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Seasonal Distribution and Habitat Associations of Bull Sharks in the Indian River Lagoon, Florida: A 30-Year Synthesis


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Many coastal shark species use shallow estuarine regions as nursery habitat, but there are considerable gaps in our understanding of the seasonal distribution and habitat use patterns of sharks within these systems. We compiled all available sampling data from the Indian River Lagoon (IRL) along Florida's central Atlantic coast to examine the distribution of bull sharks Carcharhinus leucas. The data synthesized in this study spanned the 30-year period 1975–2005 and included information on the seasonal distribution, size structure, and habitat associations of 449 bull sharks. For comparison, data from an additional 106 bull sharks captured in shelf waters adjacent to the IRL were also examined. The IRL is dominated by young-of-the-year (age-0) and juvenile bull sharks, which were most abundant during spring, summer, and autumn. Shark captures were most often associated with shallow freshwater creeks, power plant outfalls, ocean inlets, and seagrass habitats with temperatures greater than 20°C, salinities of 10–30‰, and dissolved oxygen concentrations between 4 and 7 mg/L. Juvenile bull sharks were found in waters with higher mean salinities than were age-0 sharks. Although the IRL is one of the most important bull shark nursery areas on the U.S. Atlantic coast, catch-per-unit-effort data indicate that bull shark abundance decreases with increasing latitude within and north of the IRL, suggesting that the IRL is the northern limit of functional nursery habitat for this species in the northwest Atlantic Ocean.Received August 12, 2010; accepted March 3, 2011
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Transactions of the American Fisheries Society 140:1213–1226, 2011
American Fisheries Society 2011
ISSN: 0002-8487 print / 1548-8659 online
DOI: 10.1080/00028487.2011.618352
Seasonal Distribution and Habitat Associations of Bull
Sharks in the Indian River Lagoon, Florida: A 30-Year
Tobey H. Curtis*1
Florida Program for Shark Research, Florida Museum of Natural History, University of Florida,
Museum Road, Gainesville, Florida 32611, USA
Douglas H. Adams
Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute,
1220 Prospect Avenue, Suite 285, Melbourne, Florida 32901, USA
George H. Burgess
Florida Program for Shark Research, Florida Museum of Natural History, University of Florida,
Museum Road, Gainesville, Florida 32611, USA
Many coastal shark species use shallow estuarine regions as nursery habitat, but there are considerable gaps in
our understanding of the seasonal distribution and habitat use patterns of sharks within these systems. We compiled
all available sampling data from the Indian River Lagoon (IRL) along Florida’s central Atlantic coast to examine
the distribution of bull sharks Carcharhinus leucas. The data synthesized in this study spanned the 30-year period
1975–2005 and included information on the seasonal distribution, size structure, and habitat associations of 449
bull sharks. For comparison, data from an additional 106 bull sharks captured in shelf waters adjacent to the IRL
were also examined. The IRL is dominated by young-of-the-year (age-0) and juvenile bull sharks, which were most
abundant during spring, summer, and autumn. Shark captures were most often associated with shallow freshwater
creeks, power plant outfalls, ocean inlets, and seagrass habitats with temperatures greater than 20C, salinities of
10–30‰, and dissolved oxygen concentrations between 4 and 7 mg/L. Juvenile bull sharks were found in waters with
higher mean salinities than were age-0 sharks. Although the IRL is one of the most important bull shark nursery areas
on the U.S. Atlantic coast, catch-per-unit-effort data indicate that bull shark abundance decreases with increasing
latitude within and north of the IRL, suggesting that the IRL is the northern limit of functional nursery habitat for
this species in the northwest Atlantic Ocean.
The identification of critical habitats for marine species is
essential for sound management of populations; however, habi-
tat use data are lacking for many species. Although information
regarding specific habitats used during all life stages is impor-
tant, recent emphasis has been placed on delineation of nursery
areas for early life stages (e.g., Beck et al. 2001; McCandless
*Corresponding author:
1Present address: National Marine Fisheries Service, Northeast Regional Office, 55 Great Republic Drive, Gloucester, Massachusetts 01930,
Received August 12, 2010; accepted March 3, 2011
et al. 2007). Many continental shelf-associated fishes and inver-
tebrates use coastal and estuarine systems as nursery areas ow-
ing to their relatively high productivity and shallow, protected
waters (Beck et al. 2001). These inshore and nearshore sys-
tems, however, suffer from dramatic anthropogenic alteration
and habitat loss, potentially affecting the survival of species
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that rely on coastal habitats during their most vulnerable life
stages. Baseline distribution and habitat use data and continued
monitoring are necessary so that future impacts can be properly
Along the Atlantic coast of the United States, numerous car-
charhiniform sharks use coastal areas as nursery habitat (e.g.,
Castro 1993; Merson and Pratt 2001; McCandless et al. 2007;
Reyier et al. 2008). Shark nursery areas have traditionally been
defined as regions where parturition occurs or where young
sharks spend the first months or years of their lives, or both
(Castro 1993). “Primary” nurseries are areas where parturition
occurs and young-of-the-year (age-0) sharks are abundant, while
“secondary” nurseries are areas that juveniles (>1 year old) in-
habit (Bass 1978). Recently, more tangible criteria for defining a
region as a shark nursery were proposed by Heupel et al. (2007).
These criteria are (1) density of juvenile sharks is greater in the
putative nursery area relative to other areas, (2) juvenile sharks
exhibit higher-than-average site fidelity to these areas (i.e., not
transient), and (3) the area is used repeatedly by juvenile sharks
across years (Heupel et al. 2007). If these criteria are met, then
the area is likely to support increased production of the shark
population in question, and can be considered a functional nurs-
One species in need of more information on nursery area
delineation and habitat associations is the bull shark Carcharhi-
nus leucas. The bull shark is a circumglobal, macropredatory
species in tropical and subtropical coastal waters (Compagno
1984). It is one of the largest carcharhinid sharks, reaching 400
cm total length (TL) and weighing up to 600 kg (Compagno
1984; McCord and Lamberth 2009). Size at birth is 60–80 cm
TL (Snelson et al. 1984). Males become reproductively mature
at 210–220 cm, while females mature at lengths greater than
225 cm (Branstetter and Stiles 1987). It is one of the few com-
pletely euryhaline sharks, readily occurring in brackish estuaries
and freshwater rivers throughout its range (Thorson 1971, 1972;
Thomerson et al. 1977; Compagno 1984). In the western North
Atlantic Ocean, bull sharks range from Massachusetts to Brazil
and are taken in longline, gill-net, and sport fisheries throughout
that region (Compagno 1984; Morgan et al. 2009). In U.S. wa-
ters, they are managed in conjunction with other coastal sharks
by the National Marine Fisheries Service (NMFS) through
the Atlantic Highly Migratory Species Fishery Management
As with many species of sharks, slow growth rates, late matu-
rity, and low fecundity (Compagno 1984; Branstetter and Stiles
1987; Neer et al. 2005) make bull sharks particularly suscepti-
ble to overexploitation by fisheries. Although bull shark stock
status in the United States is presently unknown (NMFS 2006),
there is evidence of localized population declines in the Gulf of
Mexico (Jones and Grace 2002; O’Connell et al. 2007), and the
World Conservation Union (IUCN) categorizes the species as
“Near Threatened” worldwide. Compounding the effects of fish-
ing pressure is their coastal and inshore distribution, which may
disproportionately expose bull sharks to adverse anthropogenic
environmental impacts such as wastewater pollutants, contami-
nants, and habitat loss. Overfishing of some coastal sharks in the
northwest Atlantic Ocean has prompted the need for updated bi-
ological information on essential shark habitats, including nurs-
ery areas that are important for juvenile survival (NMFS 2006,
Potential bull shark nursery areas on the U.S. Atlantic coast
extend from North Carolina to Texas and typically include shal-
low, brackish, intracoastal lagoons, bays, and riverine systems
(Snelson et al. 1984; McCandless et al. 2007; Froeschke et al.
2010). One such area is the Indian River Lagoon (IRL) system
on Florida’s central Atlantic coast. The occurrence and diet of
bull sharks in the northern IRL was previously documented by
Snelson et al. (1984), and baseline information regarding bull
shark occurrence in the IRL have been reported (Adams and Pa-
perno 2007), but information on spatial distribution and habitat
use patterns in the lagoon were not discussed in detail. Snelson
et al. (1984) also only surveyed a relatively small portion of the
IRL, so data have not been reported for much of the remainder
of the expansive lagoon system. The objective of this study was
to combine new shark sampling data with available historic bull
shark records from the IRL, including records from the scien-
tific literature, unpublished fishery-independent data, and other
verified observations, to provide a more comprehensive descrip-
tion of their seasonal distribution and habitat use patterns in this
region. This information will help to assess the extent to which
the IRL functions as a bull shark nursery area and provide new
data to support effective fisheries management and delineation
of essential fish habitat for this species.
Study site.—The IRL is a shallow, estuarine barrier island
system that stretches over one-third of Florida’s central Atlantic
coast between the latitudes of 2904N (Ponce de Leon Inlet) and
2656N (Jupiter Inlet) (Figure 1). This system comprises three
main basins: Mosquito Lagoon, Indian River Lagoon proper,
and Banana River Lagoon, which are interconnected by canals
or channels. There are five inlets along the length of the system
that connect these bodies to the ocean, as well as a hydraulic lock
system at Port Canaveral that provides intermittent access be-
tween the lagoon and the Atlantic Ocean. The study area spanned
from Ponce de Leon Inlet at the north end of Mosquito Lagoon
(Volusia County) to St. Lucie Inlet, 225 km to the south (St.
Lucie County) (Figure 1). The National Aeronautics and Space
Administration (NASA) Kennedy Space Center and Canaveral
Air Force Station are located within the study site on Merritt
Island, and security measures prohibit entry by fishers and other
unauthorized personnel into security zones located in the north-
ern reaches of Banana River Lagoon and Banana Creek, which
creates de facto marine reserves (Tremain et al. 2004).
The IRL spans a climatic transition zone between tropical
and warm–temperate environments, and therefore contains a di-
verse ichthyofauna (Gilmore 1995). It is home to at least 397
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FIGURE 1. Map of the Indian River Lagoon study site. The boxes delineate
the four lagoon subregions described in Methods: ML =Mosquito Lagoon, NIR
=northern Indian River and Banana Riverlagoons, MS =Melbourne–Sebastian
area, and SIR =southern Indian River Lagoon.
species of temperate to tropical fishes, many of these being
juvenile phases of offshore species (Gilmore 1977, 1995). In
addition, the IRL contains a variety of habitat types includ-
ing seagrass beds, fringing mangroves, salt marshes, oyster
bars, open sand bottom, lagoon reefs, tidally influenced fresh-
water tributaries, and ocean inlets (Gilmore 1977; Kupschus
and Tremain 2001). The mean annual salinity for the entire
IRL is 25.6‰ (Gilmore 1977), but it varies with precipitation
rates and proximity to freshwater inputs (0‰) or ocean in-
lets (>30‰). Hypersaline conditions (>40‰) also commonly
occur in Mosquito Lagoon during summer months when evap-
oration rates are high (Snelson et al. 1984). Water tempera-
tures generally annually range from 11C to 32.5C (Gilmore
1977; Tremain and Adams 1995), but occasionally drop to
less than 5C during cold periods (e.g., Snelson and Bradley
1978). It is a heavily used and economically important water-
way, with fishing, boating, and other recreational water uses
and expenditures valued at US$2.1 billion in 2007 (Johns et al.
