Article

Sex-based spatial segregation of adult bull sharks, Carcharhinus leucas, in the New Caledonian great lagoon

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Abstract

Conservation of threatened large sharks and management of shark-human interactions requires an understanding of shark occurrence and movement patterns. Here, we present the first catch, movement and behaviour data of adult bull sharks, Carcharhinus leucas, in New Caledonia. Amongst six adult C. leucas tagged with passive acoustic tags, four females were caught in coastal waters while males were only found at an isolated oceanic barrier coral reef over 100 km from the nearest river mouth. Two females were monitored in the southern New Caledonia lagoon for 707 and 208 days respectively and displayed classical transient behaviour and sporadic short-term residency around a coastal reef bay, with movements in and out a river detected prior to spring. Adult C. leucas in New Caledonia may develop a sex-based spatial segregation with an atypical presence of adult males in oceanic environments, probably influenced by the unique estuarine-marine continuum of the New Caledonian great lagoon.

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... Sea surface temperature (SST) and lunar phase can also be used to predict white shark movement (Weltz et al., 2013;Werry et al., 2012b). Whereas, bull sharks (Carcarhinus leucas) appear to be driven by their link to the river-estuarine-marine continuum (Heupel et al., 2010;Werry et al., 2012aWerry et al., , 2011Werry and Clua, 2013) and the amount of available estuarine habitat (Haig et al., 2018). The temporal use of critical habitats (such as beach areas) will vary between species, and also between ontogenetic stages of the same species. ...
... Bull sharks have a lifecycle closely linked to the freshwater-estuarine-marine continuum; distributed between equatorial and subtropical coastal habitats Werry and Clua, 2013). Bull sharks use upper river reaches for pupping (Werry et al., 2012a and neonates and juveniles are physiologically adapted to occupy estuarine areas as nursery habitats (Heupel and Simpfendorfer, 2008;Pillans et al., 2005). ...
... Bull sharks use upper river reaches for pupping (Werry et al., 2012a and neonates and juveniles are physiologically adapted to occupy estuarine areas as nursery habitats (Heupel and Simpfendorfer, 2008;Pillans et al., 2005). Adult bull sharks readily occur in the nearshore and open marine environments (Brunnschweiler and Barnett, 2013;Cliff and Dudley, 1991;Werry et al., 2011;Werry and Clua, 2013) with large scale coastal migration (Espinoza et al., 2016;Heupel et al., 2015). These unique euryhaline abilities result in variable movement patterns for males and females and also for different age cohorts in coastal habitats. ...
Article
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Climate and weather-based drivers of shark movement are poorly known, yet vital for determining measures for effective conservation of shark populations and the management of shark-human interactions at different time scales. The bull shark, Carcharhinus leucas, an IUCN ‘Near-threatened’ species, is globally distributed in subtropical to tropical regions and is implicated in many attacks on humans because of its euryhaline habitat-use. However, drivers determining rapid transitions of this species among habitats along the freshwater-estuarine-marine continuum are yet to be fully understood. To identify triggers for movement by this species into beach areas we used conditional (binomial and gamma) generalised linear modelling (CGLM) of historical bull shark catches from the Queensland Shark Control Program (QSCP) collected from 1996 to 2007 across 1783 km of the Queensland coastline, Australia. We then compared catches before and after key weather events (such as floods) between 2006 and 2014 and used passive long-term acoustic telemetry to monitor movements of bull sharks into beach areas to test the model predictions. The CGLM showed that bull shark catch (occurrence) in beach areas is driven by rainfall and further influenced by sea surface temperature. Our model suggests that ≥100 mm total rainfall in the catchment associated with each beach area is significantly correlated with increased bull shark catch 1–8 days after the rainfall, a relationship also confirmed by the movements of acoustic tagged sharks between estuarine and beach areas. These trends provide the first predictive relationship for identifying increased risk of bull shark-human interactions in beach areas. They also suggest that the activity patterns of bull sharks are correlated with rainfall and this makes them particularly susceptible to localised, short-term changes in weather and long-term climate change.
... The fact that these systems are also of high productivity could be one of the reasons why adult female bull sharks prefer these sites to give birth (Curtis, Parkyn, & Burgess, 2013). Several authors have reported bull sharks using coastal ecosystems for protection and food during their early life stages (Simpfendorfer, Greitas, Wiley, & Heupel, 2005;Werry & Clua, 2013). Nursery grounds for bull sharks have been documented in the Gulf of Mexico S247 Revista de Biología Tropical, ISSN: 2215-2075: S246-S255, October 2021(Published Oct. 30, 2021 and the east coast of Florida (Curtis, Adams, & Burgess, 2011). ...
... Numerous studies have also reported the preference or habitat fidelity of adult bull sharks to shallow waters of coastal habitats, particularly to those with a high input of freshwater, such as river mouths and estuaries (Bangley, Paramore, Shifman, & Rulifson, 2018;Brunnschweiler, Queiroz, & Sims, 2010;Carlson, Ribera, Conrath, Heupel, & Burgess, 2010;Graham et al., 2016;Heupel et al., 2015). The permanence of bull sharks in coastal ecosystems makes them vulnerable to anthropogenic actions like habitat degradation, pollution, and fishing exploitation (Curtis et al., 2013;Thorson, 1971;Werry & Clua, 2013). ...
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The North Pacific and the South Pacific of Nicaragua is a region of great biological, geological, economic and social wealth. There the dry forest mixes with the rain forest, the sea with the islands and nature with the people. It is a region influenced by the Papagayo upwelling, the emergence of cold marine waters during the dry season, which generates an abundance of life in the sea. As a sample of this marine wealth, in this Special Issue, more than half of the contributions are dedicated to advances in the knowledge of the marine biodiversity of the region. Contributions from the social sciences, geology and physics of the region are also included. Within these areas, the publications provide information on maritime border management, archaeology and sustainable tourism, coastal geology, projected climate changes, as well as various oceanographic aspects of the area. We hope that this Special Issue on the North Pacific of Costa Rica and the South Pacific of Nicaragua will promote more research in the region and help inform decision-making processes and educational activities. We thank the authors for their manuscripts, as well as the more than sixty reviewers who with their comments and suggestions helped to improve the quality of the manuscripts.
... Results showed that both bull sharks were regularly found inshore, suggesting possible fidelity to Reunion Island. This insular fidelity is similar to that described in previous studies on bull sharks [8,16]. The coastal fidelity of the female was also observed in New-Caledonia [16] and could be Hawaii as suggested by [12]. ...
... This insular fidelity is similar to that described in previous studies on bull sharks [8,16]. The coastal fidelity of the female was also observed in New-Caledonia [16] and could be Hawaii as suggested by [12]. ...
