travel (MANOVA, df = 5, F = 5.034, p = .007). All sharks moved at an elevated speed as they traveled upriver (mean = 17.5 m/min) but ther e was no difference in ROM between travel downriver (mean = 13.5 m/min) or toward the shoreline (12.5 m/min). Spearman correlation analysis showed si gnificant relationships betwee n tidal stage and shark depth (p = .004), linearity (p < .001), and direction of travel (p = .027), but no relationship was present with ROM (p = 0.637). Sharks swam s lightly deeper in the water column during a falling tide (mean = 1.1 m) versus a rising tide (mean = 0.9 m). Individuals displayed more random movements during a rising tid e (mean = 0.262) than during a falling tide (mean = 0.321). All individuals followed the tide, traveling upriver during a rising tide and downriver during a falling tide. Relationships between shark depth and linearity (Spearman, p = .005) and ROM and linearity (Sp earman, p = .011) were also significant. Each individual showed a hi gher degree of random movement s at shallower depths and more linear travel in deeper depths. An imals also displayed a faster ROM when
23 swimming a linear trajectory than when trave ling a random pattern. No other movement variables showed signi ficant correlations. Water Quality There were significant differences for al l surface and bottom values of salinity, temperature, dissolved oxygen (DO), turbidit y, and pH when compared across tracks. Water conditions therefore cha nged significantly over time (Tab le 3). All top and bottom water quality variables, excep t bottom pH, showed significan t diel differences within each track (Table 3). However, when water quality from all tracks was analyzed together, it became evident that few variab les exhibited clear diel trends. Surface temperature was always higher and surface a nd bottom pH was always lower during the day.
24 Table 3: Results from univariate general linear models testing whether there was a significant difference in surface and bottom wa ter quality variables between each track and for diel differences in surface and botto m water quality variables within each track. Â“TÂ” denotes surface of the water column and Â“BÂ” denotes the bottom of the water column. VariableMinMaxMeandfF p dfF p T Salinity2.412.87.5545.300.00167.130.001 B Saliinity5.417.010.7542.300.001611.860.001 T Temp27.037.330.4568.900.001618.700.001 B Temp28.032.030.45629.000.001617.000.001 T DO126.96.36.19932.810.00166.900.001 B DO188.8.131.5272.710.00162.750.012 T Turbidity1.45.93.05117.920.001615.830.001 B Turbidity1.715.24.4511.190.00162.550.020 T pH184.108.40.20668.000.001615.000.001 B pH7.48.98.0513.820.00160.750.606 Water quality statistics Difference in WQ between tracks Diel difference in WQ within tracks Linear regression showed a significant relationship between shark location and salinity (p < .0001), temperature (p < .0001), a nd dissolved oxygen (p = .012) for tracks 1-2, and the model accounted for 0.609 of the sa mple variation. Tracks 4 through 7 were related to temperature (p = .017), dissolved oxygen (p < .0001), turbidity (p < .0001) and pH (p < .0001), and the model accounted for 0.560 of the sample variation (Tables 4, 5). No significant relationship wa s yielded between shark swim ming depth and water quality variables (p > .05).
25 Table 4: Linear regression model summa rizing water quality in fluence on actively tracked bull sharks 1 Â– 2. VariablesCoefficientt Value p Salinity-0.003-8.2070.001 Temperature-0.003-4.1980.001 Diss. Oxygen0.0012.5620.012 Turbidity0.0011.5480.124N125 pH-0.004-1.0780.283 R20.609 Diel-0.008-7.2980.001Standard error of est.0.005 Constant26.783915.2130.001Significance (p-value)0.001 Model Summary Table 5: Linear regression model summa rizing water quality in fluence on actively tracked bull sharks 4 Â– 7. VariablesCoefficientt Value p Salinity0.00005480.3490.727 Temperature0.0012.4000.017 Diss. Oxygen0.0014.0850.001 Turbidity-0.001-4.5900.001N284 pH-0.016-5.7970.001 R20.560 Diel-0.006-11.9560.001Standard error of est.0.004 Constant26.644951.0040.001Significance (p-value)0.001 Model Summary
26 Discussion Utilization of estuarine regions by euryhali ne sharks has largely gone unstudied. Bull sharks, C. leucas are widely reported to utilize fresh and brackish waters throughout their range (Bass et al. 1973; Compagno 1984) Recently, Pillans and Franklin (2004) reported the presence of bull sharks along an en tire salinity gradient in Australia with smallest individuals found in the freshwater reaches of the system. They suggested that physiological limitations (the inability to upregulate urea) may explain the lack of juvenile bull sharks in fully marine water. However, they also suggested that the movement of bull sharks should be investigat ed to further define use of habitats and potential physiological implicat ions of use of estuarine re gions. We have found that young bull sharks in a dynamic Florida estuar ine system display distinct movement patterns within that habitat and may respond to environmental changes which affect their movements and distribution. The home range size of young bull sharks in the Caloosahatchee River (1.2 Â– 4.3 km2) were similar to that described for ot her juvenile sharks including scalloped hammerheads, Sphyrna lewini (0.5 Â– 3.5 km2; Holland et al. 1993a) and sandbar sharks, C. plumbeus (1.9 Â– 14.7 km2; Medved and Marshal 1983). Results were also similar to the home range size found for juvenile bull sh arks in FloridaÂ’s Indian River Lagoon (0.02 Â– 3.49 km2; Curtis 2008). Similarity among juven ile sharks suggests that this space utilization may be typical for young sharks. In addition, home range size was consistent among individuals within the study despite the fact that individuals were collected in different portions of the estuary.