Data sources.—To synthesize bull shark distribution infor-
mation from the IRL, verified bull shark capture and observa-
tion records were compiled from three main sources: (1) the
scientific literature (Dodrill 1977; Snelson and Williams 1981;
Snelson et al. 1984; Schmid and Murru 1994); (2) fishery-
independent sampling data from the Florida Fish and Wildlife
Conservation Commission’s (FWC) Fishery-Independent Mon-
itoring Program (Tremain et al. 2004; Adams and Paperno
2007); green Chelonia mydas and loggerhead Caretta caretta
sea turtle netting studies at the University of Central Florida
(UCF) and Kennedy Space Center (KSC); sampling conducted
as part of an ongoing shark-tagging study by the Univer-
sity of Florida (UF); and (3) personal communications from
cooperating scientists and local fishers. The data sources,
the gears used, the years represented, and the numbers of
sharks captured in each of these studies are summarized in
Table 1.
Capture methods.—Sampling effort, spatial distribution, and
techniques varied considerably across the studies from which the
data were compiled. The primary capture gears were gill net and
bottom longline, but also included rod and reel and haul seine
(Table 1). Direct visual observations of free-swimming sharks
and occasional strandings were also included in this synthe-
sis. Environmental observations, including depth, temperature,
salinity, dissolved oxygen (DO) concentration, and Secchi disk
TABLE 1. Summary of Indian River Lagoon bull shark data sources, 1975–2005. Gear types are defined as follows: LL =bottom longline, RR =rod and reel,
GN =large-mesh gill net, HS =haul seine, and OB =visual observation.
Data source Sharks (n) Locations (n) TL Range (cm) Years Gear
Dodrill (1977) 19 19 70–250 1975–1977 LL, RR
Snelson et al. (1984) 150 31 73–249 1975–1979 GN
Schmid and Murru (1994) 5 1 61–68 1985 LL, RR
FWC 50 34 72–144 1991–2001 GN, HS
UCF–KSC turtle bycatch 52 4 130–200 1996–2005 GN
Cape Canaveral Scientific 49 49 75–172 1992–2004 GN, RR
Personal communications 34 8 70–250 2003–2005 RR, OB
UF 90 90 66–130 2003–2005 LL, RR, OB
Total 449 236 61–250 1975–2005
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depth, were available from Snelson et al. (1984), FWC, KSC,
and UF, although information for all features were not available
from all studies.
Hook gear, including bottom longlines and rod and reel, was
used by Dodrill (1977) to collect sharks in the IRL and off
Melbourne Beach between 1974 and 1977. The longline was
200 m in length and consisted of 1,590-kg-test nylon mainline.
Ten baited hooks were evenly spaced along the length of the
mainline on 3.7-m-long steel chain gangions. The hooks alter-
nated in size and included Mustad 51-mm and 63-mm shark
hooks, and 14/0 tuna hooks.
Snelson et al. (1984) used large-mesh gill nets designed to
capture sea turtles in the northern IRL between 1975 and 1979,
and the elasmobranch bycatch was recorded. The gill nets were
composed of braided nylon twine with stretch mesh of 30.5–40.6
cm, net lengths from 90 to 230 m, and a net depth of 3.7 m. The
nets were deployed monthly in Mosquito Lagoon and the north-
ern IRL for periods of 24–147 h, as described by Snelson et al.
(1984). Catch per unit effort (CPUE) was calculated as the num-
ber of sharks captured per 24-h net-day [(m net deployed/100) ×
(h net deployed/24)].
The bull shark data contributed by FWC were from gill-net
captures in the IRL between 1990 and 1997 and haul seine cap-
tures between 2000 and 2005. The gill nets used were multipanel
monofilament gill nets 198 m in length and 1.8 m deep, with
a 12.7-mm-diameter polypropylene float line and a 12.7-mm-
diameter lead-core lead line. The net consisted of five panels: a
15.2-m-long panel of 50-mm stretch mesh, and four 45.7-m pan-
els with 76-, 102-, 127-, and 152-mm stretch mesh (Adams and
Paperno 2007). Soak times ranged from 1.53 to 4.25 h (mean =
2.65 h). The haul seine was 183 m long and 3 m deep, with
38-mm stretch mesh. Water temperature, salinity, Secchi disk
depth, and DO concentration were measured at all sampling
sites. Locations of sampling sites are provided in Adams and
Paperno (2007).
Bull sharks captured by UCF and KSC were bycatch in
ongoing sea turtle studies in the IRL. The fishing gear used
by staff from KSC and UCF consisted of anchored entangle-
ment nets, 3.6 m deep and made from 40-cm stretch mesh
nylon twine, with a lead-core lead line and foam-core float
line. The length of nets fished by KSC was 490 m, while the
length of nets fished by UCF varied between 192 and 455
m (Ehrhart et al. 2007). The area surveyed by KSC included
Mosquito Lagoon and the northern IRL, while the primary sam-
pling site for UCF was in the vicinity of Sebastian Inlet (Fig-
ure 1). No detailed information on fishing effort or habitat was
Schmid and Murru (1994) used small bottom longlines to
capture neonate bull sharks from the IRL for captive study and
display at Sea World of Florida in Orlando. Collecting occurred
during the summer of 1985 near the power plant outfalls by
Port St. John (F. Murru, Sea World, personal communication).
Data provided by Cape Canaveral Scientific were from bull
sharks captured either by rod and reel or gill net (of various
configurations) within the IRL. Sampling effort and habitat data
were also not available from these sources.
In the UF study, sampling gear included a 50-hook bottom
longline composed of 305 m of 6.4-mm braided nylon mainline
and 1.5-m gangions of braided nylon with 1.6-mm stainless steel
cable leader attached to a baited 12/0 circle hook (with barbs
depressed for easier release). On some occasions only half of the
mainline was set with 25 hooks. Bait included fresh or frozen
fish (mullets Mugil spp., herrings and shads Alosa spp., threadfin
shad Dorosoma petenense, ladyfish Elops saurus, hardhead cat-
fish Ariopsis felis, stingrays Dasyatis spp., and jacks Caranx
spp.). Soak time (defined as the time between the setting of the
last hook and the retrieval of the first hook) varied depending
on environmental conditions, and ranged from 20 to 65 min, but
the majority of sets soaked for 45 min. Rod and reel was also
used at many locations where setting the longline was not fea-
sible and employed the same terminal tackle and bait, attached
to 13.6-kg (30-lb)-test monofilament fishing line. The CPUE
was measured as the number of sharks captured per 100 hook
hours (hh) of effort (1 hh =1 baited hook soaking for 1 h).
At each sampling location the water depth, bottom type, water
temperature, salinity, DO concentration, and Secchi disk depth
were recorded.
To compare the population composition of bull sharks from
the IRL with that of bull sharks captured in adjacent ocean
waters, data on bull sharks captured along the Atlantic coast in
the commercial bottom longline (BLL) fishery were obtained
from the Commercial Shark Fishery Observer Program (e.g.,
Morgan et al. 2009) and the National Marine Fisheries Service’s
Southeast Fisheries Science Center. This fishery targets larger
sharks using large hooks, but the gear still frequently captures
small juvenile sharks.
Data analysis.—For the purposes of this synthesis, the study
site was divided into four subregions: (1) Mosquito Lagoon
(ML), (2) northern Indian River and Banana River lagoons
(NIR), (3) the Melbourne–Sebastian area (MS), and (4) the
southern Indian River Lagoon (SIR) (Figure 1). All known bull
shark capture and sighting locations were plotted with geo-
graphic information systems (GIS) software (ArcGIS 9; ESRI,
Redlands, California). Owing to the lack of standardization of
fishing effort across studies and to zero-inflated data, over-
all results are largely presented in a descriptive manner, with
quantitative results presented for specific data sets. Descriptive
statistics were used to characterize the catch and environmental
variables observed from each of these subregions and for the
study site overall. Two-sample Student’s t-tests assuming equal
variances were used to test for significant differences (P<
0.05) between the habitat use of age-0 (including neonates)
and juvenile sharks. The CPUE was summarized by region and
season for each study from which sampling effort information
was available (i.e., Snelson et al. 1984 and UF). The desig-
nation of seasons follows that of Snelson et al. (1984): win-
ter =December–February; spring =March–May; summer =
June–August; and fall =September–November.
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FIGURE 2. Length frequency distribution of bull sharks captured in the Indian
River Lagoon, 1975–2005 (black bars), and off the Atlantic coast of Florida,
1994–2010 (gray bars).
Population Composition
All the data sources combined provided a total of 449 in-
dividual bull shark records from the IRL from 1975 to 2005
(Table 1) and 106 from adjacent coastal waters from 1994 to
2010. The population of bull sharks in the IRL was dominated
by immature sharks (mean ±SD =112 ±33 cm), including
neonates (less than 75 cm), age-0 sharks (less than 90 cm),
juveniles (less than 190 cm), and subadults (less than 210
cm) (Figure 2). There was a difference in gear selectivity, with
the mean ±SD TL of sharks captured by longline and rod-and-
reel gear (98 ±31 cm; n=97) being significantly less than the
length of sharks captured by gill net (135 ±34 cm; n=160)
(t-test: P<0.0001). Of the sharks for which sex was noted, 116
were male and 129 were female, yielding a male : female ratio
of 1:1.1. Juvenile bull sharks (90–190 cm) were the dominant
size-class in the lagoon (72.7% of the total) and were captured
year-round in the IRL. The few adult-sized sharks were only cap-
tured during the late spring and early summer. Neonate-sized
sharks with fresh, open or partially-healed umbilical scars were
only captured between May and August, with a peak in June.
The smallest free-swimming bull shark captured in the IRL, 61
cm, was captured in August 1985.
The only mature bull sharks captured to date within the IRL
(n=5) have been mature-sized females, some of which were
confirmed as gravid. Mature male bull sharks have not been
documented within the IRL. Dodrill (1977) discussed a large
(>250-cm) female bull shark harpooned by a fisher north of
Sebastian Inlet in late spring–early summer 1976. Snelson et al.
(1984) captured two large mature females (225–249 cm) in
ML, one in May 1975 and one in May 1979. One shark was not
fully examined but appeared gravid according to those authors.
The other shark carried 12 near-term embryos, 60.8–70.6 cm.
An anecdotal report from a local sport fisher also indicated
the observation of a large (>250-cm) shark, presumably a bull
shark, of unknown sex at the southern end of ML in May 2005.
A large bull shark was also reported from the southern IRL
near Ft. Pierce Inlet in August 2002 (Capt. S. Bachman, Ft.