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Two bull sharks ( Carcharhinus leucas ) were tagged in coastal waters off Reunion Island in the tropical Indian Ocean and where tracked for 174 and 139 days using both popup satellite archival tags (pSAT) and acoustic tags. Both sharks spent a majority of their time inshore (58.1% and 89.9% in the male and the female respectively). The female performed short excursions. The male alternated residence time along the coast with wide ranging movements and performed one extensive open-ocean excursion near a seamount situated at more than 200 km from the island. The differences in the residency and home range of both sharks probably reflect different patterns of foraging and mating behaviors in the male and the female. These results underline the importance of developing risk-mitigation management taking into account the movements of sharks, and of double tagging in telemetry studies that attempt to measure the degree of fidelity of a species.
... However, it was more surprising that no clear sexual segregation was found as this is common in many shark populations including bull sharks (Werry & Clua, 2013;Espinoza et al., 2016). Sexual segregation can emerge from females trying to avoid male harassment, especially during the mating period (Jacoby, Busawon & Sims, 2010). ...
Article
• Knowledge about spatial and temporal variability in the distribution and abundance of predators is necessary to adapt measures to mitigate human–wildlife interactions. • Acoustic telemetry and network analyses were used to investigate the spatial ecology of bull sharks, the species responsible for most shark bites in Reunion Island, one of the world's shark bite hotspots. • The west coast of the island was not used uniformly by every individual, with size predicting the movements of sharks along the coast. • Node-based metrics – closeness, node strength, and cumulated continuous residency times – derived from up to 181 monthly movement networks from 20 individuals, revealed that smaller sharks (<250 cm total length) primarily used the south-west coast while larger individuals spent most of their time in the northern region with regular visits to multiple areas along the coast. • This study provides essential knowledge on bull shark behaviour and central areas used at different periods of the year, which correlates well with the dynamics of observed shark bites. Our approach provides a non-invasive alternative to help predicting and anticipating human–shark conflicts and avoid shark culling programmes detrimental to the conservation of large predators such as sharks.
... As such, long-distance migration of adult bull sharks may genetically link ecosystems within these regions. Each movement study also highlights the fidelity of bull sharks to specific sites at discrete times, as shown in Reunion Island , in New Caledonia (Werry & Clua, 2013), in Australia , in Fiji, and in the Bahamas (Brunnschweiler & Baensch, 2011;Brunnschweiler et al., 2010 ...
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Knowledge of population structure, connectivity, and effective population size remains limited for many marine apex predators, including the bull shark Carcharhinus leucas. This large-bodied coastal shark is distributed worldwide in warm temperate and tropical waters, and uses estuaries and rivers as nurseries. As an apex predator, the bull shark likely plays a vital ecological role within marine food webs, but is at risk due to inshore habitat degradation and various fishing pressures. We investigated the bull shark's global population structure and demographic history by analyzing the genetic diversity of 370 individuals from 11 different locations using 25 micros-atellite loci and three mitochondrial genes (CR, nd4, and cytb). Both types of markers revealed clustering between sharks from the Western Atlantic and those from the Western Pacific and the Western Indian Ocean, with no contemporary gene flow. Microsatellite data suggested low differentiation between the Western Indian Ocean and the Western Pacific, but substantial differentiation was found using mitochon-drial DNA. Integrating information from both types of markers and using Bayesian
... Less understood is how the habitat characteristics associated with near-shore environments influence the occurrence of different ontogenetic life history stages of the bull shark Werry and Clua, 2013). For example, distance to the 60 and 120 m depth contour and orientation of beach to the north-south migratory corridor has been shown to influence the occurrence of juvenile white sharks (Carcarodon carcharias) and hence CPUE trends in near shore habitats and shark control programs in Queensland and New South Wales, Australia (Werry et al., 2012b). ...
Article
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... 20% of these single species capture events occurred among blacktip sharks across a range of salinities (0.0-40.3 psu), suggesting that blacktip sharks are not completely restricted from entering low salinity waters and using habitats in traditional bull shark nurseries. Interactions among blacktip shark and bull sharks in brackish waters may lead to an increase in competition between these two species, and increasing salinities in brackish waters could promote such niche width expansion by blacktip sharks (Compagno 1984;Heithaus 2007;, while constricting low-moderate salinity habitats used by bull sharks as nurseries (e.g., Norden 1966;Heuter and Tyminski 2007;Werry et al. 2011;Curtis et al. 2013;Werry and Clua 2013;Drymon et al. 2014). ...
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As top predators, sharks play important ecological roles in coastal marine ecosystems. Yet, environmental changes in many coastal regions are likely altering the composition of ecological communities, the interspecifc interactions sharks have within coastal waters, and subsequently the ecological roles sharks play within these ecosystems. As such, understanding interactions among sharks, and how extrinsic factors shape these interactions, is important for predicting the consequences of future human impacts and environmental changes. Elucidating the contexts under which species co-occur and the implications of co-occurrence is an important step in developing such a predictive framework. Blacktip shark (Carcharhinus limbatus) and bull shark (Carcharhinus leucas) concentrations were quantifed using long-term coastal gill net survey data across five bay systems in Texas. Relationships between co-occurrence, and environmental factors and shark sizes were examined within and across species. Co-occurrence of blacktip sharks and bull sharks varied spatially and temporally, with a significant increase in interspecifc co-occurrence from the 1970s to 2010s, and a significant decrease in bull shark concentrations through time. Changes in environmental conditions, specifcally increasing salinities, may have been responsible for increased blacktip and bull shark co-occurrence, and potential interspecifc competition, which in turn may have led to decreased bull shark concentrations to reduce intraspecifc competition. However, more refined questions are needed before predictive frameworks can be developed concerning the contexts under which co-occurrence is prevalent. Quantifying resource use among coastal sharks will help elucidate the drivers and implications of co-occurrence, and the potential for competitive interactions within and across species.