27 When examined using cluster analysis, the six tracks were divided into two distinct groups. This groupi ng indicated movement patterns of individuals tracked in the northern part of the study site were differe nt from those tracked approximately 10 km downriver in the southern region, suggesti ng that either indivi dual differences or differences in location may be influencing movement patterns. Heupel and Simpfendorfer (2008) suggested that changes in environmental c onditions within the Caloosahatchee River caused synchronous downriver movement of an entire monitored population (c. 18 sharks per year) of bull shar ks. Movement in relation to changes in salinity was also documented on FloridaÂ’s east coast, where indivi duals moved between a creek and an open lagoon, dependi ng on precipitation levels, to remain within a preferred salinity range (Curtis 20 08). If this movement pattern is consistent, individuals captured in the southern portion of the site during this study may have been displaced from further upriver due to the large freshwater influx th at occurred between tracks 3 and 4. Thus, differences in movement patterns between th e two clusters may be related to location within the river (at a given point in time) and differences in ha bitat in those regions. The upriver portion of the estuar y is slightly wider, shallower and more natural than downriver areas. Examination of diel patterns revealed distinct differences in day and night movement patterns and locations for most i ndividuals. The first two tracked animals displayed a larger nighttime space use accompanied by an increased ROM and distance traveled during the night. This result was not unexpected since many shark species have been reported to increase home range size and swimming speed at night (Gruber et al. 1988; Klimley and Nelson 1984; Holland et al 1992; Ackerman et al. 2000; Vaudo and
28 Lowe 2006). Tracks 5 through 7, however, show ed larger daytime home ranges with a faster ROM and larger distances traveled during the day. Those individuals were also shown to utilize more random movement pa tterns in shallow water at night. A substantial shift in environmental parameters could be the impetus for behavioral change between the two groups of juvenile bull shar ks in this study. The magnitude of the freshwater influx that occurred prior to the tracking of sharks 4-7 moved the salt-wedge downriver and likely displaced many bony fish species upon which they prey (Snelson et al. 1984). Increased river discharge has been linked to behavioral changes in several bony fish species by altering thei r habitat selection (Brenden et al. 2006; Albanese et al. 2004). A positive relationship was also f ound between freshwater inflow and fish abundance in a southwest Florida estuary (Fla nnery et al. 2002). This relationship may drive predators downstream during freshwate r influx events to maintain favorable foraging conditions. The ROM, or speed of movement over ground (as opposed to swimming speed), is affected by the linearity of shark moveme nt and is an approximation since the position of the shark cannot be fixed exactly. A lthough ROM is not an accurate measure of swimming speed, it is helpful in elucidating te mporal behavioral changes. The ROM of the six sharks examined here ranged from 0 73.5 m/min (0 4.41 kmh-1), with a mean of 11.07 m/min (0.67 kmh-1). This is lower than the m ean reported for adult bat rays, Myliobatis californica (8.84 kmh-1; Matern et al. 2000), adult leopard sharks, Triakis semifasciata (4.9 kmh-1; Ackerman et al. 2000), a nd the range of 0 Â– 38.2 kmh-1 for juvenile lemon sharks, Negaprion brevirostris (Sundstrm et al. 2001). This result, however, approximates the mean ROM of 1.53 kmh-1 reported for neonate and juvenile
29 sandbar sharks, C. plumbeus (Rechisky and Wetherbee 2003) and the mean ROM of 0.55 kmh-1 (0.154 m/s) reported for juvenile and youn g-of-the-year bull sh arks on FloridaÂ’s east coast (Curtis 2008). Analysis of depth data showed that a pproximately 60% of all shark fixes were recorded when the shark was swimming wher e water depth was less than 1 m. This depth preference is similar to that of 1.0 Â– 1.5 m reported for young bull sharks in a Florida lagoon, where 65% of the positions were recorded in less than 2.0 m (Curtis 2008). This suggests shallow porti ons of the estuary ar e key habitat for this species. Bull sharks in this study exhibited a uniform tr end of spending a large amount of time during the day in the middle of the ri ver and closer to the banks at night. Correspondingly, this meant that sharks were slightly deeper in th e water column during the day than at night. Similarly, gray reef sharks ( C. amblyrhynchos ) were reported to display predictable diel patterns of utilizing d eeper water during the day and move d to shallower areas at dusk, likely for foraging (Nelson and Johnson 1980). A linearity index helps to determine if sh arks are traveling in long-ranging linear paths, or making small, random movements. Examination of this movement parameter provides information on how individuals are using habitat and ma y provide clues to behavior during those periods. Morrissey a nd Gruber (1993b) calculate d a linearity index value of 0.044 for juvenile lemon sharks and concluded they were highly site attached due to regular re-visita tion of preferred areas. Rechis ky and Wetherbee (2003) reported a linearity index of 0.2 (range = 0.02 Â– 0.62) fo r neonate and juvenile sandbar sharks indicating a more linear pattern of movement. In the Caloosahatchee River, juvenile bull sharks had a linearity index of 0.29 (range = 0.04 Â– 1.0) indicating more linear paths than
30 both juvenile lemon sharks and sandbar sharks This result is not unexpected as lemon sharks were tracked in an open lagoon, sandbar sharks in a large bay, and bull sharks in a narrow river which provided physical constraints to movement. Bull sharks in this study also exhibited more linear trav el than that of bull sharks tracked in the Indian River Lagoon, Florida (mean = 0.18; Curtis 2008). Although there were periods when C. leucas traveled a linear path, random moveme nts were more common. Individuals displayed a higher degree of ci rcular or random movements at shallower depths and more linear travel in greater de pths. Movement within a shallow nursery habitat by young sandbar sharks was attributed to predator avoidance, avoidance of currents, and distribution of prey (Rechis ky and Wetherbee 2003). Since there are no natural predators of bull sharks in the Calo osahatchee River (Heupel unpublis hed data), and generally minimal current speed, it is likely that a la rge portion of juvenile bull shark movement patterns may be attributed to the distributi on of and search for prey. Little is known about the movement patterns of their primary prey species, ariid ca tfishes and dasyatid stingrays, however, D. sabina was found to have small, restricted movements in a shallow tidal lagoon (Schmid 1988). Nursery areas provide abundant food sources and it is probable that the movement patterns of bu ll sharks are reflecti ng the distribution and movement patterns of th eir prey species. The results demonstrated that tidal flow w ithin the estuary had a significant effect on movements. Shark depth, linearity of m ovement, and direction of travel were all significantly correlated with tidal stage. Sharks have been reported to move in relation to environmental variables in previous studies a nd may have been using tidal transport as a means of conserving energy. For example, sa ndbar sharks and Atla ntic stingrays have