Pierce, Florida, personal communication). Additionally, of the
106 observed bull sharks caught off the Atlantic coast of Florida
by the BLL fishery, 26 were mature-sized females (>225 cm)
and included one 244-cm-TL female caught in September 2003
that was carrying five pups.
Although all size-classes less than 210 cm are represented
in the lagoon catch, the length frequency of IRL bull sharks
appears to be bimodal (Figure 2) with peaks at the 70–109-cm
and 150–169-cm size-classes. Fewer juveniles in the 110–129-
cm size-class were observed. In contrast, bull sharks caught by
commercial BLL vessels along the Atlantic coast outside the
lagoon ranged from 138 to 286 cm, with a peak in the 230–249-
cm size-class (Figure 2).
Seasonal Distribution
Bull shark occurrence was lower in the SIR and ML regions
than in the NIR and MS regions, with 72 sharks being caught
in ML, 149 sharks in the NIR, 147 sharks in the MS area, and
10 sharks in the SIR area. Shark capture locations are shown in
Figures 3–5. The spatial distribution of catches was not evenly
distributed throughout the lagoon. Catches of sharks tended to
be clustered at specific sites within each subregion (Figures
3–5). In ML, most sharks were captured south of Haulover
Canal along the western shoreline of the lagoon (Figure 3). In
the NIR, shark catches were clustered near power plant outfalls
at Frontenac and Delespine (Figure 3). In the MS area, most
bull shark catches occurred within or adjacent to the freshwater
creeks that flow into the lagoon from its western shoreline, and
also at Sebastian Inlet (Figure 4). In the SIR region, all 10 sharks
were captured in the area where the St. Lucie River flows into
the IRL (Figure 5). Few sharks were captured in deeper mid-
lagoon waters, or from the Intracoastal Waterway (ICW) which
runs through the entire study site.
Bull sharks were captured year-round in the IRL, although
based on all available catch data there appears to be variation in
their seasonal distribution and occurrence. In ML, sharks were
only captured between March and November. During winter
months, no sharks were captured in ML despite various levels
of sampling effort (Tables 2, 3). In the NIR and MS areas, sharks
TABLE 2. Gill-net effort (net-days, i.e., [m net deployed/100] ×[h net de-
ployed/24]), catch of bull sharks (n), and CPUE (sharks per 24-h net-day) in
the Indian River Lagoon by subregion (ML =Mosquito Lagoon and NIR =
northern Indian River Lagoon) and season, 1976–1979, from Snelson et al.
Season Effort Sharks CPUE Effort Sharks CPUE
Winter 25.50 0.00 34.50 0.00
Spring 102.47 0.07 39.823 0.58
Summer 160.421 0.13 61.316 0.26
Fall 131.024 0.18 46.232 0.69
Total 419.352 0.12 181.871 0.39
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FIGURE 3. Locations of bull shark captures and observations in Mosquito Lagoon and northern Indian River and Banana River lagoons, 1975–2005. Asterisks
indicate the locations of power plants; MINWR =Merritt Island National Wildlife Refuge and NASA =the Kennedy Space Center.
were documented year-round, though with less frequency during
the winter. Only five bull sharks were documented in the NIR
during winter, three of which were dead or moribund following
severe hypothermal events in the lagoon. The other two sharks
were individuals tagged by FWC which were recaptured in the
NIR. The MS area, however, yielded 13 juvenile bull sharks
(144–150 cm) during the winter, primarily near Sebastian Inlet.
This is the only area sampled where nonmoribund bull sharks
were regularly captured during winter. In the SIR, sharks were
only caught between July and September.
During spring months, juvenile bull sharks were documented
in the ML, NIR, and MS areas, though they were most common
in the NIR. During the summer, neonate and age-0 sharks first
appeared in the catch, and numbers peaked in ML and MS areas
and dropped somewhat in the NIR. Sharks remained present in
all regions into the fall, particularly in the NIR and MS areas,
TABLE 3. Hook gear effort (hh =hook hours), catch of bull sharks (n), and CPUE (sharks per 100 hh) in the Indian River Lagoon by subregion (ML =Mosquito
Lagoon, NIR =northern Indian River Lagoon, and MS =Melbourne–Sebastian region) and season during the University of Florida study, 2003–2005.
Season hh Sharks CPUE hh Sharks CPUE hh Sharks CPUE
Winter 7.00 0.00 565.30 0.00 0.00 0.00
Spring 2.00 0.00 572.77 1.22 210.73 1.42
Summer 383.84 1.04 685.53 0.44 184.012 6.52
Fall 472.20 0.00 116.30 0.00 0.00 0.00
Total 865.04 0.46 1,939.710 0.52 394.715 3.80
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FIGURE 4. Locations of bull shark captures and observations in the Melbourne–Sebastian subregion, 1975–2005.
until numbers dropped off as winter approached. The available
CPUE data reflect these patterns of occurrence in both Snelson
et al. (1984) and UF studies (Tables 2, 3). In the UF study,
CPUE was greatest in the MS region and lowest in ML. Snelson
et al. (1984) also found that CPUE was lower in ML than in the
NIR. Across seasons, catch rates ranged from 0.0 to 0.18 sharks
per 24-h net-day in ML, and from 0.0 to 0.69 sharks per 24-h
net-day in the NIR (Table 2). Seasonal catch rates in the UF
study were highest (6.52 sharks per 100 hh) during summer in
the MS region. In the UF study CPUE was substantially lower
(0–1.42 sharks per 100 hh) in all other regions and seasons
(Table 3). Snelson et al. (1984) reported the highest seasonal
CPUE of their study (1.15 sharks per 24-h net day) during fall
1976 in ML and NIR.
Habitat Use
There are broad ranges of physical and biological habitats
in the IRL system (e.g., Table 4), most of which were used by
bull sharks at least occasionally. The bull sharks captured in the
IRL were found in a variety of habitat types including ocean in-
lets, brackish and freshwater creeks, around piers, over seagrass
flats, open sand and muddy bottoms, and in dredged channels.
However, there did appear to be increased occurrence in and
near freshwater creeks (15.6% of observations), seagrass beds
(18.3% of observations), ocean inlets (14.5% of observations),
and power plant outfalls (10.0% of observations) (Figures 3–5).
The physical characteristics of habitats where bull sharks were
captured was also broad (Table 5), and represent a large por-
tion of habitats available in the IRL. However, some variation
existed in environmental characteristics between lagoon subre-
gions (Table 4).
Excluding hypothermal (<10C) events (during which mori-
bund and dead bull sharks were recovered), the water tem-
peratures encountered during sampling ranged from 12.1Cto
37.0C, and bull sharks were captured in waters of 20.0–37.0C
(Table 5). The mean ±SD temperature of occurrence was
29.7 ±3.5C. Age-0 bull sharks (including neonates) were
captured in a narrower temperature range than were juve-
niles (Table 5), but the difference in temperature was not
significant. The mean temperature of capture was 30.4 ±
1.8C for age-0 sharks and 29.7 ±3.3C for juveniles
The range of salinities encountered during sampling was
0.7–42.0‰, with bull sharks captured at salinities of 1.1–42.0‰
(mean ±SD =23.2 ±10.0‰) (Table 5). Age-0 bull sharks
tended to be captured in lower salinity waters (17.0 ±7.7‰)
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FIGURE 5. Locations of bull shark captures and observations in the southern Indian River Lagoon subregion, 1999–2005.
than were juveniles (21.6 ±7.9‰) (P<0.01) (Table 5). The
low salinity areas sampled included freshwater creeks and rivers,
such as Crane Creek, Turkey Creek, and the Eau Gallie River
near Melbourne (Figure 4), and the tidally influenced St. Sebas-
tian and St. Lucie rivers (Figures 4, 5).
The range of DO concentrations encountered during sam-
pling was 2.4–10.2 mg/L (Table 4), and bull sharks were cap-
tured at concentrations of 3.2–9.2 mg/L (Table 5). The mean
DO at sites where sharks occurred was 5.5 ±1.5 mg/L. Age-
0 bull sharks were captured in areas that had lower mean DO
TABLE 4. Mean (SD) and range of environmental variables sampled in the Indian River Lagoon by subregion during the University of Florida study, 2003–2005.
Variable ML NIR MS
Depth (m) Mean (SD) 1.9(1.4) 1.2 (0.7) 1.4 (0.7)
Range 0.4–6.0 0.4–4.0 0.4–3.4
Temperature (C) Mean (SD) 25.0(2.5) 25.6 (6.4) 30.1 (3.13)
Range 20.3–31.9 12.1–37.0 17.4–33.4
Salinity (‰) Mean (SD) 30.1(4.0) 24.8 (3.0) 10.08 (7.3)
Range 20.6–36.6 16.4–29.4 0.7–31.1
Dissolved oxygen (mg/L) Mean (SD) 5.6(1.5) 6.3 (1.6) 5.0 (1.2)
Range 2.4–9.9 3.7–10.2 3.2–8.3
Secchi disk depth (m) Mean (SD) 1.1(0.2) 1.1 (0.7) 1.0 (0.3)
Range 0.8–1.5 0.2–3.0 0.2–1.5
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TABLE 5. Mean and range of environmental variables at sites where bull sharks were captured in the Indian River Lagoon, by life stage, 1975–2005.
Life stage
Variable Age 0 (n=41) Juvenile (n=71) Total (n=136)
Depth (m) Mean (SD) 1.0(0.6) 1.0(0.7) 1.0(0.6)
Range 0.2–3.4 0.3–3.0 0.2–3.4
Temperature (C) Mean (SD) 30.4(1.8) 29.7(3.3) 29.7(3.5)
Range 27.9–33.5 20.0–37.0 20.0–37.0
Salinity (‰) Mean (SD) 17.0(7.7) 21.6(7.9) 23.2(10.0)
Range 1.6–34.2 1.1–42.0 1.1–42.0
Dissolved oxygen (mg/L) Mean (SD) 4.8(1.5) 6.0(1.2) 5.5(1.5)
Range 3.2–9.0 3.8–7.9 3.2–9.2
Secchi disk depth (m) Mean (SD) 0.9(0.2) 1.0(0.4) 0.9(0.3)
Range 0.3–1.1 0.3–1.5 0.3–1.5
levels (4.8 ±1.5 mg/L) than did juveniles (6.0 ±1.2 mg/L)
(P<0.001; Table 5).
The complete depth range in the IRL (0.2–10.0 m) was sam-
pled for bull sharks. Despite this, sharks were only captured in
depths of 0.2–3.4 m (Table 5). The mean depth of occurrence
was 1.0 ±0.6 m for both age-0 and juvenile sharks (P=0.73;
Table 5).
The water clarity levels encountered during sampling, as
measured by Secchi disk depth, ranged from 0.2 to 3.0 m (Table
4). Bull sharks were captured at Secchi disk depths of 0.3–1.5
m (Table 5). The mean Secchi disk depth where sharks occurred
was 0.9 ±0.3 m, and was similar for both age-0 and juvenile
sharks (P=0.72; Table 5).