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The bull shark (Carcharhinus leucas Valenciennes, 1839) is a large, primarily coastally distributed shark famous for its ability to penetrate far into freshwater bodies in tropical, subtropical, and warm-temperate climates. It is a cosmopolitan species with a geographical range that includes the coastlines of all major ocean basins (Atlantic Ocean, Indian Ocean, Pacific Ocean). As a consequence, freshwater occurrences of C. leucas are possible everywhere inside its geographic range. Carcharhinus leucas is a fully euryhaline, amphidromous species and possibly the widest-ranging of all freshwater tolerating elasmobranchs. This species is found not only in river systems with sea access that are not interrupted by human impediments but in hypersaline lakes as well. Rivers and estuaries are believed to be important nursery grounds for C. leucas, as suggested by observations of pregnant females in estuaries and neonates with umbilical scars in rivers and river mouths. Due to the physical capability of this species to enter riverine systems, the documentation of its occurrence in fresh and brackish water is essential for future conservation plans, fishery inspections, and scientific studies that focus on the link between low salinity habitats, shark nurseries, and feeding areas. The author’s review of the available literature on C. leucas revealed the absence of a comprehensive overview of fresh and brackish water localities (rivers and associated lakes, estuaries) with C. leucas records. The purpose of this literature review is to provide a global list of rivers, river systems, lakes, estuaries, and lagoons with records and reports of this species, including a link to the used references as a base for regional, national, and international conservation strategies. Therefore, the objective of this work is to present lists of fresh and brackish water habitats with records of C. leucas as the result of an extensive literature review and analysis of databases. This survey also took into account estuaries and lagoons, regarding their function as important nursery grounds for C. leucas. The analysis of references included is not only from the scientific literature, but also includes semi-scientific references and the common press if reliable. The result of 415 global fresh and brackish water localities with evidence of C. leucas highlights the importance of these habitats for the reproduction of this species. Moreover, gaps in available distribution maps are critically discussed as well as interpretations and conclusions made regarding possible reasons for the distribution range of C. leucas, which can be interpreted as the result of geographic circumstances, but also as a result of the current state of knowledge about the distribution of this species. The results of the examination of available references were used to build a reliable and updated distribution map for C. leucas, which is also presented here.
Thesis
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Shark-based tourism that uses bait to reliably attract certain species to specific sites so that divers can view them is a growing industry globally, but remains a controversial issue. We evaluate multi-year (2004-2011) underwater visual (n = 48 individuals) and acoustic tracking data (n = 82 transmitters; array of up to 16 receivers) of bull sharks Carcharhinus leucas from a long-term shark feeding site at the Shark Reef Marine Reserve and reefs along the Beqa Channel on the southern coast of Viti Levu, Fiji. Individual C. leucas showed varying degrees of site fidelity. Determined from acoustic tagging, the majority of C. leucas had site fidelity indexes >0.5 for the marine reserve (including the feeding site) and neighbouring reefs. However, during the time of the day (09:00-12:00) when feeding takes place, sharks mainly had site fidelity indexes <0.5 for the feeding site, regardless of feeding or non-feeding days. Site fidelity indexes determined by direct diver observation of sharks at the feeding site were lower compared to such values determined by acoustic tagging. The overall pattern for C. leucas is that, if present in the area, they are attracted to the feeding site regardless of whether feeding or non-feeding days, but they remain for longer periods of time (consecutive hours) on feeding days. The overall diel patterns in movement are for C. leucas to use the area around the feeding site in the morning before spreading out over Shark Reef throughout the day and dispersing over the entire array at night. Both focal observation and acoustic monitoring show that C. leucas intermittently leave the area for a few consecutive days throughout the year, and for longer time periods (weeks to months) at the end of the calendar year before returning to the feeding site.
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Despite accelerated global population declines due to targeted and illegal fishing pressure for many top-level shark species, the impacts of coastal habitat modification have been largely overlooked. We present the first direct comparison of the use of natural versus artificial habitats for the bull shark, Carcharhinus leucas, an IUCN 'Near-threatened' species - one of the few truly euryhaline sharks that utilises natural rivers and estuaries as nursery grounds before migrating offshore as adults. Understanding the value of alternate artificial coastal habitats to the lifecycle of the bull shark is crucial for determining the impact of coastal development on this threatened but potentially dangerous species. We used longline surveys and long-term passive acoustic tracking of neonate and juvenile bull sharks to determine the ontogenetic value of natural and artificial habitats to bull sharks associated with the Nerang River and adjoining canals on the Gold Coast, Australia. Long-term movements of tagged sharks suggested a preference for the natural river over artificial habitat (canals). Neonates and juveniles spent the majority of their time in the upper tidal reaches of the Nerang River and undertook excursions into adjoining canals. Larger bull sharks ranged further and frequented the canals closer to the river mouth. Our work suggests with increased destruction of natural habitats, artificial coastal habitat may become increasingly important to large juvenile bull sharks with associated risk of attack on humans. In this system, neonate and juvenile bull sharks utilised the natural and artificial habitats, but the latter was not the preferred habitat of neonates. The upper reaches of tidal rivers, often under significant modification pressure, serve as nursery sites for neonates. Analogous studies are needed in similar systems elsewhere to assess the spatial and temporal generality of this research.
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The Zambezi or bull shark Carcharhinus leucas is a large, predatory shark that occurs in warm-temperate, tropical and subtropical coastal and estuarine systems worldwide. To confirm reports of Zambezi sharks in the Breede Estuary on the south-west coast of South Africa, a survey was undertaken during 20-26 January 2009. On 24 January, a large female Zambezi shark was caught on rod and reel. Measuring 400 cm total length and 320 cm precaudal length, it is the largest recorded Zambezi shark. Furthermore, its occurrence in the Breede Estuary is the southernmost record of the species, extending its previously documented range by 366 km. The shark was tagged with a continuous acoustic tag and tracked for 43 consecutive hours. During that period, it swam as far as 20 km upstream, but it also briefly exited the estuary and travelled 2 km out to sea. Most of the time (24%) was spent 11-13 km upstream, where it actively inspected boats and shore-anglers, a behaviour considered to be an opportunistic foraging strategy. Estuaries appear to represent critical habitats in the life history of Zambezi sharks.
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A total of 772 bull sharks Carcharhinus leucas was caught in Natal's protective "shark nets" between 1978 and 1990. Confusion in distinguishing C. leucas from C. amboinensis resulted in their catch data being combined from 1966, when data collection began, to 1977. The catch rate of the species pair declined until 1977, recovered until the mid 1980s, but subsequently declined again. The trend in catch rate of C. leucas alone for the period 1978-1990 was similar, with minima of 0,70 sharks·km-net−1·year−1 in 1978 and 0,95 in 1990, and a maximum of 2,08 in 1986. Recaptures of six tagged sharks suggest that the species is not highly migratory. Catches, particularly of immature sharks, were highest at the northernmost beaches. Most bull sharks were caught in summer and in turbid water (mean water clarity 2,0 m). The sex ratio of the catch was 1 male to 1,3 females. Sizes ranged from 74 to 213 cm precaudal length, with modes of 141-145 cm (males) and 171-175 cm (females). Size at maturity for both sexes was between 180 and 190 cm. The mating season was prolonged but with a summer peak. Seven gravid females were examined; the mean litter size was 8,7 embryos and size at birth was approximately 55 cm. Fluke infections were observed on 9 per cent of animals examined. As size increased there was a shift in diet, in terms of frequency of occurrence, from teleost to elasmobranch predominance. There was a high incidence of benthic and demersal species in the stomachs. Minor prey groups included mammals, birds, turtles, molluscs and crustaceans. Scavenging appeared to be important.