31 been reported to move with tidal flow (Teaf 1978; Medved and Marshall 1983; Rechisky and Wetherbee 2003). Leopard sharks also used currents for movement to and from muddy littoral zones that contained an abundanc e of food (Ackerman et al. 2000). Based on this pattern, Ackerman et al. (2000) de termined that leopard sharks potentially conserved 6% of their total energy expenditure by swimming with currents. This would suggest that bull sharks may be utilizing pa ssive transport in order to conserve or reallocate energy. A second explanation for this behavior could lie in environmental variables within the region. If bull sharks have preferences for specific environmental conditions as suggested by Simpfendorfer et al. (2005) Heupel and Simpfendorfer (2008), and Curtis (2008), then this movement may be a means of remaining in a desired environmental regime. Movement downriver and swimming closer to the bottom on a falling tide would allow indivi duals to remain in potential ly more saline or well-mixed water and avoid freshwater in the upper por tion of the water column. Movement with tides may further be an indirect result from foraging for prey which are likely to be tidally influenced. Many bony fish have exhibited mo vement patterns that correlate to tidal activity (Kanou et al 2005; Krumme 2004; Dresser and Kneib 2007). Therefore, movement is likely a means of optimizing ener gy allocation either via passive transport and/or maintenance of favorable e nvironmental and foraging conditions. Movements of juvenile C. leucas may also be directly related to changes in water quality, specifically salinity, in order to decrease energy expende d for osmoregulation. The process of osmoregulation in seawater wa s determined to require 6 to 10% of the total energy budget of th e euryhaline killifish, Fundulus heteroclitus (Kidder et al. 2006). This expenditure was suggested to be enough for behavioral osmoregulation, a process of
32 seeking a medium isotonic with body fluids, to be a significant driving force in killifish movement (Kidder et al. 2006). While bull sh arks are capable of osmoregulating in a wide salinity range (Pillans and Franklin 2004; Pillans et al. 2005; Pillans et al. 2006), Heupel and Simpfendorfer (2008) reported that young bull sharks remained within a salinity range of 7 to 20 Â‰, a nd avoided areas of less than 7 Â‰. Curtis (2008) reported that despite a range of availa ble habitats, bull sharks select ed locations with salinities above 11 Â‰. In previous studies, salinity and temperature were found to be the most important factors determini ng the distribution and abundanc e of four elasmobranch species including the bull shark in Florida (Heupel and Simpfendorfer 2008), and the bat ray, M. californic a, leopard shark, T. semifasciata and brown smoothhound shark, Mustelis henlei, in California (Hopkins and Cech 2003). These two environmental factors were also determined to in fluence the location of cownose rays, Rhinoptera bonasus in the Chesapeake Bay (Smith and Merriner 1987). Thus, changes in these variables may be key to juve nile bull shark movement and distribution. Water quality parameters may also play an indirect role in bull shark movement due to the influence of fluctuating conditions on the distribution of prey species. Temperature and salinity were found to be primary factors influencing m ovement (Harrison and Whitfield 2006) and community assemblages (Vega-Cendejas and Hern ndez de Santillana 2004) of fishes in estuaries. These water quality variables were specifically determined to be important in structuring estuarine assemblages on Florid aÂ’s east coast (Kupschus and Tremain 2001; Paperno and Brodie 2004). Arius felis and B. marinus two primary food items for bull sharks, are known to prefer salinity a bove 10 Â‰ (Muncy and Wingo 1983). Therefore, during high freshwater influx ev ents, these species may be displaced downriver to remain
33 within a certain salinity range. The downriver movement of the sharks may have been in efforts to follow the prey population, the distri bution of which changed in order to fulfill physiochemical requirements. Although significant differences in water quality variables over time and diel differences within tracks were reported, as ide from the large freshwater influx event, changes were generally subtle. Locations of the first two sharks were related to salinity, temperature, and dissolved oxygen, while locations for sharks 4 through 7 were related to temperature, dissolved oxygen, turbidity and pH These differences are likely due to the different locations in the river. Although dissolved oxygen, pH and turbidity influenced shark distribution it is unclear what role these factors play in influencing movement patterns. It is difficult to define the role that small variations in water quality have on movement over a short period of time, especial ly for a species that is known to have considerable environmental tolerances. This makes it necessary to examine short-term movement patterns with long-term trends. Fo r example, temperature is more likely to have a role in shark presence over a longe r period, showing seasonal variation (e.g. Simpfendorfer et al. 2005; Grubbs et al. 2007; Heupel 2007). Although there is evidence that salinity plays a role in long-term distribution of bull sh arks in this estuary (Heupel and Simpfendorfer 2008), small variability in water quality made it impossible to determine movement drivers in the short-te rm. Therefore, mechanisms driving shortterm movement patterns of bull sharks within the Caloosahatchee River may be dependent on a number of confounding variable s and conditions. It did appear, however that a large influx of freshwater changed the location of individuals within this habitat
34 supporting the conclusion of Heupel and Simpfendorfer (2008) and suggesting water management practices causing large cha nges should be carefully examined. Habitat alterations in the Caloosahatchee River due to canals, locks, and dams have been substantial in the last century. Increasing urbani zation of the lower sections has resulted in a loss of mangroves and other native vegeta tion (Barnes 2005; Simpfendorfer et al. 2005). Water flow in the Caloosahatchee River is managed by the South Florida Water Management District to maintain water supply for agricultural purposes, to sustain appropriate levels in Lake Okeechobee and to supply water to the Everglades (South Florida Water Management District 2000). An thropogenic hydrologic modifications have altered not only the water quality throughout the river, but also the magnitude, timing, and distribution of flows to the estuary (Haunert et al. 2000). Historically, rainfall runoff was contained within the unde veloped watershed during the wet season which prevented heavy, fast fres hwater flows into the river. Thus, populations of C. leucas were probably not frequently exposed to large environmental fluctuations. Water management practices currently create large, rapid changes in salinity which appear to have a direct effect on the distri bution of juvenile C. leucas. The flow rate documented between tracks 3 and 4 was 86 m3s-1, more than double the average flow of 36 m3s-1 during 2006. There appears to be a cr itical threshold between these flow rates which influences the population distri bution. In periods of high rainfall and augmented river flow, salinity in the rive r may drop low enough to require young sharks to move out of the protected nur sery areas into the bay where they will face a higher risk of predation (Simpfendorfer et al. 2005). Th is may have severe consequences for this population if continued for an extended peri od. The Caloosahatchee River is likely to
35 undergo restoration as part of the Greater Ev erglades Region and it will be necessary to have an understanding of how resident sp ecies are affected by water management practices. In order to deve lop less invasive use of wate r resources, the behavior and biology of mobile estuarine species should be considered. An understanding of how these species respond to environmental stresso rs will provide a basis for well-informed management decisions.