Based on 30 years of capture data, it is clear that the northern
IRL (ML, NIR, and MS regions) is commonly used by immature
bull sharks and functions as an important primary and secondary
nursery area in the region. Insufficient data were available from
the southern IRL to make the same determination for that region.
However, the northern IRL meets all of the criteria for a shark
nursery area under the Heupel et al. (2007) definition. Based
upon the observations compiled in this study within the lagoon,
and the uncommon occurrence of juvenile bull sharks in adjacent
coastal and offshore areas (Figure 2; Aubrey and Snelson 2007;
Adams and Paperno 2007; Reyier et al. 2008), there is a higher-
than-average density of juvenile bull sharks in the northern IRL.
Bull sharks occur in the IRL repeatedly across years, and have
been regularly documented within the lagoon since the 1970s
(Table 1). Finally, recent acoustic tracking studies in the northern
IRL indicate significant levels of site fidelity spanning days
to months by bull sharks to specific lagoon habitats (Curtis
2008; J. Imhoff, Florida Museum of Natural History, personal
communication). Collectively, this information supports the role
of the northern IRL as a functional bull shark nursery area, the
most significant bull shark nursery on the U.S. Atlantic coast.
Immature bull sharks are uncommon in other Atlantic coast
estuaries and coastal areas that have been sampled (Castro 1993;
McCandless et al. 2007).
By synthesizing several data sets spanning multiple decades,
a more complete picture of the seasonal distribution and habitat
use patterns of IRL bull sharks was obtained. These results im-
prove upon previous studies on bull sharks in the IRL (Dodrill
1977; Snelson et al. 1984; Adams and Paperno 2007) and pro-
vide a comprehensive review of bull shark distribution in this
productive system. This information may prove useful for the
management of bull shark populations, help in delineation of
essential fish habitat, and guide future research efforts in this
Examination of bull shark distribution patterns in other re-
gions using fishery-independent sampling has been described in
several studies (e.g., Bass 1978; Simpfendorfer et al. 2005; Mc-
Candless et al. 2007; Wiley and Simpfendorfer 2007; Heithaus
et al. 2009; Froeschke et al. 2010), and the habitat use patterns
identified in this study are largely consistent with the results of
these studies. In general, age-0 and juvenile bull sharks tend to
use shallow tropical and subtropical estuarine intracoastal re-
gions as nursery areas. Their seasonal distribution within these
systems can be influenced by numerous physical and biolog-
ical conditions (e.g., temperature, salinity, DO, prey distribu-
tion). The increased frequency of bull sharks in specific habitats
and apparent avoidance of certain physical conditions, however,
suggest preferences for certain areas (Table 5; Figures 3, 4).
However, more research will be necessary to confirm potential
habitat selection patterns.
The reduced frequency in the IRL of bull sharks in the
110–129-cm size-class may be an artifact of gear bias and varia-
tion in the sampling techniques and effort of the different studies
included in this synthesis. The reduced frequency of bull sharks
in that size-class could indicate that sampling gears were inef-
ficient at capturing sharks of that size, rather than their reduced
occurrence in the study site. Snelson et al. (1984), who used
large-mesh nets, failed to capture any sharks in the 86–116-cm
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length range and postulated that this was also a result of gear
bias, rather than absence of sharks from the lagoon. Bull shark
data from the other sources compiled herein corroborate that
conjecture. In fact, sharks 90–109 cm long appear to represent
the dominant size-class in the lagoon (Figure 2). These findings
underscore the importance of using multiple gears that select
for various sizes for characterizing shark populations in nursery
The low catches of sharks greater than 190 cm, however, were
not considered to be influenced by gear bias, as large bull sharks
were readily taken by certain gears (Dodrill 1977; Snelson et al.
1984) and would probably have been observed visually if they
were more abundant. Sharks greater than about 190 cm appear
to have reached the size at which they leave the nursery and fully
transition to offshore adult habitats. According to age-at-length
estimates for bull sharks (Neer et al. 2005), individuals of that
size are approximately 9 years of age. In the Caloosahatchee
River estuary on Florida’s Gulf of Mexico coast, Simpfendor-
fer et al. (2005) captured bull sharks 68–189 cm stretched TL
and estimated their ages at 0–10 years. Wiley and Simpfendor-
fer (2007) sampled elasmobranchs offshore from the Florida
Everglades and captured bull sharks 73–210 cm stretched TL,
although most sharks were less than 190 cm. Blackburn et al.
(2007) also reported that bull sharks in coastal Louisiana waters
ranged from 44 to 136 cm fork length (55–166 cm TL), and
Steiner et al. (2007) reported that bull sharks in Florida’s Ten
Thousand Islands estuary were 43–120-cm precaudal length
(60–162 cm TL). Based on these length ranges (see Neer
et al. 2005 for length conversions), bull sharks from the Atlantic
coast of Florida and the Gulf of Mexico appear to transition from
nursery to adult habitats at lengths of approximately 160–180
cm. Bull shark length frequency data from the offshore large
coastal shark fishery reflect this pattern (Figure 2). However,
smaller juvenile bull sharks (<95 cm) have occasionally been
documented in the offshore region as well (Dodrill 1977; D. H.
Adams, unpublished data).
Bull sharks primarily use the northern half of the IRL (Se-
bastian Inlet to Ponce de Leon Inlet) during spring, summer,
and autumn. In the late spring, gravid adult female bull sharks
enter the lagoon via inlets (Snelson et al. 1984), and parturition
occurs between May and August, after which time the adult
sharks probably exit the lagoon (Dodrill 1977; Snelson et al.
1984). The occurrence of gravid female bull sharks in nursery
areas before parturition was also observed by Bass (1978) in
South Africa. Some of the mature-sized female bull sharks from
Atlantic Ocean waters adjacent to the IRL documented in this
study may have been preparing to enter the lagoon to give birth.
Beginning in October or November, age-0 and juvenile sharks
appear to migrate out of the northern portions of ML and the
NIR. It is not currently known whether the bull sharks exit ML
via Ponce de Leon Inlet to the north, or through Haulover Canal
and into the NIR. Bull sharks begin to migrate back into the
northern IRL around March of each year.
The northernmost reaches of the IRL (i.e., the ML and NIR
regions) appear not to serve as an overwintering ground for bull
sharks. While some sharks remain in the NIR during winter,
particularly near thermal refugia like heated power plant outfalls
or in the deep (6–10 m) human-made basins of the northern
Banana River Lagoon (Snelson and Bradley 1978; J. Imhoff,
Florida Museum of Natural History, personal communication;
D.H.A., unpublished data), it is speculated that most sharks leave
the area, either moving offshore or south in the IRL. The higher
catches of bull sharks near Sebastian Inlet in winter support
this hypothesis. Dodrill (1977) also captured age-0 bull sharks
off the ocean beaches outside of the IRL in November and
January. Additionally, a 105-cm female juvenile bull shark was
caught and tagged about 31 km (19 mi offshore from Cocoa
Beach, Florida, in February and was subsequently recaptured
within the IRL in the St. Sebastian River the following July
(D.H.A., unpublished data). Owing to limited sampling, only
a small number of bull sharks were documented in the SIR.
It is not known whether sharks also occur during winter in
this subregion, but based on the increases in winter catches
as latitude decreases, it is likely that they use or transit the
southern portions of the lagoon during this time. Future fishery-
independent sampling efforts should expand to focus more on
the southern IRL, particularly in the area around the St. Lucie
River (Figure 5).
Since the northern IRL is located in a climatic transition
zone (Gilmore 1995) and temperatures fluctuate seasonally and
sometimes dramatically (e.g., Snelson and Bradley 1978), the
occurrence of bull sharks in the system will probably fluctuate
in response. Based on data from other nursery areas, young bull
sharks tend to avoid water temperatures below 18–21C (Black-
burn et al. 2007; Wiley and Simpfendorfer 2007; Froeschke et
al. 2010) and unusually low water temperatures (<10C) can
be lethal (Dodrill 1977; Snelson and Bradley 1978; D.H.A., un-
published data). Temperatures below this threshold commonly
occur in the shallow water of ML and the NIR during winter pe-
riods (Gilmore 1977), and cold-killed juvenile bull sharks have
been documented along deep basins of the northern Banana
River Lagoon near KSC during winter (Figure 3). Many car-
charhinid shark species migrate south along the Atlantic coast
of the United States during winter (Kohler et al. 1998), and im-
mature bull sharks do not appear to be an exception. Bull sharks
are frequently observed at heated power plant outfalls, but it is
unknown whether these thermal effluent plumes alter or disrupt
normal migration patterns within estuarine waters.
Given the low catch rates of bull sharks in ML (even during
warmer summer and fall months) and the low reported catches
of immature bull sharks in estuaries north of ML (McCand-
less et al. 2007), ML may represent the northern extent of bull
shark nursery habitat along the Atlantic coast. In general, catch
rates of immature bull sharks appear to be greater in Gulf of
Mexico estuaries than in the IRL (e.g., Simpfendorfer et al.
2005; Blackburn et al. 2007; Froeschke et al. 2010). However,
if Atlantic coast bull shark stocks are distinct (e.g., geographi-
cally, genetically) from Gulf of Mexico stocks, the IRL nursery
area could be vital to the survival of the Atlantic stock. Recent
population genetics research on Northwest Atlantic bull sharks
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suggests significant female philopatry to natal nursery areas, but
Atlantic and Gulf of Mexico stocks are homogenized primarily
through male dispersal (Karl et al. 2011). Additionally, juvenile
bull sharks appear to be uncommon in the Bahamas, although
data are lacking on some potential nursery habitats (B. Franks,
Bimini Biological Field Station, and D. Grubbs, Florida State
University, personal communications). Therefore, the IRL may
be the most accessible nursery area for bull sharks that occur
in the Bahamas and could also function as a source of recruits
for that region. The apparent movement of a pop-up satellite
archival-tagged female bull shark from the Bahamas to the St.
Lucie Inlet area (Brunnschweiler et al. 2010) provides some
support to this hypothesis, although more tagging and genetics
data are needed.
In addition to temperature, salinity influenced the distribu-
tion of bull sharks in the IRL. Even though the sharks occurred
in freshwater, brackish water, and even hypersaline conditions
(Table 5), a finding consistent with numerous bull shark studies
(e.g., Snelson et al. 1984; McCandless et al. 2007; Froeschke
et al. 2010), there was evidence of preferences for salinities rang-
ing from approximately 10‰ to 30‰ (Table 5). The observed
size segregation of bull sharks by salinity was also observed in
estuarine waters of southwestern Florida by Simpfendorfer et al.