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This study examines life history patterns and correlations between traits related to body size, reproduction, age, and growth in sharks, using data from 230 populations representing 164 species, 19 families, and 7 orders. The analysis focused on interspecific life history variability, but intraspecific and intrapopulation variation were also considered. Interspecifically, body size correlated positively with litter size and offspring size, and a tradeoff between litter size and offspring size was found after factoring out the effects of body size. Offspring size correlated negatively with growth completion rate (K), but the correlation became positive after correcting for the effects of body size. Parental size for males and females was negatively correlated with K. Parental size and size at maturity exhibited a strong positive correlation, with sexual maturity occurring at about 75% of maximum size in both sexes. Males were 10% smaller than females and reached their maximum length 34% faster than females on average. Females tend to mature later and live longer than males, but age at maturity is reached at about 50% of maximum age in both sexes. Maximum size and empirical longevity were not significantly correlated in females, but were positively correlated in males. Size and age at maturity also exhibited a moderate positive correlation in males, especially after excluding data for Squalus acanthias. Principal component and cluster analyses were used to reflect similarities among life history traits of 40 populations from 34 species, and at least three separate life history strategies were identified.
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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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The performance of an array of data-logging single frequency acoustic receivers in the Caloosahatchee River (Florida, USA) was examined and the results incorporated into a positioning algorithm for animals tracked within the system. The mean code detection efficiency across all individual receivers and all download periods was 0.414 detections per synchronization code. On average, the code rejection coefficient was approximately 4%, indicating that it was only a minor factor in reducing code detection efficiency. There were significant performance differences between stations and download periods, but no interaction between these two factors for all three metrics. Code detection efficiency, the rejection coefficient, and the noise quotient all showed significant variations with distance from the river mouth and time since deployment. Comparison of position estimates with and without efficiency produced small differences for bull sharks (Carcharhinus leucas) and cownose rays (Rhinoptera bonasus) monitored via this system. Root mean square errors were higher for cownose rays (48 m) than for bull sharks (23 m). Mean differences for individuals were always slightly downstream because of the increasing code detection efficiency of upriver receivers. The results of this comparison indicated that the inclusion of code detection efficiency did not significantly improve the results of the positioning algorithm.
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The space utilization and distribution of young (<2 yr old) bull sharks Carcharhinus leucas within a 27 km stretch of the Caloosahatchee River estuary in Southwest Florida was examined using an array of acoustic monitors to define influences of environmental variables. A total of 56 young sharks from 3 cohorts (2003, 2004, 2005) were fitted with acoustic tags and monitored for up to 460 d. Sharks did not remain within the estuary continuously, but on average approximately one-third were present at any one time from each cohort. Salinity and freshwater inflow showed greatest influence on shark distribution, with temperature appearing to play a limited role. Although individuals occurred in salinities from 0.1 to 34, electivity analysis indicated that they generally avoided areas with salinity <7 and had an affinity for areas with salinities from 7 to at least 20. There were significant relationships between the mean location of a cohort within the estuary and salinity, with sharks occurring further up river when the river was more saline. These relationships were more pronounced for the youngest sharks, and strength of the relationship decreased with age. Since bull sharks are euryhaline, these results suggest that they may select environmental conditions via movement, possibly to reduce energetic costs associated with osmoregulation.
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Predators can impact ecosystems through trophic cascades such that differential patterns in habitat use can lead to spatiotemporal variation in top down forcing on community dynamics. Thus, improved understanding of predator movements is important for evaluating the potential ecosystem effects of their declines. We satellite-tagged an apex predator (bull sharks, Carcharhinus leucas) and a sympatric mesopredator (Atlantic tarpon, Megalops atlanticus) in southern Florida waters to describe their habitat use, abundance and movement patterns. We asked four questions: (1) How do the seasonal abundance patterns of bull sharks and tarpon compare? (2) How do the movement patterns of bull sharks and tarpon compare, and what proportion of time do their respective primary ranges overlap? (3) Do tarpon movement patterns (e.g., straight versus convoluted paths) and/or their rates of movement (ROM) differ in areas of low versus high bull shark abundance? and (4) Can any general conclusions be reached concerning whether tarpon may mitigate risk of predation by sharks when they are in areas of high bull shark abundance? Despite similarities in diet, bull sharks and tarpon showed little overlap in habitat use. Bull shark abundance was high year-round, but peaked in winter; while tarpon abundance and fishery catches were highest in late spring. However, presence of the largest sharks (>230 cm) coincided with peak tarpon abundance. When moving over deep open waters (areas of high shark abundance and high food availability) tarpon maintained relatively high ROM in directed lines until reaching shallow structurally-complex areas. At such locations, tarpon exhibited slow tortuous movements over relatively long time periods indicative of foraging. Tarpon periodically concentrated up rivers, where tracked bull sharks were absent. We propose that tarpon trade-off energetic costs of both food assimilation and osmoregulation to reduce predation risk by bull sharks.
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Female scalloped hammerhead sharks move offshore at a smaller size than do males to form schools composed primarily of intermediate size female sharks. This movement results in smaller females feeding more on pelagic prey than do males and with greater predatory success. It is contended that this change in habitat causes females to grow more rapidly to reproductive size. Intermediate size females grow at a more rapid rate than males. Female scalloped hammerhead sharks mature at a size larger than males. For many elasmobranch species, females: (1) occupy a different habitat, (2) grow more rapidly prior to maturity and continue growth following maturation, (3) feed on different prey with increased feeding success, and (4) reproduce at a size larger than males. It is suggested that female segregation increases fitness, resulting in more rapid growth for the former sex. The females reach maturity at the larger size necessary to support embryonic young, yet similar age to males, matching the female reproductive lifetime to that of males.
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Understanding the factors that influence the distribution and abundance of predators, including sharks, is important for predicting the impacts of human changes to the environment. Such studies are particularly important in Florida Bay, USA where there are planned large-scale changes to patterns of freshwater input from the Everglades ecosystem. Studies of many marine predators suggest that links between predator and prey habitat use may vary with spatial scale, but there have been few studies of the role of prey distribution in shaping habitat use and abundance of sharks. We used longline catches of sharks and trawls for potential teleost prey to determine the influence of teleost abundance on shark abundance at the scale of regions and habitats in Florida Bay. We found that shark catch per unit effort (CPUE) was not linked to CPUE ofteleosts at the scale of sampling sites, but shark CPUE was positively correlated with the mean CPUE for teleosts within a region. Although there does not appear to be a strong match between the abundance of teleosts and sharks at small spatial scales, regional shark abundance is likely driven, at least partially, by the availability of prey. Management strategies that influence teleost abundance will have cascading effects to higher trophic levels in Florida Bay.