36 CHAPTER THREE: HABITAT USE OF JUVENILE BULL SHARKS, Carcharhinus leucas IN A FLORIDA NURSERY AND THE INFLUE NCE OF WATER QUALITY AND QUANTITY MODIFICATION Abstract There is a population dec line in many coastal shar k species accompanied by a lack of information on which to base appropr iate management policies. Estuarine areas serve as nursery habitat for many plant and an imal species and are at high risk from anthropogenic modification. This study examin ed the relationship between changes in water quality and quantity and the distributi on and residency pattern s of juvenile bull sharks in a Florida estuarine nursery. Individuals displayed distinct movement patterns in relation to alterations in flow rates and salinity. Sharks were distributed throughout the river at low flow rates, but we re located only near the river m outh, or exited the river, at discharges rates above 75 m3s-1. This paper examines current water management policy and makes recommendations based on the phys iological preferences of this top-level predator. Introduction There has been a drastic popul ation decline in many coas tal shark species of the Atlantic over the past 30 years (NMFS 1993; Burgess et al. 2005). The 1993 Fisheries Management Plan for Sharks of the Atlant ic Ocean (FMP) recognized a disturbing lack
37 of information regarding shark fisheries a nd biological data re quired for appropriate fisheries management. Estuarine nurseries we re specifically identified by the FMP as areas of great concern for coastal sharks becau se these locations are directly exposed to anthropogenic alteration (NFMS 1993). The in creasing level of coastal development makes it necessary to determine what consti tutes estuarine shark nursery areas and how these habitats are used by juvenile sharks. The definition of a shark nursery has been vague and these areas have ge nerally been considered as locations in which protection from predation and adequate food supp lies are provided (Springer 1967; Bass 1978; Branstetter 1990). However, in a recent effort to define nurseries in a way that allows for quantitative determination, these sites were attr ibuted with three testable criteria; they contain a higher density of juve niles in relation to other areas individuals tend to remain or return for extended periods, and the habita t is used repeatedly over a period of years (Heupel et al. 2007). The Caloosahatchee Estu ary in southwest Flor ida qualifies as a nursery for bull sharks and is highly impact ed by anthropogenic alterations in flow rates and salinity levels. The purpose of this res earch was to examine in closer detail the relationship between distribut ion and abundance of juvenile bull sharks in a coastal estuarine environment and physical factors, specifically flow rate and salinity. The degradation of water quality in estu aries has been identified as a potential threat to coastal shark populations, howeve r the effects of this alteration is unknown (National Marine Fisheries Service (NMFS 1993). High levels of variation in river flow discharge and salinity have been found to affect the distribu tion of aquatic species (Bain and Finn 1988; Moser and Gerry 1989; Paper no and Brodie 2004; Harrison and Whitfield 2006), and specifically on bull sharks (He upel and Simpfendorfer 2008, Curtis 2008).
38 This study was conducted to complement previ ous research (Chapter 1) which examined short term movement patterns of juvenile bull sharks in relation to environmental fluctuations. That study also found a di rect relationship between significant modifications in water quality and quantity a nd the distribution of bu ll sharks over a short period of time due to a direct physiological or indirect prey-driven influence. However, due to the short-term nature of the study, it was difficult to in vestigate in detail the role that water quality played on shark distri bution. Those results necessitated further research to determine how fluctuations in environmental parameters affect the distribution of C. leucas over a longer period of time. Bull sharks are cosmopolitan in tropical and subtropical coastal marine waters (Garrick 1982). This elasmobranch speci es has unique physiology and osmoregulatory capabilities which enable it to reside in both freshwater and saltwater for extended periods (Thorson 1971; Thorson et al. 1973; Montoya and Thorson 1982). Females are thought to give birth in estuar ies or in proximity to river mouths and juveniles tend to move upstream after parturition (Thorson 1972; Last and Stevens 1994). Juveniles have exhibited a preference for fresh or estuarine water following birth and are rarely found in marine environments (Branstetter and Stile s 1987; Thorson et al. 1973; Thorson 1976), possibly due to an inability to up-regulate urea in marine environments (Pillans and Franklin 2004). Bull sharks are a common shark species in FloridaÂ’s coastal and estuarine areas, many of which are believed to provide important nur sery habitat (Snelson et al. 1984; Michel 2002; Si mpfendorfer et al. 2005). The Caloosahatchee River in southwest Florid a served as the site of this study and is a nursery for bull sharks in their first ye ar of life (Simpfendorfer et al. 2005). Water
39 flow into the river is managed by the S outh Florida Water Management District (SFWMD) and water quality parameters can change rapidly, on a scale from hours to days (Simpfendorfer et al. 2005), dependi ng on precipitation levels and discharge regimes. In heavily regulated systems, such as the Caloosahatchee River, it is essential to understand the effect that modifications to environmental parameters have on resident species. An understanding of how these animals are affected by anthropogenic modification within the estuary will aid in developing informed management decisions. Materials and Methods Study Site This study was conducted in the Caloosahatch ee River, in southwest Florida. The Caloosahatchee River serves as an artif icial hydrologic connection between Lake Okeechobee and the Gulf of Mexico and has been highly impacted over the previous 100 years due to heavy urbanization and channeli zation. Water flow throughout the system is currently regulated by the SFWMD via dams for hydrologic and agricu ltural purposes. Episodic freshwater discharge from the dams, up to 1278 m3s-1 (SFWMD et al. 2008), can create rapid, dramatic fluctuations in wa ter quality variables down stream, particularly salinity. During periods of hi gh freshwater discharge, salinity may drop to less than 5 Â‰ at the river mouth, and conve rsely, salinity may exceed 10 Â‰ at the head of the river during periods of low discharge. Time need ed to transition between these extremes may be less than a week (SFWMD 2002). This st udy was completed in the estuarine section of the river and encompassed approx imately 26 km of river habitat.