(2005), and was thought to be associated with either the need
for age-0 sharks to avoid larger predators, or a physiologically
driven preference to reduce the metabolic costs of osmoregu-
lation. The bull shark’s ability to osmoregulate in low salinity
environments (Thorson et al. 1973; Pillans et al. 2005), and the
use of such areas by immature sharks as nurseries, may give
this species a distinct survival advantage compared with other
coastal species that are less euryhaline. The risk of predation
by larger sharks is reduced in such shallow, low salinity regions
(Branstetter 1990; Simpfendorfer et al. 2005), and potentially
further reduced in freshwater areas. The only natural preda-
tors of juvenile bull sharks in the IRL are larger bull sharks
(Snelson et al. 1984) and possibly American alligators Alligator
mississippiensis (Curtis 2008). More research is needed to ad-
dress the salinity preferences of age-0 and juvenile bull sharks
in the IRL and to understand the factors that influence salinity
The influence of DO on elasmobranch distribution has been
examined in a few studies, but it has not been found to be a
significant predictor of habitat use in most cases (Grubbs and
Musick 2002; McCandless et al. 2007; Heithaus et al. 2009).
Since oxygen is often not limited in shallow, well-mixed, sub-
tropical estuaries like the IRL, temperature and salinity tend to
be more important factors for elasmobranchs in coastal environ-
ments (Matern et al. 2000; Grubbs and Musick 2002; Hopkins
and Cech 2003; Simpfendorfer et al. 2005). It is therefore not
surprising that there was no discernible pattern or correlation
between bull shark distribution and DO concentrations in this
study (Table 5). However, in Florida’s Shark River Estuary,
where oxygen levels can fluctuate more dramatically, it was ob-
served that age-0 bull sharks avoided DO levels below 2.9 mg/L
and were more abundant where DO was above 5.95 mg/L
(Heithaus et al. 2009). These authors caution that DO obser-
vations should not be ignored when examining the habitat use
of sharks in estuaries.
While temperature, salinity, and DO are physical environ-
mental conditions that are selected at least in part based on phys-
iological requirements, other environmental factors, including
water clarity, bottom substrate, depth, and habitat complexity,
are less influenced by physiological tolerance than by biological
factors such as predator avoidance and the distribution of prey.
The prevalence of age-0 and juvenile bull sharks in waters less
than 2 m deep may be linked to the distribution of prey species
that use productive seagrass beds, one of the dominant habitat
types at those depths. Primary prey species such as stingrays,
catfishes, and mullet are abundant in these shallow areas of the
IRL (Snelson and Williams 1981; Snelson et al. 1989; Tremain
and Adams 1995; T. H. Curtis, unpublished data). Prey den-
sity may also influence the bull shark’s frequent use of power
plant effluents and the freshwater creeks in the IRL. Few studies
have investigated the relationships between shark distribution
and prey distribution, and those that have found mixed results
(Heupel and Hueter 2002; Torres et al. 2006; Wirsing et al.
2007). Similar investigations in the IRL, a nursery area with
few shark predators, may provide greater insights into the habi-
tat selection patterns of bull sharks relative to their prey.
Fishing mortality is often identified as a threat to shark pop-
ulations; however, the effects of habitat loss and water pollution
on shark nursery areas have received very little attention, even
though these anthropogenic impacts could negate the beneficial
functions of such habitats. Fishing pressure (directed and inci-
dental catch) is a concern for juvenile bull sharks in the IRL
that should be carefully monitored, but they are also potentially
vulnerable to an array of natural and anthropogenic stressors.
The bull shark’s proximity to coastal development, habitat al-
teration, and degradation in inshore regions like the IRL, may
expose young bull sharks to adverse water quality conditions or
reduced suitable habitat for foraging.
We found a greater occurrence of bull sharks in and near
freshwater creeks within the IRL that have been historically
degraded. The overall habitat quality of these IRL tributaries
has been drastically reduced in recent years owing to increased
anthropogenic influences including shoreline development, nu-
trient loading, manipulated freshwater flow rates, direct contam-
inant input, accumulated muck deposits, and seasonal hypoxia
(IRLNEP 2008). These stressors may directly influence neonate
and juvenile bull sharks by increasing their exposure to detri-
mental contaminants and suboptimal habitat conditions. Simi-
larly, seagrass habitats, frequently used by age-0 and juvenile
bull sharks, have experienced an overall density reduction within
the IRL, with observed reduced densities and instability within
certain areas of the IRL (e.g., from Melbourne to Vero Beach)
(Virnstein 1999; Steward et al. 2006; Virnstein et al. 2007).
Although seagrasses in some areas are stable or increasing, sea-
grasses have declined up to 70% in some areas over a 50-year
period (Virnstein 1999). The effects of seagrass habitat alteration
on bull shark distribution and abundance are currently unknown.
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Bull sharks in the IRL can also bioaccumulate high con-
centrations of contaminants such as mercury, polychlorinated
biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs),
and other brominated flame-retardant chemicals (Adams and
McMichael 1999; Johnson-Restrepo et al. 2005, 2008). The con-
centrations of PCBs and PBDEs in bull shark tissues increased
exponentially between 1993 and 2004, but the possible effects of
such contamination on shark health (e.g., endocrine disruption)
remain unknown (Johnson-Restrepo et al. 2005). Marine toxins
may also serve as additional stressors to bull sharks in the IRL
and adjacent coastal waters. The accumulation of brevetoxins
from red tide events in coastal waters of Florida is common and
widespread across multiple shark species, and can be lethal in
some instances (Flewelling et al. 2010). Significant sublethal
effects were also found in juvenile lemon sharks Negaprion
brevirostris exposed to brevetoxins in nearshore waters directly
adjacent to the IRL (Nam et al. 2010).
Further research is needed to address the relative produc-
tivities of different shark nurseries and to determine whether
certain nursery areas have become compromised by detrimental
effects on habitat. Given the vulnerability of bull sharks, and the
depleted status of some large coastal shark stocks (NMFS 2006,
2009), protection and possible restoration of nursery areas, in-
cluding mitigation of contamination and other stressors, should
be a high priority.
We express our gratitude to the individuals and organizations
that shared bull shark records for this study including F. Snelson,
F. Murru, M. Stolen, L. Ehrhart, R. Paperno, S. Kubis, D. Bagley,
J. Provancha, S. Tyson, and Cape Canaveral Scientific, Inc. For
assistance with field work we acknowledge T. Vigliotti, T. Ford,
E. Reyier, B. Delius, and numerous other volunteers. F. Snelson,
D. Parkyn, M. Heupel, and E. Phlips provided helpful guidance
over the course of this study. A. Morgan and L. Hale provided
bull shark data from the BLL observer program. For logistical
support and permitting we additionally thank the Florida Fish
and Wildlife Conservation Commission (permit 02R-718), Mer-
ritt Island National Wildlife Refuge (permit SUP 35 Burgess),
and Canaveral National Seashore (Permit No. CANA-2002-
SCI-0007). This research was supported by a grant from the
National Marine Fisheries Service (NMFS) Highly Migratory
Species Division to the National Shark Research Consortium,
and tagging supplies were provided by C. McCandless and the
NMFS Apex Predators Program, Cooperative Atlantic States
Shark Pupping and Nursery Survey (COASTSPAN). Comments
provided by three anonymous reviewers greatly improved the
Adams, D. H., and R. E. McMichael. 1999. Mercury levels in four species of
sharks from the Atlantic coast of Florida. U.S. National Marine Fisheries
Service Fishery Bulletin 97:372–379.
Adams, D. H., and R. Paperno. 2007. Preliminary assessment of a nearshore
nursery ground for the scalloped hammerhead off the Atlantic coast of Florida.
Pages 165–174 in C. T. McCandless, N. E. Kohler, and H. L. Pratt, Jr., editors.
Shark nursery grounds of the Gulf of Mexico and the east coast waters of
the United States. American Fisheries Society, Symposium 50, Bethesda,
Aubrey, C. W., and F. F. Snelson. 2007. Early life history of the spinner shark in
a Florida nursery. Pages 175–189 in C. T. McCandless, N. E. Kohler, and H.
L. Pratt Jr., editors. Shark nursery grounds of the Gulf of Mexico and the east
coast waters of the United States. American Fisheries Society, Symposium
50, Bethesda, Maryland.
Bass, A. J. 1978. Problems in studies of sharks in the southwest Indian Ocean.
Pages 545–594 in E. S. Hodgson and R. F. Matthewson, editors. Sensory
biology of sharks, skates, and rays. Office of Naval Research, Department of
Navy, Arlington, Virginia.
Beck, M. W., K. L. Heck, K. W. Able, D. L. Childers, D. B. Eggleston, B. M.
Gillanders, B. Halpern, C. G. Hays, K. Hoshino, T. J. Minello, R. J. Orth,
P. F. Sheridan, and M. P. Weinstein. 2001. The identification, conservation,
and management of estuarine and marine nurseries for fish and invertebrates.
BioScience 51:633–641.
Blackburn, J. K., J. A. Neer, and B. A. Thompson. 2007. Delineation of bull
shark nursery areas in the inland and coastal waters of Louisiana. Pages
331–343 in C. T. McCandless, N. E. Kohler, and H. L. Pratt Jr., editors.
Shark nursery grounds of the Gulf of Mexico and the east coast waters of
the United States. American Fisheries Society, Symposium 50, Bethesda,
Branstetter, S. 1990. Early life-history implications of selected carcharhinoid
and lamnoid sharks of the Northwest Atlantic. NOAA Technical Report
NMFS 90:17–28.
Branstetter, S., and R. Stiles. 1987. Age and growth of the bull shark, Car-
charhinus leucas, from the northern Gulf of Mexico. Environmental Biology
of Fishes 20:169–181.
Brunnschweiler, J., N. Queiroz, and D. W.Sims. 2010. Oceans apart? Short-term
movements and behavior of adult bull sharks Carcharhinus leucas in Atlantic
and Pacific oceans determined from pop-off satellite archival tagging. Journal
of Fish Biology 77:1343–1358.
Castro, J. I. 1993. The shark nursery of Bulls Bay, South Carolina, with a
review of the shark nurseries of the southeastern coast of the United States.
Environmental Biology of Fishes 38:37–48.
Compagno, L. J. V. 1984. FAO species catalogue, volume 4. Sharks of the world.
An annotated and illustrated catalogue of shark species known to date, part 2,
Carcharhiniformes. FAO (Food and Agricultural Organization of the United
Nations) Fisheries Synopsis 125;251–655.
Curtis, T. H. 2008. Distribution, movements, and habitat use of bull sharks
(Carcharhinus leucas,M
uller and Henle 1839) in the Indian River Lagoon
system, Florida. Master’s thesis. University of Florida, Gainesville.
Dodrill, J. W. 1977. A hook and line survey of the sharks found within five
hundred meters of shore along Melbourne Beach, Brevard County, Florida.
Master’s thesis. Florida Institute of Technology, Melbourne.
Ehrhart, L. M., W. E. Redfoot, and D. A. Bagley. 2007. Marine turtles of the
central region of the Indian River Lagoon system, Florida. Florida Scientist
Flewelling, L. J., D. H. Adams, J. P. Naar, K. E. Atwood, A. A. Granholm,
S. N. O’Dea, and J. H. Landsberg. 2010. Brevetoxins in sharks and rays
(Chondrichthyes, Elasmobranchii) from coastal waters of Florida. Marine
Biology 157:1937–1953.