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The distribution and salinity preference of immature bull sharks (Carcharhinus leucas) were examined based on the results of longline surveys in three adjacent estuarine habitats in southwest Florida: the Caloosahatchee River, San Carlos Bay, and Pine Island Sound. Mean sizes were significantly different between each of these areas indicating the occurrence of size-based habitat partitioning. Neonate and young-of-the-year animals occurred in the Caloosahatchee River and juveniles older than 1 year occurred in the adjacent embayments. Habitat partitioning may reduce intraspecific predation risk and increase survival of young animals. Classification tree analysis showed that both temperature and salinity were important factors in determining the occurrence and catch per unit effort (CPUE) of immatureC. leucas. The CPUE of <1 year oldC. leucas was highest at temperatures over 29°C and in areas with salinities between 7‰ and 17.5‰ Although they are able to osmoregulate in salinities from fresh to fully marine, youngC. leucas may have a salinity preference. Reasons for this preference are unknown, but need to be further investigated.
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Although marine protected areas (MPAs) are a common conservation strategy, these areas are often designed with little prior knowledge of the spatial behaviour of the species they are designed to protect. Currently, the Coral Sea area and its seamounts (north-east Australia) are under review to determine if MPAs are warranted. The protection of sharks at these seamounts should be an integral component of conservation plans. Therefore, knowledge on the spatial ecology of sharks at the Coral Sea seamounts is essential for the appropriate implementation of management and conservation plans. Acoustic telemetry was used to determine residency, site fidelity and spatial use of three shark species at Osprey Reef: whitetip reef sharks Triaenodon obesus, grey reef sharks Carcharhinus amblyrhynchos and silvertip sharks Carcharhinus albimarginatus. Most individuals showed year round residency at Osprey Reef, although five of the 49 individuals tagged moved to the neighbouring Shark Reef (~14 km away) and one grey reef shark completed a round trip of ~250 km to the Great Barrier Reef. Additionally, individuals of white tip and grey reef sharks showed strong site fidelity to the areas they were tagged, and there was low spatial overlap between groups of sharks tagged at different locations. Spatial use at Osprey Reef by adult sharks is generally restricted to the north-west corner. The high residency and limited spatial use of Osprey Reef suggests that reef sharks would be highly vulnerable to targeted fishing pressure and that MPAs incorporating no-take of sharks would be effective in protecting reef shark populations at Osprey and Shark Reef.
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Reproductive philopatry in bull sharks Carcharhinus leucas was investigated by comparing mitochondrial (NADH dehydrogenase subunit 4, 797 base pairs and control region genes 837 base pairs) and nuclear (three microsatellite loci) DNA of juveniles sampled from 13 river systems across northern Australia. High mitochondrial and low microsatellite genetic diversity among juveniles sampled from different rivers (mitochondrial φ(ST) = 0·0767, P < 0·05; microsatellite F(ST) = -0·0022, P > 0·05) supported female reproductive philopatry. Genetic structure was not further influenced by geographic distance (P > 0·05) or long-shore barriers to movement (P > 0·05). Additionally, results suggest that C. leucas in northern Australia has a long-term effective population size of 11 000-13 000 females and has undergone population bottlenecks and expansions that coincide with the timing of the last ice-ages.
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Impacts of chronic overfishing are evident in population depletions worldwide, yet indirect ecosystem effects induced by predator removal from oceanic food webs remain unpredictable. As abundances of all 11 great sharks that consume other elasmobranchs (rays, skates, and small sharks) fell over the past 35 years, 12 of 14 of these prey species increased in coastal northwest Atlantic ecosystems. Effects of this community restructuring have cascaded downward from the cownose ray, whose enhanced predation on its bay scallop prey was sufficient to terminate a century-long scallop fishery. Analogous top-down effects may be a predictable consequence of eliminating entire functional groups of predators.
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Shark tourism has become increasingly popular, but remains controversial because of major concerns originating from the need of tour operators to use bait or chum to reliably attract sharks. We used direct underwater sampling to document changes in bull shark Carcharhinus leucas relative abundance at the Shark Reef Marine Reserve, a shark feeding site in Fiji, and the reproductive cycle of the species in Fijian waters. Between 2003 and 2009, the total number of C. leucas counted on each day ranged from 0 to 40. Whereas the number of C. leucas counted at the feeding site increased over the years, shark numbers decreased over the course of a calendar year with fewest animals counted in November. Externally visible reproductive status information indicates that the species' seasonal departure from the feeding site may be related to reproductive activity.
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Despite an Indo-Pacific wide distribution, the movement patterns of grey reef sharks (Carcharhinus amblyrhynchos) and fidelity to individual reef platforms has gone largely unstudied. Their wide distribution implies that some individuals have dispersed throughout tropical waters of the Indo-Pacific, but data on large-scale movements do not exist. We present data from nine C. amblyrhynchos monitored within the Great Barrier Reef and Coral Sea off the coast of Australia. Shark presence and movements were monitored via an array of acoustic receivers for a period of six months in 2008. During the course of this monitoring few individuals showed fidelity to an individual reef suggesting that current protective areas have limited utility for this species. One individual undertook a large-scale movement (134 km) between the Coral Sea and Great Barrier Reef, providing the first evidence of direct linkage of C. amblyrhynchos populations between these two regions. Results indicate limited reef fidelity and evidence of large-scale movements within northern Australian waters.
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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.
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Knowledge of migratory movements and depth/temperature-related use of coastal waters by sharks can lead to more sustainable fisheries and assist in managing the long-term conservation of those species now considered threatened. Pop-up archival satellite tags (PATs) provide an alternative to conventional tagging for documenting migratory movements. This study focussed on the migratory movements of Carcharias taurus, a critically endangered shark found along the east coast of Australia. From October 2003 to July 2008, 15 C. taurus individuals were tagged with PATs with varying deployments (60-150 days) and acoustic tags linked to an acoustic monitoring system providing accurate geo-location. Distances moved by C. taurus individuals ranged from 5 to 1550 km and varied according to sex and season. Migrations north and south were punctuated en route by occupation of sites for varying periods of time. The deepest depth recorded was 232 m off South West Rocks on the New South Wales mid-north coast. On average, C. taurus males and females spent at least 71% of their time in waters <40 m and 95% of their time in waters 17-248 degrees C. By mainly occupying inshore waters, C. taurus is exposed to potentially adverse fishing-related interactions that may be difficult to mitigate.