40 Field Methods Twelve C. leucas were collected between June and July 2006 via long line fishing. An 800 m longline was employed, wh ich consisted of an 8-mm braided nylon rope anchored at both ends. Frozen mullet ( Mugil cephalus ), ladyfish ( Elops saurus ), and fresh catfish ( Arius felis, Bagre marinus ) were used as bait on Mustad tuna circle hooks ranging from 12/0 to 14/0 in size. Soak time was approximately one to two hours. The location, date, time, duration of set, a nd environmental conditions were recorded at each fishing location. Once on board, sharks were weighed, m easured, sexed, and tagged externally with both a dart and rototag for identification. Dart tags were inserted at the base of the first dorsal fin and rototags were attached to the dorsal fin. All individuals were surgically fitted with Vemco V13P transmitters. Surgical procedures were identical to those described by Heupel and Hueter (2001) where a 2-3 cm incision was made in the abdomen and the transmitter inserted. These transmitters were identical in size and shape to those used for active tracking (see Chapter 1). The long-term transmitters pulsed once per minute at 69.0 kHz and at randomly spaced intervals between 45 and 75 seconds. Transmitters also reported the depth at wh ich the individual wa s swimming based on a pressure sensor. Each transmitter was programmed to produce a unique pulse series per individual and had a battery life of at least 12 months. Random signal transmission times helped to avoid signal overlap and the blocki ng of detections. After insertion of the transmitter, the incision was closed via runni ng nylon sutures in both the muscle and skin layers. Individuals were revived on boa rd and released in good condition. The long-term presence and movement patterns of sharks fitted with transmitters were
41 monitored via a series of 25 acoustic rece ivers (Vemco VR2) m oored within the Caloosahatchee Estuary (Figure 5). Met hods for deploying receiver stations are described in Heupel & Heuter (2001). Receivers were located in the es tuarine section of the river between the mouth and approximately 26 km upstream. Each receiver recorded the time, date, and transmitter code, which identified the individual, when an animal swam within range of a receiv er. Receivers were omnidirect ional, single frequency, and had a detection range of approximately 600 m (Heupel unpublished data), depending on variables such as ambient noise, depth, and wa ter clarity. This de tection range often allowed sharks to be detected at more than one station simultaneousl y. The array allowed for continuous monitoring of sharks for most of the time they were present in the study location. Data was downloaded from the rece ivers once every month at which time any necessary maintenance was conducted. At each receiver station, surface and bottom temperature, salinity, and disso lved oxygen were measured us ing a YSI 85 water quality meter.
42 Figure 5: The location of the Caloosahat chee River study site, with inset showing location of study site in Florida. Filled ci rcles indicate locations of acoustic receiver stations. Data Analysis Residency Data from the acoustic monitoring array were used to determine the residence period and movement patterns of C. leucas within the estuary. A shark was considered to be present on a particular day if more than one signal was detected from that individual. The total number of days that an individual was present in the study site, as well as the number of consecutive days, was calculated to determine if any pattern was evident between individuals. Three individuals were not included in anal yses due to early mortality; two animals died w ithin days of release and the third within six weeks.
43 Home Range and Depth Data from acoustic receivers was condensed and analyzed using a custom written FORTRAN program (see Simpfendor fer et al. 2008 for details). The program yielded shark position in the estuary every 30 minutes on a linear scale, with the river position of 2 km located at the mouth and 26 km located at the northern section of the study site. Position estimates were used to determine daily minimum, maximum, and mean river location for each animal. The extent of rive r space used per day was calculated as the difference between the maximum and minimum mean river locations (in river kilometer) and served as a proxy for home range. This was the most accurate method of calculating spatial usage due to the linear configuration of recei vers in the river, which would have influenced the size of an area measurement. Kruskal-Wallis determined if there was a difference in daily home range size by individu al. Position within the river was separated into day and night bins and Wilcoxon paired samples tests were performed to determine if there were diel differences in either rive r location or home range size. Fixes that were recorded between the hours of 0700 and 1900 were considered daytime and between 1900 and 0700 were considered nighttime. A Kruskal-Wallis test determined if home range size changed significantly over weekly or monthly periods. Data for these analyses were examined from the onset of monitoring, in June or July, 2006 and concluded in November 2006 since only two animals remain ed in the estuary past that month. To determine if there was a difference am ong individuals in average depth within the water column, a Kruskal-Wallis test was used. A Wilcoxon paired samples test was conducted to determine if individuals disp layed a diel depth pattern. A Pearson Correlation determined if there was a relations hip between location in the river and depth
44 in the water column. A Kruskal-Wallis test was performed to determine if depth changed significantly over time. Four individuals were not used in depth analyses because three did not survive and depth was not accurately recorded by the receiver for another individual. Statistical tests were performed with SPSS, version 16.0. Water Quality and Quantity The number of sharks present each day was calculated and compared to daily river flow and salinity levels us ing linear regression. Flow ra te data was recorded at the Franklin Locks, an area upriver from the study area and 35 km from the mouth of the river. Salinity was a daily mean of the con tinuously measured value recorded at the Cape Coral Bridge, 10.5 km from the river mouth. Sa linity and flow data were obtained from the SFWMD and the values at Cape Coral were used as indicators of the regimes present in the river on each day. These daily valu es have been determined to be accurate measures of conditions throughout the river (P. Doering, Pers comm.). To examine relationships between shark loca tion with salinity and flow, th e daily mean river distance for each animal was also compared to these variables using linear regression. Regression analysis was also conducted to determine if there was a significant relationship between salinity and flow rate. To further investigate the relationship between water quality and shark location within the estuary, the mean river distance of each shark on days when receivers were downloaded, thus when water quality parameters were recorded, was compared to water quality variables. Values betw een all stations were highly co rrelated, as determined by a Pearson correlation, therefore, water quality da ta were compared to a single station.
45 Linear regression was used to elucidate pot ential relationships between shark location with salinity, temperature, di ssolved oxygen, and turbidity. Results Residency The movements of twelve neonate bull shar ks were recorded beginning in June (n = 5) and July (n = 7), 2006 and ending in Ma rch, 2007. Seven individuals were females, five were males, and the animals ranged in size from 74 to 83 cm stretch total length (STL); sampling data is shown in Table 6. Two individuals remained within the study site until March 2007, however th e remainder of the sharks were absent after November 2006. Duration of residency throughout the study ranged from 13 to 285 days, with sharks utilizing the nursery for an average of 138 days (Figure 6). Individuals regularly moved in and out of the detec tion range of the acoustic syst em, with sharks leaving the estuary for varying amounts of time. Shar ks were present in the study site for consecutive periods of 181 days, with a m ean of ten days con tinuously present.