Froeschke, J., G. W. Stunz, and M. L. Wildhaber. 2010. Environmental influ-
ences on the occurrence of coastal sharks in estuarine waters. Marine Ecology
Progress Series 407:279–292.
Gilmore, R. G. 1977. Fishes of the Indian River Lagoon and adjacent wa-
ters, Florida. Bulletin of the Florida State Museum, Biological Sciences
Gilmore, R. G. 1995. Environmental and biogeographic factors influencing
ichthyofaunal diversity: Indian River Lagoon. Bulletin of Marine Science
Downloaded by [NOAA Central Library], [Tobey H. Curtis] at 06:02 23 September 2011
Grubbs, R. D., and J. A. Musick. 2002. Shark nurseries of Virginia: spatial and
temporal delineation, migratory patterns, and habitat selection; a case study.
Pages 25–60 in C. T. McCandless, H. L. Pratt Jr., and N. E. Kohler, editors.
Shark nursery grounds of the Gulf of Mexico and the east coast waters of the
United States: an overview. National Marine Fisheries Service, Silver Spring,
Heithaus, M. R., B. K. Delius, A. J. Wirsing, and M. M. Dunphy-Daly. 2009.
Physical factors influencing the distribution of a top predator in a subtropical
oligotrophic estuary. Limnology and Oceanography 54: 472–482.
Heupel, M. R., J. K. Carlson, and C. A. Simpfendorfer. 2007. Shark nursery
areas: concepts, definition, characterization and assumptions. Marine Ecology
Progress Series 337:287–297.
Heupel, M. R., and R. E. Hueter. 2002. The importance of prey density in
relation to the movement patterns of juvenile sharks within a coastal nursery
area. Marine and Freshwater Research 53:543–550.
Hopkins, T. E., and J. J. Cech. 2003. The influence of environmental variables
on the distribution and abundance of three elasmobranchs in Tomales Bay,
California. Environmental Biology of Fishes 66:279–291.
IRLNEP (Indian River Lagoon National Estuary Program). 2008. Indian River
Lagoon comprehensive conservation and management plan, update 2008.
IRLNEP, St. Johns River Water Management District, Palatka, Florida.
Johns, G. M., J. Kiefer, S. Blacklocke, and D. Sayers. 2008. Indian River
Lagoon economic assessment and analysis update. Indian River Lagoon
National Estuary Program, Contract 24706, Final Report, Hollywood,
Johnson-Restrepo, B., D. H. Adams, and K. Kannan. 2008. Tetrabromo-
bisphenol A (TBBPA) and hexabromocyclododecanes (HBCDs) in tissues
of humans, dolphins, and sharks from the United States. Chemosphere
Johnson-Restrepo, B., K. Kannan, R. Addink, and D. H. Adams. 2005.
Polybrominated diphenyl ethers and polychlorinated biphenyls in a ma-
rine food web of coastal Florida. Environmental Science and Technology
Jones, L. M., and M. A. Grace. 2002. Shark nursery areas in the bay systems of
Texas. Pages 209–219 in C. T. McCandless, H. L. Pratt Jr., and N. E. Kohler,
editors. Shark nursery grounds of the Gulf of Mexico and the east coast waters
of the United States: an overview. National Marine Fisheries Service, Silver
Spring, Maryland.
Karl, S. A., A. L. F. Castro, J. A. Lopez, P. Chavet, and G. H. Burgess. 2011.
Phylogeography and conservation of the bull shark (Carcharhinus leucas)
inferred from mitochondrial and microsatellite DNA. Conservation Genetics
Kohler, N. E., J. G. Casey, and P. A. Turner. 1998. NMFS cooperative shark
tagging program, 1962–93: an atlas of shark tag and recapture data. Marine
Fisheries Review 60:1–87.
Kupschus, S., and D. M. Tremain. 2001. Associations between fish assemblages
and environmental factors in nearshore habitats of a subtropical estuary.
Journal of Fish Biology 58:1383–1403.
Matern, S. A., J. J. Cech, and T. E. Hopkins. 2000. Diel movements of bat rays,
Myliobatis californica, in Tomales Bay, California: evidence for behavioral
thermoregulation? Environmental Biology of Fishes 58:173–182.
McCandless, C. T., N. E. Kohler, and H. L. Pratt Jr. 2007. Shark nursery grounds
of the Gulf of Mexico and the east coast waters of the United States. American
Fisheries Society, Symposium 50, Bethesda, Maryland.
McCord, M. E., and S. J. Lamberth. 2009. Catching and tracking the world’s
largest Zambezi (bull) shark Carcharhinus leucas in the Breede Estuary,
South Africa: the first 43 hours. African Journal of Marine Science 31:107–
Merson, R. R., and H. L. Pratt Jr. 2001. Distribution, movements and growth
of young sandbar sharks, Carcharhinus plumbeus, in the nursery grounds of
Delaware Bay. Environmental Biology of Fishes 61:13–24.
Morgan, A., P. W. Cooper, T. H. Curtis, and G. H. Burgess. 2009. Overview
of the U.S. East Coast bottom longline shark fishery, 1994–2003. Marine
Fisheries Review 71:23–38.
Nam, D. H., D. H. Adams, L. J. Flewelling, and N. Basu. 2010. Neurochemical
alterations in lemon shark, Negaprion brevirostris, brains in association with
brevetoxin exposure. Aquatic Toxicology 99:351–359.
Neer, J. A., B. A. Thompson, and J. K. Carlson. 2005. Age and growth of
Carcharhinus leucas in the northern Gulf of Mexico: incorporating variability
in size at birth. Journal of Fish Biology 67:370–383.
NMFS (National Marine Fisheries Service). 2006. Final consolidated Atlantic
highly migratory species fishery management plan. NMFS, Silver Spring,
NMFS (National Marine Fisheries Service). 2009. Final Amendment 1 to the
consolidated Atlantic highly migratory species fishery management plan es-
sential fish habitat. NMFS, Silver Spring, Maryland.
O’Connell, M. T., T. D. Shepherd, A. M. U. O’Connell, and R. A. Myers. 2007.
Long-term declines in two apex predators, bull sharks (Carcharhinus leucas)
and alligator gar (Atractosteus spatula), in Lake Pontchartrain, an oligohaline
estuary in southeastern Louisiana. Estuaries and Coasts 30:567–574.
Pillans, R. D., J. P. Good, W. G. Anderson, N. Hazon, and C. E. Franklin.
2005. Freshwater to seawater acclimation of juvenile bull sharks (Carcharhi-
nus leucas): plasma osmolytes and Na+,K+-ATPase activity in gill, rec-
tal gland, kidney and intestine. Journal of Comparative Physiology B 175:
Reyier, E. A., D. H. Adams, and R. H. Lowers. 2008. First evidence of a high
density nursery ground for the lemon shark, Negaprion brevirostris, near
Cape Canaveral, Florida. Florida Scientist 71:134–148.
Schmid, T. H., and F. L. Murru. 1994. Bioenergetics of the bull shark, Car-
charhinus leucas, maintained in captivity. Zoo Biology 13:177–185.
Simpfendorfer, C. A., G. G. Freitas, T. R. Wiley, and M. R. Heupel. 2005.
Distribution and habitat partitioning of immature bull sharks (Carcharhinus
leucas) in a southwest Florida estuary. Estuaries 28(1):78–85.
Snelson, F. F., and W. K. Bradley. 1978. Mortality of fishes due to cold on the
east coast of Florida, January, 1977. Florida Scientist 41:1–12.
Snelson, F. F., T. J. Mulligan, and S. E. Williams. 1984. Food habits, occurrence,
and population structure of the bull shark, Carcharhinus leucas,inFlorida
coastal lagoons. Bulletin of Marine Science 34:71–80.
Snelson, F. F., and S. E. Williams. 1981. Notes on the occurrence, distribution,
and biology of elasmobranch fishes in the Indian River lagoon system, Florida.
Estuaries 4:110–120.
Snelson, F. F., S. E. Williams-Hooper, and T. H. Schmid. 1989. Biology of
the bluntnose stingray, Dasyatis sayi, in Florida coastal lagoons. Bulletin of
Marine Science 45:15–25.
Steiner, P. A., M. Michel, and P. M. O’Donnell. 2007. Notes on the occur-
rence and distribution of elasmobranchs in the Ten Thousand Islands Estuary,
Florida. Pages 237–250 in C. T. McCandless, N. E. Kohler, and H. L. Pratt Jr.,
editors. Shark nursery grounds of the Gulf of Mexico and the east coast waters
of the United States. American Fisheries Society, Symposium 50, Bethesda,
Steward, J. S., R. W. Virnstein, M. A. Lasi, L. J. Morris, L. M. Hall, and W.
A. Tweedale. 2006. The impacts of the 2004 hurricanes on hydrology, water
quality, and seagrass in the central Indian River Lagoon, Florida. Estuaries
Thomerson, J. E., T. B. Thorson, and R. L. Hempel. 1977. The bull shark,
Carcharhinus leucas, from the upper Mississippi River near Alton, Illinois.
Copeia 1977:166–168.
Thorson, T. B. 1971. Movements of bull sharks, Carcharhinus leucas,be-
tween Caribbean Sea and Lake Nicaragua demonstrated by tagging. Copeia
Thorson, T. B. 1972. The status of the bull shark, Carcharhinus leucas,inthe
Amazon River. Copeia 1972:601–605.
Thorson, T. B., C. M. Cowan, and D. E. Watson. 1973. Body fluid solutes of
juveniles and adults of the euryhaline bull shark, Carcharhinus leucas, from
freshwater and saline environments. Physiological Zoology 46:29–42.
Torres, L. G., M. R. Heithaus, and B. K. Delius. 2006. Influence of teleost
abundance on the distribution and abundance of sharks in Florida Bay, USA.
Hydrobiologia 569:449–455.
Downloaded by [NOAA Central Library], [Tobey H. Curtis] at 06:02 23 September 2011
Tremain, D. M., and D. H. Adams. 1995. Seasonal variations in species diversity,
abundance, and composition of fish communities in the northern Indian River
Lagoon, Florida. Bulletin of Marine Science 57:171–192.
Tremain, D. M., C. W. Harnden, and D. H. Adams. 2004. Multidirectional move-
ments of sportfish species between an estuarine no-take zone and surrounding
waters of the Indian River Lagoon, Florida. U.S. National Marine Fisheries
Service Fishery Bulletin 102:533–544.
Virnstein, R. W. 1999. Seagrass meadows: fish and wildlife factories. Florida
Naturalist 72:18–19.
Virnstein, R. W., J. S. Steward, and L. J. Morris. 2007. Seagrass cover-
age trends in the Indian River Lagoon system. Florida Scientist 70:397–
Wiley, T. R., and C. A. Simpfendorfer. 2007. The ecology of elasmobranchs
occurring in the Everglades National Park, Florida: implications for conser-
vation and management. Bulletin of Marine Science 80:171–189.