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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.
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Tiger Sharks, Galeocerdo cuvier, are large top-level predators usually solitary as adults. Observation of their scavenging activity on the carcass of a dead whale offered a rare opportunity for better understanding the pattern of intra-specific behaviour within the aggregations of these large predators. In January 2002, the stranding, subsequent death and consumption of a 17.4m total length (TL) blue whale, Balaenoptera musculus, was observed and filmed in Prony Bay, southern New Caledonia. After three weeks of confinement in the bay, the cetacean was killed by adult bullsharks Carcharhinus leucas. The first adult Tiger Shark was subsequently observed around the carcass after 36h. The fat slicks from the carcass attracted further Tiger Sharks which arrived after an additional 24h. The use of photo-identification on video footage collected during four observation sessions over an eight-day period identified 46 individual Tiger Sharks (primarily adult females between 3.3 and 4m TL) participating in the feeding aggregation. Only four animals were identified in two seperate observation sessions (over two consecutive days), suggesting a short-term residency pattern of several hours (<36h) around the carcass. As the arrival time of Tiger Sharks to the carcass differed, most arrivals of a new participant were followed by a frenzied period of intense intra-specific interaction. Different biting and agonistic behaviours were demonstrated by the Tiger Sharks on the carcass, including three new behaviours previously undescribed for this species. Size and level of aggressiveness appeared to be the determining factors of dominance amongst Tiger Sharks. These observations and analysis demonstrate that systematic study of feeding aggregations supported by photo-identification could contribute to knowledge of large shark ecology when coupled with capture-recapture, genetic fingerprinting and tagging techniques.
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Increases in standardized catch per unit effort (CPUE) and mean length of bull shark (Carcharhinus leucas) were observed in coastal estuaries over a 35-year period (1976–2010). Trends in abundance and size were examined using fisheries-independent data from a long-term monitoring survey in Texas, USA. Catch, effort, and environmental covariates that affect bull shark distribution were used to create a standardized index of abundance. Increases in abundance and mean length were detected, potentially due to the initiation of federal management and restrictions on the use of gill nets in nearby Louisiana, USA, waters in 1995. This study provides a long-term perspective of two important demographic indicators (abundance and mean size) of bull shark and provides an encouraging signal in the Gulf of Mexico for a species whose stock status is unknown yet considered near threatened on the International Union for Conservation of Nature red list. Continuing research is needed to gauge effects of management and environmental impacts on shark resources as well as investigations into ecosystem effects of increasing predatory density in coastal waters.
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Understanding the ontogenetic habitat linkages of sharks is important for conservation and managing human interactions. We used acoustic telemetry, catch data, elemental and stable isotope signatures and dietary analyses to investigate ontogenetic habitat use in south-east Queensland, Australia, by the bull shark Carcharhinus leucas, a IUCN ‘near-threatened’ species that is implicated in many shark attacks on humans in urban estuaries. Sequential analyses for δ15N and δ13C of vertebrae from five adult C. leucas and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) for elemental composition from 23 C. leucas, including a pregnant female, were also used to trace ontogenetic habitat dependence. Acoustic telemetry indicated large juvenile and subadult C. leucas remained in estuarine habitats. δ15N values across shark vertebrae showed an ontogenetic shift in diet with total length (TL), confirmed by stomach contents. LA-ICPMS data reflected the ontogenetic movements of C. leucas from natal habitats. Differences among adults were gender related. Shifts in habitat use by subadults were correlated with a sigmoidal δ13C relationship with TL. C. leucas have a multipartite, stage-specific dependency in their transition between habitats along the freshwater–estuarine–marine continuum, making them particularly susceptible to the habitat alteration that is occurring globally.
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Previous research suggests that nursery areas provide an abundant food source as well as protection from predation for young sharks, and that these benefits are the reasons they use these areas. This study examined the abundance of prey species within a known nursery area, Terra Ceia Bay, Florida, and compared those data with the amount of time blacktip sharks spent within various geographic zones within the nursery. The most abundant prey species within the study site were pinfish, Lagodon rhomboides, pigfish, Orthopristis chrysoptera, spotfin mojarra, Eucinostomus argenteus, and silver perch, Bairdiella chrysoura. Prey species were found to be most abundant in the mid to southern portion of the nursery area, whereas sharks spent the majority of their time within the northern portion of the study site. There was no correlation between the amount of time sharks (as a whole and by individual) spent within a geographic zone and the abundance of prey species within that area. These results suggest that prey abundance is not the main factor directing the movement patterns and habitat choice of juvenile Carcharhinus limbatus within Terra Ceia Bay. Predator avoidance may be more important in the use of the nursery grounds by these young animals than prey abundance.
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Lagoon fish in New Caledonia are mainly caught by artisanal fisheries and subsistence fishing. Reef fish are the major component of this catch. The present study aimed at estimating these reef fish standing stocks and at finding the main factors influencing the distribution of these fish. Sampling of 904 stations was stratified according to three zones (north, east and west) and three reef types (barrier, intermediate and fringing). Fish communities exhibited strong heterogeneity in their distribution, showing higher biomass (maximum of 447 g·m–2) and total standing stock (43 000 tonnes) in the north zone than in the east and west zones. Similarly, observed patterns were dependent on reef types: higher biomass and total standing stock being observed on barrier reefs than on intermediate or fringing reefs. The total standing stocks, which were about 65 000 t, were mainly composed of herbivorous fish families such as the Acanthuridae and Scaridae. The differences in the patterns of distribution of species, individuals and standing stocks between reef types may be explained by variations in terrestrial influences and reef morphology, whereas differences among zones were most likely due to accessibility of fishing areas and fishing pressure. The latter is almost non-existent in the north zone, which can thus be considered to be almost unexploited commercially. This most likely explains the high proportion, 77 %, of long-lived species in the biomass of this zone. The results might have implications in management of reefs elsewhere in the South Pacific, for which similar data are only scarcely available.