46 Table 6: Biological data for bull sharks monito red within the Caloosah atchee estuary. ID indicates transmitter number, size is indicated as Standard Total Length (STL), Fate is defined as S = apparent survival and D = i ndividuals that died within the study site. Shark IDSexSize STLFateMonitoring DatesTotal days monitored 241F83S6/17/06 3/28/07285 242F70S6/14/06 11/06/06146 243F74D6/16/06 6/16/061 244M76S6/19/06 3/18/07273 245F78S7/03/06 11/01/06122 246F81S6/15/06 7/31/0647 247M82S7/03/06 10/23/06113 248M77S7/03/06 7/15/0613 249F82S7/03/06 11/13/06134 250F82D7/03/06 7/13/0611 251M82D7/03/06 8/17/0646 252M76S7/03/03 11/07/06128
47 Figure 6: Presence and absence of neona te bull sharks monitored within the Caloosahatchee River during 2006 and 2007. In order to elucidate patterns in re sidency, the proportion of the monitored population that was present in the estuary throughout the study period was examined. The proportion of individuals present ranged from 0 to 89 % per day (mean = 35 %, median = 22 %), with the highest number of individuals being detected in July, August, and October of 2006, and few from Nove mber 2006 Â– March 2007, (Figure 7a,c).
48 Figure 7: Proportion of neonate bull sharks present, shown in grey, within the Caloosahatchee River relative to a) freshwater inflow as i ndicated by rates measured at Franklin Locks and c) salinity as indicated by values measured at the Cape Coral Bridge. Flow and salinity are each represented by black lines. Scatterplots show the proportion of sharks present versus b) flow rate and d) salinity. a.
49 b. c.
50 d. Home Range and Depth Linear distance traveled served as a pr oxy for home range of the nine individuals, and ranged from 0.1 to 8.83 km per day (mean = 1.8 km, median = 1.4 km), as shown in Figure 8. Results showed a significant di fference in home range size by individual (Kruskal-Wallis, df = 8, p < 0.0001). Daily home range size differed significantly based on diel period (Wilcoxon, Z = -10.203, p < 0.0001); sharks displayed an increased home range size at night (mean = 1.94 km) than during the day (mean = 1.27 km). Home range size changed significantly on a weekly basi s (Kruskal-Wallis, df = 40, p < 0.0001), but did not exhibit any uniform change over ti me. Home range size did not change significantly based on the month (Kruskal-Wallis, df = 5, p = 0.146). However, there was a population wide expansion of home ra nge size in October, 2006 (mean = 3.23 km) with the smallest average home range si ze observed the following month, in November, 2006 (mean = 0.62 km). Throughout the study, i ndividuals spent 46 % of their time
51 within 14 km of the river mouth; mean river location is shown in Figure 9. The difference in daily mean river location for all sharks based on diel period was negligible (day = 13.74 km, night = 13.58 km). Figure 8: Daily home range of animals, s hown in grey, present from June 2006 to November 2006 relative to a) freshwater flow and b) salinity. Flow and salinity each represented by black lines. a.
53 Figure 9: Relationship between distribution of acoustically ta gged bull sharks within the Caloosahatchee River relative to a) freshwat er influx and c) daily average salinity. Scatterplots show daily mean river location rela tive to b) freshwater and d) salinity. Mean river location is shown in the grey sh aded area and the black lines represent the water quality variable. a.
54 b. c.
55 d. Daily average swimming depth ranged fr om 0.0 to 3.3 m. There was a significant difference between individuals in daily av erage depth (t-test, df = 732, p < 0.0001). Individuals exhibited a diel trend in average depth (Wilcoxon, Z = -13.518, p < 0.0001), with animals swimming deeper in the water column during the day. The mean daily day depth was 1.07 m and the averag e daily night depth was 0.81 m. There was a significant correlation between location in the river a nd average daily depth (Pearson, p < 0.0001), with sharks swimming deeper in the water co lumn as they approach ed the river mouth. There was no difference in shark depth ove r time (Kruskal-Wallis, df = 5, p = 0.493).
56 Water Quality and Quantity From June 14, 2006 to March 28, 2007, indi viduals were exposed to a salinity range of 0.15 Â– 28.64 Â‰ (mean = 16.48 Â‰, median = 19.67 Â‰) and flow rates from 0 Â– 596 m3s-1 (mean = 31 m3s-1, median = 7 m3s-1). There was a significant positive relationship between the daily proportion of animals resident and average daily salinity (R2 = 0.434, slope = 0.707, p < 0.0001) and a signi ficant negative relationship with daily mean flow (R2 = 0.115, slope = -3.926-5, p < 0.0001). A higher proportion of individuals were present in the river during conditions of low flow (< 50 m3s-1) and salinities approximately 512 Â‰. There was also a significant negative relationship between the log of salinity and the log of river flow (Figure 10) which confirmed that increased flow rates throughout the river were accompanied by decrea ses in salinity levels (R2 = 0.618, slope = -2.058, p < 0.0001).
57 Figure 10: Scatterplot of daily salinity versus river flow. Examination of shark distribution within the estuary in relation to daily salinity and flow data showed a strong relationship. Analysis of shark location in the river yielded a significant positive lin ear relationship for salinity (R2 = 0.516, slope = 1.469, p < 0.0001) indicating that as salinity in the rive r increased, sharks traveled away from the river mouth. Conversely, as salinity decrea sed, sharks moved towards the mouth of the river. Shark position in the river had a negative linear relationship with flow (R2 = 0.341, slope = -1.106, p < 0.0001) showing th at at flow rates below 50 m3s-1, sharks were distributed throughout the river. However, as flows increased above approximately 75 m3s-1 animals were located in close proximity to the mouth of the river. Although the strength of the relationship differed, both sa linity and flow rate exhibited a significant influence on shark location.