Wirsing, A. J., M. R. Heithaus, and L. M. Dill. 2007. Can measures of prey
availability predict the abundance of large marine predators? Oecologia
Downloaded by [NOAA Central Library], [Tobey H. Curtis] at 06:02 23 September 2011
... temperature, salinity, dissolved oxygen) can vary spatially, with latitude, inlet proximity, or freshwater influence, or temporally, by day, month, or season. Use of estuarine nurseries by young sharks can be influenced by these fluctuations (Froeschke et al. 2010, Curtis et al. 2011, Drymon et al. 2014 or other factors including body size (Simpfen dorfer et al. 2005, Lear et al. 2019. Shark species with slow growth and late maturity also typically display residency or fidelity to a nursery area for many years prior to maturity (Castro 1993, Heupel et al. 2007) compared to sharks with faster life-history characteristics (Carlson et al. 2008). ...
... The Indian River Lagoon (IRL; Fig. 1a) is an estuary of national significance located along the east coast of Florida, USA, with adjacent coastal waters designated as essential fish habitat for Atlantic coast bull sharks Carcharhinus leucas (NMFS 2017). Im ma ture bull sharks, especially young-of-year (YOY) and juvenile size classes (<190 cm total length [TL]), are common in the IRL between the Ponce De Leon and St. Lucie Inlets ( Fig. 1a; Snelson et al. 1984, Curtis et al. 2011, Roskar et al. 2021). In addition, the IRL north of Sebastian Inlet has been confirmed as bull shark nursery habitat (Curtis et al. 2011), where parturition takes place and both YOY (≤ 1 yr of age) and juvenile sharks (immature sharks >1 yr of age) occur. ...
... Im ma ture bull sharks, especially young-of-year (YOY) and juvenile size classes (<190 cm total length [TL]), are common in the IRL between the Ponce De Leon and St. Lucie Inlets ( Fig. 1a; Snelson et al. 1984, Curtis et al. 2011, Roskar et al. 2021). In addition, the IRL north of Sebastian Inlet has been confirmed as bull shark nursery habitat (Curtis et al. 2011), where parturition takes place and both YOY (≤ 1 yr of age) and juvenile sharks (immature sharks >1 yr of age) occur. Bull sharks in this region of the IRL have been shown to display small activity spaces (Curtis et al. 2013), which can be defined as the area in which an animal spends its time (Burt 1943, Grubbs 2010, Powell & Mitchell 2012, based on utilization distributions (50 or 95%). ...
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Although portions of the Indian River Lagoon (IRL), Florida (USA), serve as essential fish habitat for US Atlantic coast bull sharks Carcharhinus leucas , past studies were short-term (days to months) and encompassed only small parts of this expansive estuarine system. In this study, 29 immature bull sharks were tracked in the IRL between Port St. John and Port Salerno, Florida, and in adjacent shelf waters for up to 4 yr using passive acoustic telemetry. Dynamic Brownian bridge movement models showed small daily (50% utilization distribution [UD] = 1.00 km ² , 95% UD = 4.36 km ² ) and monthly (50% UD = 4.88 km ² , 95% UD = 24.67 km ² ) mean activity spaces that seasonally shifted (October-March) to include adjacent coastal waters. Tracked bull sharks were found to display residency in the IRL and in distinct subregions of the system. Analysis confirmed that bull shark nursery habitat extends south of Sebastian Inlet to Port Salerno, approximately 86 km farther south than previously described, and that adjacent shelf waters, which had not been studied, are important to immature bull sharks during cooler months. This study provides the first multi-year assessment of bull shark space use in the IRL, with improved resolution and over a greater expanse of the system than past studies. These movement data will be important to understanding how young bull sharks may be affected by anthropogenic stressors in this highly impacted lagoonal estuary.
... Bull sharks inhabit coastal waters from 1 to 50 m depth and once a year invade brackish waters, bays, estuaries of large rivers, and lagoons for the purpose of breeding (IMARPE, 2015: 46;Compagno et al, 1984: 479-480). In these brackish waters, bull sharks give birth to young, which spend their juvenile life in this environment until the season when sea water temperatures drop (Curtis et al, 2011). Although it should be noted that the current range of bull sharks is limited to the extremely north tropic area of the Peru, the possibility that this species inhabited south part of the north coast during the Archaic period cannot be ruled out. ...
... El cazón de leche habita en aguas costeras de 1 a 50 m de profundidad y una vez al año invaden aguas salobres, bahías, estuarios de grandes ríos y lagunas con el fin de reproducirse (IMARPE, 2015: 46;Compagno et al, 1984: 479-480). En estas aguas salobres, el cazón de leche da a luz a las crías, que pasan su vida juvenil en este entorno hasta la temporada en que la temperatura del agua del mar baja (Curtis et al, 2011). Aunque hay que señalar que el área de distribución actual del tiburón toro se limita a la zona del trópico extremadamente septentrional del Perú, no se puede descartar la posibilidad de que esta especie haya habitado la parte sur de la costa norte durante el período arcaico. ...
... This species is found mainly at depths between 1 and 30 m and is euryhaline, occupying a wide range of habitats, including coastal habitats, e.g., hypersaline lagoons, bays, river mouths, upstream in warm rivers, and turbid freshwater lakes (Ebert et al. 2013). Females give birth in estuaries (Heupel and Simpfendorfer 2011;Ebert et al. 2013), and immature individuals tend to remain in estuaries for several years (Curtis et al. 2011). According to the IUCN Red List of Threatened Species, bull shark populations are declining worldwide, and the species' threat category has changed from "Near Threatened" to "Vulnerable" according to the latest assessment in 2021 (Rigby et al. 2021). ...
... Therefore, obtaining information through this "small fishery industry" represents an important achievement, also considering that some individuals are captured during the ban season and most of them are not reported. Other bull shark nursery areas in the Caribbean and the Atlantic coast have reported, in the Gulf of Venezuela, 128 bull sharks caught in a 5-year period (representing 25 sharks/year; Tavares and Sánchez 2012), and for the Indian River Lagoon (Florida) a total of 449 bull sharks caught in a 30-year period (representing 15 sharks/year, Curtis et al. 2011). Also, Schwartz (2012) documented a total of 113 individuals captured (juveniles and adults) from 1965 to 2011 for the coastal and estuarine waters of North Carolina. ...
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Nursery areas are essential habitats for sharks, offering protection and increasing the survival of newborns. We conducted interviews with local fishers and collected data from artisanal fishery landings between January 2013 and December 2019 to investigate Chetumal Bay as a nursery area for the bull shark (Carcharhinus leucas) in the Mesoamerican Reef region. The bull shark is a coastal euryhaline shark that inhabits temperate and tropical waters worldwide. In the Mexican Caribbean, bull sharks are caught mainly as bycatch in a multi-specific artisanal fishery using nylon bottomset gillnets, longlines, and hand lines. We record 63 bull sharks in the catches ranging from 67 to 125 cm TL corresponding to immature individuals, 23 neonates with either open or healing umbilical scars (67 and 78 cm TL), and 40 YOY with present but healed umbilical scars (79 to 125 cm TL), with a notable absence of large size juveniles and adults in the catches. Bull sharks were present in landings between May and November; the highest abundance was during July. Our data provide evidence to recognize Chetumal Bay as a nursery area for bull sharks, meeting all the criteria proposed to identify nursery areas. These findings constitute the first documented evidence of a bull shark nursery area in an estuary within the Mexican Caribbean as well as within the entire region of the Mesoamerican Reef System. Moreover, we discuss the importance of this nursery in light of a newly described distinct lineage of bull sharks in Chetumal Bay.
... Significant associations with rivers were observed across all sizes; however, juvenile bull sharks were not usually detected by coastal shark listening stations until they were 157 cm total length. These size-based results support previous studies on the distribution patterns of juvenile bull sharks [37,[51][52][53][54][55][56][57] with individuals remaining in their natal river for up to five years [56] or until 160 to 180 cm TL [58] due to the suggested increased availability of prey [59] and lower associated predation risk [60] compared to coastal environments [61]. Beyond this size, individuals transitioned from estuaries to marine areas [36] and this is reflected in changes in their diets and dentition [48,62]. ...
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Simple Summary In Australia, bull sharks are one of the species implicated most regularly in shark–human interactions. The factors affecting bull shark presence in nearshore waters of New South Wales (NSW), Australia were examined to determine periods of increased overlap with beach-users and thereby potentially increased risk of shark–human interactions. We investigated the spatial ecology of 233 juvenile and large (including sub-adult and adult, >175 cm total length) bull sharks acoustically tagged and monitored over a 5.5-year period (2017–2023) along 21 coastal beaches of NSW. Our study highlights that large bull sharks were present more in waters north of 32° S with a southward distribution during summer and autumn. The occurrence of large bull sharks in nearshore waters was greatest from midday to 04:00, when water temperatures were higher than 20 °C, after >45 mm of rain and when swell heights were between 1.8 and 2.8 m. We show that current shark bite mitigation educational messaging, incorporating proximity to rivers, turbidity (rainfall) and time of day, reflect heightened periods of the occurrence of large bull sharks. We concur with the current shark smart advice that nocturnal swimming and surfing, especially in warm waters and when water visibility is poor, should be avoided for many reasons, not the least of which being the potential presence of bull sharks. However, we suggest that time of day messaging for large bull shark presence should be modified from “dawn and dusk” to instead refer to afternoon and low-light periods. Abstract Unprovoked shark bites have increased over the last three decades, yet they are still relatively rare. Bull sharks are globally distributed throughout rivers, estuaries, nearshore areas and continental shelf waters, and are capable of making long distance movements between tropical and temperate regions. As this species is implicated in shark bites throughout their range, knowledge of the environmental drivers of bull shark movements are important for better predicting the likelihood of their occurrence at ocean beaches and potentially assist in reducing shark bites. Using the largest dataset of acoustically tagged bull sharks in the world, we examined the spatial ecology of 233 juvenile and large (including sub-adult and adult) bull sharks acoustically tagged and monitored over a 5.5-year period (2017–2023) using an array of real-time acoustic listening stations off 21 beaches along the coast of New South Wales, Australia. Bull sharks were detected more in coastal areas of northern NSW (<32° S) but they travelled southwards during the austral summer and autumn. Juveniles were not detected on shark listening stations until they reached 157 cm and stayed north of 31.98° S (Old Bar). Intra-specific diel patterns of occurrence were observed, with juveniles exhibiting higher nearshore presence between 20:00 and 03:00, whilst the presence of large sharks was greatest from midday through to 04:00. The results of generalised additive models revealed that large sharks were more often found when water temperatures were higher than 20 °C, after >45 mm of rain and when swell heights were between 1.8 and 2.8 m. Understanding the influence that environmental variables have on the occurrence of bull sharks in the coastal areas of NSW will facilitate better education and could drive shark smart behaviour amongst coastal water users.
... Young bull sharks spend up to five years in these low salinity environments (Pillans 2006;Heupel and Simpfendorfer 2008;Matich and Heithaus 2012), where they are exposed to lower levels of predation by larger sharks (Heupel and Simpfendorfer 2011). In this context, Curtis et al. (2011) found out for Floridaʼs Indian River Lagoon that juvenile bull sharks even remain in this nursery until they have reached an age of nine years before they make the full transition to marine offshore habitats, but this may be exceptional and restricted to this particular locality. Investigations have revealed that C. leucas uses low salinity habitats across its range and that its distribution is limited by their availability (Gaus-mann 2021), emphasizing the importance of these habitats for the life cycle and occurrence of this shark. ...