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Hammerhead sharks have experienced a drastic global reduction in abundance, yet limited data exist on their general biology and population structure. We use generalised linear models (GLMs) to analyse historical mesh net and drumline catches (1996–2006) of Sphyrna lewini from the Queensland Shark Control Program (QSCP), which covers 10 locations on the north-east coast of Australia. Results show a significant decline in S. lewini female total length over the study period and a significant increase in annual CPUE on the Gold Coast. Sexual segregation was found in Cairns, where S. lewini males were most abundant. A positive linear relationship was found between maternal total length and litter size. Birth-size embryos were found in most months. However, normal birth size (0.50m) embryos were found in pregnant females most often during spring and summer. Mesh nets were more selective for smaller S. lewini than drumlines. It is recognised that species selectivity of other, more potentially dangerous shark species varies between gear types, but replacement of mesh nets with drumlines would reduce the catch of smaller individuals of S. lewini and assist in their conservation.
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This study examined the characteristics of a blacktip reef shark Carcharhinus melanopterus population in turbid coastal habitats through a multi-year fishery-independent sampling and tag-recapture programme. Results revealed a highly structured population comprised almost entirely of juveniles and adult females with individuals between 850 and 1050 mm total length effectively absent. Mature males were also rarely encountered with adult sex ratio highly biased towards females (female:male = 7:1). Mating scars were observed on adult females between December and April, and parturition was observed from December to March. Regression analysis showed that catch rates were significantly higher during the summer wet season between November and May. Recapture data suggested a highly resident population with a recapture rate of 21% and a mean recapture distance of 0·8 km. In addition, 33% of recaptured animals were captured multiple times, indicating long-term residency. Most recaptures were, however, of adults with few juveniles recaptured. Widespread sampling at the study site and in adjacent areas suggested that the population was highly localized to a specific bay. The bimodal and sex-segregated population structure observed here differs from previous reports for this species, and in combination with reproductive observations, suggests population structuring to facilitate reproductive and recruitment success. These data also highlight the potential ecosystem functions performed by coastal habitats in sustaining C. melanopterus populations.
Article
ppropriate management strategies for coastal regions require an understanding of how ecological similarities and differences among species shape ecosystem processes. Here, we tested whether morphological similarity equated to similar age and growth patterns in two common coastal sharks in northern Australia. Vertebrae of 199 pig-eye (Carcharhinus amboinensis) and 94 bull (C. leucas) sharks were sourced principally from commercial fisheries operating along the Northern Territory coastline during 2007–2009. We sectioned vertebrae to provide estimates of age of these animals. Model averaging results indicated female pig-eye sharks matured at 13 years and lived >30 years. Theoretical asymptotic length (L∞) (±s.e.) was estimated to be 2672 (±11.94) mm with a growth coefficient (k) of 0.145 year–1. Male pig-eye sharks matured slightly earlier than females (12 years) and survived >26 years. Theoretical asymptotic length for males (L∞) (±s.e.) was also smaller (2540 ± 13.056) mm and they grew faster (k = 0.161 year–1) than females. Bull sharks matured at 9.5 years and reached a maximum theoretical size (L∞) (±s.e.) of 3119 mm (±9.803) with a similar growth coefficient (k = 0.158 year–1) to pig-eye sharks. Longevity of bull sharks was estimated to be more than 27 years. Our results indicate that these patterns of high longevity and slow growth are indicative of low resilience and high susceptibility to over-exploitation of these coastal sharks.
Article
Global declines of shark populations are of concern because of their largely assumed role as moderators of ecosystem function. Without long-term data on movement patterns for many species, it is impossible to infer relative extinction risk, which varies as a function of range, dispersal and habitat specificity and use. The past 50 yr of research on coastal sharks has revealed common movement patterns among species. In the horizontal plane, measured home range size generally increases with body size. We demonstrate meta-analytically the effects of increasing body size and monitoring time on home range size. Changes in the extent of horizontal movement might arise from ontogeny, predator avoidance or environmental tolerances. In the vertical plane, movement patterns include oscillatory vertical displacement, surface swimming, diel vertical migration and swimming at depth. These vertical movements are often attributed to foraging or navigation, but have been quantified less than horizontal patterns. Habitat specificity is often correlated with environmental conditions such as depth, salinity, substratum, and in some cases, prey availability. Site fidelity is common in species that use nursery areas. However, fidelity to mating, pupping, feeding and natal sites has only been observed in a few species. To date, few studies have examined habitat partitioning, although some general patterns have emerged: habitats appear to be subdivided by benthos type, prey availability and depth. The conservation of coastal sharks can be facilitated in some cases by the use of marine protected areas, especially for coastal resident species using specific nursery, reproduction or feeding areas. Partial protected-area closures might be effective during aggregation or migration periods to protect older size classes, but these must be applied with other management strategies such as reduced fishing and size or bag limits to protect individuals throughout different life history phases. More long-term research on habitat use, migration patterns and habitat partitioning is essential for developing successful management initiatives for coastal shark populations.
Article
The bull shark, Carcharhinus leucas, is the most common shark in the brackish Indian River lagoon system on the central east coast of Florida. The biology of the lagoon population was studied between May 1975 and May 1979. There was substantial spatial and seasonal variation in catch rates with gill nets. Bull sharks were usually most abundant in the low-salinity lagoon basins. Catch rates were generally highest in the spring and fall and were always higher at night than day. No specimens were netted during the winter although bull sharks are known to be present during that season. The permanent lagoon population was composed entirely of newborn young and juveniles up to 202 cm TL. As they approach maturity, the subadults leave the estuary. Pregnant adult females return to lagoon waters in late spring and summer to give birth. One pregnant female 249 cm TL was captured during this study. Juvenile bull sharks in the lagoon system fed primarily on stingrays and marine catfishes.
Article
The movement patterns of large juveniles are poorly known for many shark species. With increasing pressure on shark populations these data are critical for the management of large coastal species. A series of acoustic receivers were positioned in Pine Island Sound, Florida, USA, to passively track the long-term movements of large juvenile bull (Carcharhinus leucas) and lemon (Negaprion brevirostris) sharks. Nineteen C. leucas and five N. brevirostris were monitored during 2003 and 2004. Individual C. leucas were present for 8 to 89 days, while N. brevirostris were present for 12 to 83 days. Weekly minimum convex polygons and kernel utilisation distributions were calculated to demonstrate the home range and core areas of use of both species. Spectral analysis demonstrated that several N. brevirostris showed repetitive diel north-south movement patterns over periods of up to 28 consecutive days. C. leucas demonstrated regular use of backwater habitats. Long-term use of estuarine areas by these large juvenile sharks suggests that estuarine coastal lagoons provide an important habitat for this portion of their life history and as such, protection of these habitats may assist in shark management and conservation.
Article
In this investigation of the wounds occurring on blue sharks, tooth cuts were found only on females over 180 cm long, supporting the view that they are courtship scars incurred during breeding. However, it seems unlikely that these sharks normally mate in British waters. It is suggested that tooth cuts can provide information on the size at sexual maturity of the blue shark in European seas.