58 Environmental parameters were record ed at each receiver station during download events, and these valu es showed a significant rela tionship with the mean river location of the sharks. Comparison of mean river location showed a significant positive relationship with salinity (R2 = 0.869, slope = 0.351, p = 0.002), but no relationship with temperature (R2 = 0.504, slope = -0.470, p = 0.074), turbidity (R2 = 0.847, slope = -1.770, p = .080), or dissolved oxygen (R2 = 0.121, slope = 1.148, p = .445). The positive relationship between river location and salinity on days in which the receivers were downloaded supports the significant result between shark position and daily SFWMD salinity values. This further strengthens the hypothesis that salinity was a primary factor in bull shark distribution. Discussion Bull sharks are managed as part of the large coastal shark fishery complex in the Gulf of Mexico and the A tlantic Ocean. They are taken both commercially and recreationally and the complex is regarded as overfished (Corts et al. 2002). During the past 30 years, there has been a serious decl ine in the population of many shark species on the Atlantic coast due to the expansion of commercial shark fisheries (NMFS 1993; Burgess et al. 2005). The Fisheries Management Plan for Sharks of the Atlantic Ocean (FMP) stated that the abundance of large co astal species in the Atlantic Ocean has potentially declined as much as 75 % from the 1970s to the 1980s (NMFS 1993). These data are especially concerning since most shar ks have slow growth, are late maturing, and have very low fecundity compared to bony fishes (Camhi et al. 1998). Most shark species are consequently not capable of rebounding quickly to population reduction and cannot tolerate high levels of fishing without stock collapse (Camhi et al. 1998; Musick
59 1999). A high level of populat ion reduction in elasmobranchs not only affects this toplevel predator, but also their prey species and may have second and third degree affects through trophic linkages (Steve ns et al. 2000; Schindler et al. 2002). Despite these significant top-down effects, there is a seri ous lack of information on shark fisheries (NMFS 1993). It is necessary to protect immature stocks and their habitats in order to provide a sustainable fisherie s population (Cortes et al. 2002). An effective management plan will benefit by a clear understanding of what constitutes a shark nursery and how juveniles utilize these habitats. Specifi cally, how shark distri bution is affected by significant artificial modification of water cond itions within a nursery is important for both biological and management purposes. The Caloosahatchee River flows into San Carlos Bay, a shallow bay which is cut by many deep channels which run longitudina lly, and drains lower Pine Island Sound and Matlacha Pass to the Gulf of Mexico. The bay has less variation in salinity than the river due to its close proximity to the Gulf and during periods of high flow, juveniles may be forced to move into this open region and face an increased threat of predation. Typically, the smallest and youngest individuals stay with in the river and neonates are found in June and July (Simpfendorfer et al. 2005). Simpfendo rfer et al. (2005) suggested that neonate and young-of-the-year C. leucas which are found in the Caloosahatchee River remain through the summer after partur ition. After animals reach approximately 95 cm STL, they tend to move out of the river and into northern San Carlos Bay and later into Pine Island Sound (Simpfendorfer et al. 2005). Of the nine individuals which survived throughout the duration of this study, seven utilized the estuary th rough October of 2006 and two remained for ten months post captu re. Duration of residency for young bull
60 sharks within the Caloosahatchee (13 285 days) exceeded that of bonnethead sharks Sphyrna tiburo (1 173 days; Heupel et al. 2006) and cownose rays Rhinoptera bonasus (1 Â– 102 days; Collins et al. 2007) in adjacent regions. Variation in proportional resi dency appeared to relate to both fluctuations in salinity and freshwater discharg e. In many shark nurseries, it has been documented that presence is influenced by seasonal changes in temperature or photope riod (Grubbs et al. 2007; Heupel 2007; Merson and Pratt 2001). Howe ver, it appeared that presence of juvenile bull sharks within this system was largely influenced by acute fluctuations in water quantity and related changes in quality rather than by moderate changes over time. There were a reduced proportion of sharks pr esent during periods of high flow and low salinity. Conversely, periods of low flow a nd moderate salinity resulted in a greater proportion of individuals present (Fig 3). The mean daily fl ow rate during this study was 31 m3s-1, however, in August, 2006 there was an extreme discharge event of 596 m3s-1, which reduced salinity throughout the river to less than 0.5 Â‰ for approximately two weeks. During this period, the proportion of sharks present in the river dropped from 80 % to approximately 20 %. These results concur with the findings by Heupel and Simpfendorfer (2008), which determined that ha bitat use within estu ary and river systems was influenced by salinity and flow, especially for bull sharks less than one year of age. Although young bull sharks are physiologically ca pable of gradual (P illans et al. 2005) and acute (Pillans et al. 2006) transition between freshwat er and saltwater, previous research has shown they exhibit consistent salinity preferences. Based on CPUE results, Simpfendorfer et al. (2005) determined that salinity was an important variable in determining the distribution and presence of C. leucas, and individuals less than one year
61 of age were present most frequently in salinities between 7Â‰ and 17.5Â‰. This was supported by Heupel and Simpfendofer (2008) who reported a salinity preference of 7 Â‰ to at least 20 Â‰ and suggested that young bull sharks may sel ect moderate salinity levels to reduce osmoregulatory costs and allow mo re energy to be allocated to growth functions. There has been considerable resear ch regarding how euryhaline fishes physiologically cope with salinity fluctuations (Pillans and Franklin 2004; Pillans et al. 2005; Pillans et al. 2006; Kidder et al. 2006), however there ha s been little focus on how movement is used to stay within a pr eferable range of conditions (Heupel and Simpfendorfer 2008). Small bull sharks tend to remain within fresher areas whereas large individuals are more commonly found in marine waters (Pillans and Franklin 2004; Simpfendorfer et al. 2005). Small individua ls may not be able to up-regulate urea (Pillans and Frankin 2004), sugge sting that they are physiolo gically limited to fresher locations until they are further developed. Pr evious research has shown that fishes use behavioral osmoregulation to remain within iso-osmotic conditions (Kidder et al. 2006) and higher growth rates were exhibited in the teleost, Oreochromis niloticus when reared in iso-osmotic conditions rather than fresh or saltwater (Woo et al. 1997). Distribution of young bull sharks in this study appeared to be directly related to environmental change. A potential explanation for this behavior is that young sh arks may conserve energy by seeking salinity equal to that in which they were acclimated prior to a high discharge event rather than remaining to acclimate to new conditions. In either case, behavioral osmoregulation is likely due to either physio logical limitations or efforts to conserve energetic costs.