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This article addresses the history of a resident population of bull sharks (Carcharhinus leucas) in an isolated stagnant body of water in subtropical Australia. From 1996 to 2013, six bull sharks were landlocked in a golf course lake near Brisbane. The adjacent Logan and Albert rivers trapped sharks due to major floodings. When floodwaters receded, these sharks remained in the lake, which is normally isolated from the riverʼs main channel. While this event was extensively reported in the media and recieved much public attention, it has not been investigated in depth, yet it provides an opportunity for insights into the tolerance of bull sharks to low salinity habitats and euryhalinity in this species. Currently, information on the extent of the bull sharkʼs capability to endure low salinity conditions and its longevity in these environments is scarce. The case reported here provides information on the occurrence of bull sharks for 17 years, which represents the longest uninterrupted duration in a low salinity environment that ever has been recorded in this species. Bull sharks arrived first in the lake as juveniles but through time, they have reached maturity. This occurrence presents not just another ordinary bull shark record from a low salinity environment but instead a record of physiological and scientific importance. Therefore, details of the residency of C. leucas in an Australian golf course lake are reported here.
... and the east coast of Florida (Curtis, Adams, & Burgess, 2011). Bull shark juveniles can access rivers to feed, preying on species that migrate between rivers and oceans during tidal changes. ...
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Introduction: The bull shark, Carcharhinus leucas, is particularly vulnerable to anthropogenic actions because of its permanence in coastal ecosystems; populations depletion is registered in different places around the world. Aggregations of bull sharks have been reported in the North Pacific of Costa Rica, at Islas Murciélago, within the Guanacaste Conservation Area. Objective: To study the residency of bull sharks at San Pedrillo islet, Islas Murciélago. Methods: During the study period (June 2013 to February 2015) we used passive telemetry to tag 10 bull sharks. Results: All the sharks tagged were females, they were detected on 59 798 occasions by the acoustic receiver deployed in San Pedrillo. Acoustic signals from tagged sharks were received for a total period of 1 to 229 days (mean = 73.9 ± 71.3 days), with the last detections occurring on 9 January 2015. The Residency Index for each tagged shark across the entire monitoring period ranged from 0.41 to 1.00. The bull shark activity showed a significant pattern throughout the day at the receiver that specifically corresponded with the daily light cycle. Conclusions: This study concludes that San Pedrillo is an aggregation site (cleaning station) for bull sharks (C. leucas), possibly related to reproduction and not feeding behaviors.
... While the behavior of bull sharks in estuarine and riverine environments has been the subject of previous studies (Heupel and Simpfendorfer 2008;Ortega et al. 2009;Heupel et al. 2010;Curtis et al. 2011;Curtis et al. 2013), it is important to continue to monitor for behavioral changes associated with climate change for which temperature increases and sea level rises are predicted to alter or increase the frequency in which bull sharks may utilize these near-coastal brackish to freshwater environments (Heithaus et al. 2009;Matich et al. 2020). Recent studies have already demonstrated that given their reliance on these ecosystems as nurseries, bull sharks are susceptible to climate change-driven alterations of estuaries (Niella et al. 2022). ...
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A previous paper in this journal by Shell and Gardner assessed various factors around the exploration of the Mississippi River by bull sharks (Carcharhinus leucas Müller and Henle, 1839) based on two twentieth-century occurrences. Recent evidence has suggested one of these occurrences is a probable hoax. Here, we provide a correction to our earlier paper, as well as additional comments on extralimital euryhaline vertebrates in the Mississippi River system, the environmental and historical contexts for their exploration into riverine systems, and suggest steps for any future effort to detect the usage of these river systems by bull sharks.
... Many elasmobranchs exhibit repeated use of specific areas over various levels of periodicity which can equate to residency, natal or regional philopatry, and site fidelity (Chapman et al. 2015). However, detailed information regarding movement and space use patterns within these commonly used areas is difficult to obtain (Curtis et al. 2011). These data are necessary to provide ecological insight into systems of high ecological and economic importance. ...
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Large estuaries are often highly productive and biodiverse areas of ecological and economical significance. Assessing the movement behavior and space use patterns of mobile organisms in these highly dynamic areas is critical for understanding ecological dynamics within systems. Fine-scale movement data, which is useful for successful species management and conservation, is lacking for ecologically relevant species in many estuarine systems, due to the demands for fine-scale data collection methods and increased use of more passive tracking methods. Six bonnetheads (Sphyrna tiburo) and four bull sharks (Carcharhinus leucas) were tracked within the Apalachicola Bay system using active acoustic telemetry for periods up to 52 h to compare space use patterns and rates of movement across tidal and diel cycles. Tidal and diel periods had significant effects on the rate of movement (ROM) of both species, with increased ROM during crepuscular periods. Movement behavior was likely driven by a combination of optimal foraging strategies, predator avoidance, and abiotic factors. Unique movement behaviors exhibited by mature bonnetheads provide clarity for their high bycatch rates in the Gulf of Mexico shrimp trawl and should be considered during the formulation of conservation and management measures for the species. By comparing simultaneous active and passive acoustic telemetry data from bonnetheads, we determined that ROM has a negative effect on passive acoustic detection success. More investigation into the effects of environmental conditions and movement on detection efficiency is needed as passive telemetry becomes more widely used to study aquatic animal movement.
A wide variety of species depend on mangroves, making them essential habitats in tropical and subtropical regions. While the function of mangrove habitat is well studied for taxa such as teleost fishes, limited attention has been directed towards their value for elasmobranchs. Here, we review the available literature on how and why elasmobranchs use mangrove habitats based on 65 papers identified through literature searches. The use of mangrove habitat as nursery areas in combination with other coastal habitats has been well examined, although we found taxonomic and regional biases in research. Additionally, mangrove habitats are considered to offer elasmobranchs feeding opportunities and refuge from predators, yet such functions have rarely been tested. In particular, the ecological role of elasmobranchs within mangrove habitats is poorly known; as it is difficult to study their behaviour in complex mangroves, their feeding ecology is understudied as are trophic linkages between mangrove and adjacent ecosystems. For a greater understanding of the association between mangroves and elasmobranchs, direct observations of elasmobranch behaviour in mangrove habitats are needed. Given global concern regarding mangrove loss, understanding how this affects elasmobranch populations will require long-term studies, particularly in those regions where losses are greatest. We identify 8 key research questions that will help improve this understanding.
Florida's Indian River Lagoon (IRL) has experienced large-scale, frequent blooms of toxic harmful algae in recent decades. Sentinel, or indicator, species can provide an integrated picture of contaminants in the environment and may be useful to understanding phycotoxin prevalence in the IRL. This study evaluated the presence of phycotoxins in the IRL ecosystem by using the bull shark (Carcharhinus leucas) as a sentinel species. Concentrations of phycotoxins were measured in samples collected from 50 immature bull sharks captured in the IRL between 2018 and 2020. Ultra-performance liquid chromatography/tandem mass spectrometry was used to measure toxins in shark gut contents, plasma, and liver. Analysis of samples (n = 123) demonstrated the presence of multiple phycotoxins (microcystin, nodularin, teleocidin, cylindrospermopsin, domoic acid, okadaic acid, and brevetoxin) in 82 % of sampled bull sharks. However, most detected toxins were in low prevalence (≤25 % of samples, per sample type). This study provides valuable baseline information on presence of multiple phycotoxins in a species occupying a high trophic position in this estuary of national significance.
Since the discovery of the phenomenally high urea content of the body fluids of cartilaginous fishes by Staedeler and Frerichs (1858), the unique osmoregulatory system of this vertebrate class has been studied by many investigators. In broad outline, the mechanisms are essentially similar in all three of the major subtaxa: the selachians, the batoids, and the holocephalans. A single exception is the genus Potamotrygon (fresh-water stingrays of South America and Africa), which has apparently lost the ability to concentrate urea even when transferred to salt water.
The shark that occurs in the fresh water of Lake Nicaragua and the Rio San Juan was first described (as Eulamia nicaraguensis) by Gill and Bransford (1877). These authors also first proposed the theory that the sharks, as well as the sawfish and tarpon that occur in the lake, were trapped there by late Pleistocene volcanic activity, which isolated a former bay of the Pacific and resulted in the formation of the present lake. The theory of a landlocked, distinct species, of Pacific origin, has en joyed wide popular acceptance and for many years was also accepted by professional zoologists, although the idea has been questioned frequently in recent years. Carr (1953) pointed out that the closest taxonomic affinities of the marine species in Lake Nicaragua were clearly with their Atlantic, rather than their Pacific relatives. Bigelow and Schroeder (1961) concluded that the lake sharks are morphologically inseparable from the widely distributed marine bull shark, Carcharhinus leucas. Thorson (1964) and Thorson et al. (1966) confirmed the conclusion of Bigelow and Schroeder and presented circumstantial evidence that the lake shark population is not landlocked, but consists of marine bull sharks that enter from the sea. Our evidence was the occurrence, throughout the lake and river, of many sharks of the same euryhaline species that occurs at the river mouth and along the coast; that the same species occurs in similar situations in many rivers and some lakes around the world; and that all the rapids in the Rio San Juan are navigable by barges and other vessels that regularly pass up and down the river.
Distribution and habitat use of the bull shark (Carcharhinus leucas) were examined using fishery-independent sampling data, tagging, and ultrasonic telemetry to assess the potential role of the Indian River Lagoon (IRL) as a nursery area for this species. Fishery-independent sampling data were compiled and synthesized to examine patterns of seasonal occurrence, spatial distribution, and habitat associations. These data provided a comprehensive overview of bull shark ecology in the study site over a span of 30 years, based on data collected from 390 individual sharks. Tagging and acoustic telemetry methods were also employed to acquire more fine-scale information on shark movements, daily activities, and habitat utilization. A total of 50 sharks were marked with conventional tags, with four fish recaptured over the course of the study. Eleven of these sharks were tagged additionally with ultrasonic pingers, ten of which were tracked manually and one of which was monitored by moored listening stations (Vemco VR2). The manual tracking data provided fine-scale information on the patterns of movements of a small number of individuals. Integration of multiple methodologies provided a more complete picture of habitat use by this important apex predator the IRL. Bull sharks occurred over a broad range of habitats, including depths of 0.2 – 4.0 m, temperatures of 18 – 37 C, salinities of 1 – 42 ppt, dissolved oxygen concentrations of 3 – 8 mg/L, and water clarity levels of 70 – 170 cm. In addition, they were located over seagrass, sand, and mud substrates. Overall catch-per-unit-effort was low, relative to other systems. However, higher than average catch rates were observed at power plant outfalls and near freshwater creeks. These results may prove useful to the continued management and conservation of bull shark stocks in the northwest Atlantic.