62 This study was performed to complement the previous study on the short-term movements of juvenile bull sharks in the Ca loosahatchee River (Chapt er 1). That study examined general movement patterns as we ll as a relationship between movement and water quality variables. The mean da ily linear activity space (0.1 Â– 8.83 km) approximated the results from the short-te rm study (1.9 Â– 4.8 km). Animals in both studies displayed an increased use of space at night. Diel depth trends (day = 1.07 m, night = 0.81 m) also approximated that for i ndividuals in the shortterm study (day = 1.5 m, night = 0.6 m), with sharks consistently swimming deeper in the water column during the day. The short-term study examined relationshi ps between shark location and salinity, temperature, dissolved oxygen, turbidity, and pH. Although all variables were found to be significant, the strength of the relati onship varied. In this study, movement was compared to salinity and flow gathered fr om both daily values recorded by the SFWMD as well as water parameters recorded on da ys when receivers were downloaded. Flow and salinity were found to be significant in affecting the di stribution of bull sharks for each test while temperature, dissolved oxyge n, and turbidity had no influence. The results for the three water parameters were inconclusive between st udies, suggesting that further research needs to be done to determ ine what relationship, if any, these variables have on shark movement. The Caloosahatchee River and estuary is a highly impacted system with major changes in its historic hydrology due to significant modifications in land and canal development (Barnes 2005). Regulatory rele ases of freshwater from Lake Okeechobee have created large changes in the natural qua ntity, timing, and quality of flow to the
63 estuary (Barnes 2005). These hydrologic change s are associated with alterations in salinity which is a critical determinant of estuarine habitat characteristics and can affect distribution of rooted vegetation, and se ssile and motile biota (Alber 2002). Modifications in the Caloos ahatchee River are often ma de without considering the biological integrity of the syst em (Haunert et al. 2000). This system is sensitive to high salinity levels and provides an indicator of the health of the en tire watershed. The Caloosahatchee River serves as a nursery ground for many estuarin e and coastal plant and animal species (Barnes 2005; South Fl orida Water Manageme nt District 2000b). Although current data does not ye t exist to substantiate, it is expected that the river has declined in juvenile fish abundance, distribut ion, and species richness due to changes in the natural salinity regime and freshwater discharge (Barnes 2005). The state of Florida requires the five Water Management Districts to develop Minimum Flows and Levels (MFLs) for all pr iority water bodies, and the South Florida Water Management District is responsible for the Caloosahatchee River. An MFL is the limit at which further withdrawals would be si gnificantly harmful to the water resources or ecology of the area (Barnes 2005). Although the rule sp ecifically addresses MFLÂ’s in order to assess the damage to water resources from low flows, research also explores damage from high discharge even ts. Scientific deficiencies were identified in the initial effort to set MFLs for the Caloosahatchee River, including a lack of documentation on the effects of MFL flows on downstream estu arine biota (SFWMD 2002 ). In response, the SFWMD took a Valued Ecosystem Compon ent based approach for setting inflow requirements and therefore chose an im portant set of resources and tailored environmental policy based on the requirement s of specific resources (Alber 2002). For
64 the Caloosahatchee River, proposed MFLÂ’s were created by examining the salinity tolerance of three species of seagrass, Vallisneria americana Halodule wrightii and Thalassia testudinum with the assumption that salinity and flow conditions in which these species thrive will also be preferable for other organisms in the estuary (Alber 2002; Chamberlain and Doering 1998). This re search determined that flow above the 71 85 m3s-1 range (SFWMD 2002) was detrimental to tapegrass, Vallisneria americana, whereas a flow rate of approximately 8 m3s-1 was optimal. A flow rate of 8 m3s-1 yields an average daily salinity of 10 Â‰ at the Ft. My ers salinity monitoring site, however this is highly variable depending on seasonal prec ipitation levels a nd contribution from downstream tidal basin inflows (SFWMD 2002). This propos ed flow regime would be appropriate for juvenile bu ll sharks, however, MFLÂ’s are currently not being met and flow rates continue to be highly variable wh ich provides an inadequate level of resource protection (SFWMD 2002). The Caloosahatchee estuary is a connected segment of the Greater Everglades Ecosystem due to its connection with Lake Okeechobee and is theref ore included in the Comprehensive Everglades Restoration Plan (CERP) (SFWMD 2002). The CERP addresses water supply needs throughout South Florida and has projects in place that will provide total flows to the es tuary, distribute total flow between upstream and downstream areas, and affect the spatial and temporal va riability of salinity within the estuary. Specifically, the plan includes an abov e ground storage rese rvoir within the Caloosahatchee watershed to supplement Lake Okeechobee water storage which will help to reduce the effects of too mu ch or too little freshwater entering the estuary. Currently, the CERP goal is to have these structur al components in place by 2011 (SFWMD and
65 USACOE 2002). Since high disc harge rates have been shown to have an effect on the distribution of juvenile bull sharks, oftentim es forcing them out of the nursery area to remain within preferred conditions, attainment of the CERP goal woul d beneficial for this species. The Lake Okeechobee Watershed Constructi on Project Phase II Technical Plan was created by state agencies to consolidat e the numerous initia tives which addressed restoration of the Everglades, including CE RP (SFWMD et al 2008). This Plan redefined the optimal flow rate range as 13 Â– 798 m3s-1 and reported that levels have largely remained in that range since the incepti on of MFLs in 2000, however extreme high flow events of greater than 1278 m3s-1 have continued to occur (SFWMD et al. 2008). Although this range was determined optimal, data from this research suggests that this adjusted flow regime may not be appropriate for juvenile bull sharks. Young bull sharks in this study all exhibited be havior of moving towards the river mouth during an extreme discharge event, with most sharks exiting the study area. Juveniles were shown to exhibit behavioral osmoregulation at flow rates above 75 m3s-1. These data substantiate the findings of Heupel and Simpfendof er (2008) who reported that juvenile bull sharks were found throughout the river at lo w flow rates, (< 57 m3s-1), but were only near the mouth at flows over 113 m3s-1. The Caloosahatchee system provides nursery habitat to many species of animals and plants. In order to ensure protection for these estuarine communities, further research will be necessary to develop wa ter management practices which establish appropriate freshwater inflow criteria. Current water management practices have been found to be inadequate for juvenile C. leucas as there continues to be highly variable flow
66 rates and releases that are high enough to affect the dist ribution of the population. We recommend that the physiological requirements of this top-level pred ator are considered in the formulation and enforcement of comprehensive water management policies.
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