Article
The quantification of spatial and temporal movement patterns of coral reef sharks is important to understand their role in reef communities and to aid the design of conservation strategies for this predatory guild. We observed 4 species of reef sharks aggregating in an inshore bay in the north of Western Australia for over 2 yr, using acoustic telemetry and visual censuses to examine how they partitioned this site in space and time. We fitted 58 sharks with acoustic transmitters: Carcharhinus melanopterus (36), C. amblyrhynchos (11), Negaprion acutidens (7) and Triaenodon obesus (4). Aggregations consisted primarily of C. melanopterus, although C. amblyrhynchos and N. acutidens were often present. We observed aggregations by visual census in summer (maximum of 44 sharks). Detections were highest during warmer months (Sep to Mar) for all species, although some individuals showed year-round residency. C. melanopterus, C. amblyrhynchos and N. acutidens had strong diel patterns of attendance at the aggregation site. Peak daily detections occurred from 13: 00 to 14: 00 h local time for C. melanopterus and C. amblyrhynchos; juvenile C. melanopterus and N. acutidens peaked at 05:00 and 10:00 h, respectively. There was considerable spatial overlap of core areas of use (50% kernel density estimates) at the northern end of the bay by all species; the southern end was used primarily by C. melanopterus and N. acutidens. Aggregations of C. melanopterus and C. amblyrhynchos consisted mainly of adult females, some of them pregnant. Courtship behaviour in C. melanopterus and T. obesus suggests that these aggregations are related to reproduction. All species displayed inter-annual site fidelity. The long-term presence of juvenile C. melanopterus and N. acutidens also suggests that this bay provides suitable conditions for younger age classes.
Article
The movements of 24 hatchery-reared Atlantic salmon Salmo salar smolts, with miniature acoustic transmitters (pingers) implanted surgically, were determined after release in the coastal waters of Passamaquoddy Bay (mean tide range 6 m), New Brunswick, Canada, to describe the first stages of seaward migration. Automated pinger detection at fixed sites, and pinger location and tracking by boat were used. Post-smolts left the release area rapidly, and the majority moved to open waters of the bay within several tidal cycles. Initially, post-smolts moved with a seaward orientation on ebb tides and held positions on flood tides. Their movements into open waters were diurnal, and the timing corresponded with the state of the tide during which they moved through a narrow channel. Post-smolts moved preferentially through this passageway with the aid of the tidal stream. Successful movements out through the channel occurred during ebb tides and any movements back in were during flood tides. Ground speed of fish moving through the channel was 4·2 body lengths s−1 and faster than the tidal stream velocities in the channel. The relative velocity of fish swimming through the channel was 2 body lengths s−1. Post-smolt movement was indicative of active, directed swimming with a reliance on ebb-tide transport for migration through a coastal area with strong tidal currents. Some post-smolts moved seaward directly with no apparent period of acclimation for the transfer to the marine environment, whereas others delayed their departure. These differences in behaviour were probably related to asynchrony in smolting when fish were released.
Article
Historical abundances of many large marine vertebrates were tremendously greater than today. However, while pelagic sharks are known to have declined rapidly in the northwest Atlantic in recent years, there, as elsewhere, little is known about the former natural abundances of these species. Here, we compare initial (1950s) and recent (late-1990s) standardized catch rates of pelagic sharks in the Gulf of Mexico, the area where methods of exploitation between these two periods were most comparable. We estimate that oceanic whitetip and silky sharks, formerly the most commonly caught shark species, have declined by over 99 and 90%, respectively. That the former prevalence of oceanic whitetip sharks in this ecosystem is unrecognized today is clear evidence of shifting baselines. Our analysis provides the missing baseline for pelagic sharks in the Gulf of Mexico that is needed for the rational management and restoration of these species.
Article
Grey reef sharks (Carcharhinus amblyrhynchos) are apex predators found on many Indo-Pacific coral reefs, but little is known about their movement patterns and habitat requirements. We used acoustic telemetry to determine movements and habitat use of these sharks at the isolated Rowley Shoals atolls, 250km off the coast of north-western Australia. We equipped 12 male and 14 female sharks ranging from 0.79 to 1.69m in total length with transmitters that were detected by an array of 11 strategically placed receivers on two atoll reefs. Over 26,000 detections were recorded over the 325days of receiver deployment. No sharks were observed to move between reefs. Receivers on the outer slopes of reefs provided nearly all (99%) of the detections. We found no differences in general attendance parameters due to size, sex or reef, except for maximum period of detection where larger sharks were detected over a longer period than smaller sharks. Male and female sharks were often detected at separate receivers at the outer slope habitat of one reef, suggesting sexual segregation, but this pattern did not occur at the second reef where males and females were detected at similar frequencies. We identified two patterns of daily behaviour: (1) sharks were present at the reef both day and night or (2) sharks spent more time in attendance during day than at night. Fast Fourier transforms identified 24-h cycles of attendance at the reef and a secondary peak of attendance at 12h for most sharks, although no individuals shared the same attendance patterns. Our study provides baseline data that can be used to optimise the minimum area and habitat requirements for conservation of these apex predators. KeywordsAcoustic telemetry–Habitat use–Site fidelity–Marine protected areas–Grey reef shark
Article
Les poissons de lagon de Nouvelle-Calédonie sont exploités par les pêcheries artisanales, dont les poissons de récifs constituent la majeure partie de ces captures. Ce travail vise à estimer les stocks de ces poissons récifaux et à déterminer les principaux facteurs qui influencent leur distribution. L’échantillonnage de 904 stations a été stratifié en fonction de trois zones (nord, est et ouest) et de trois types de récif (barrière, intermédiaire et frangeant). Les peuplements de poissons ont montré une très forte hétérogénéité dans leur répartition de la biomasse et des stocks. Dans la zone nord, ces valeurs ont été bien supérieures (jusqu’à 447 g·m–2, et stocks de 43 000 tonnes) à celles des zones est et ouest. La distribution de ces peuplements est également fonction du biotope considéré, de plus fortes densités et biomasses étant relevées sur les récifs barrières, par contraste avec les récifs intermédiaires et frangeants. La majeure partie du stock total, qui est de l’ordre de 65 000 t, est constituée d’espèces herbivores d’Acanthuridés et de Scaridés. Les principales causes des différences observés dans la répartition des espèces et des individus, ainsi que des stocks entre types récifaux seraient liées à des différences d’apports terrigènes et de morphologie récifale, tandis que les différences relevées entre les trois zones sont davantage liées à l’accessibilité des ressources et à la pression de pêche. Cette dernière est très faible dans la zone nord, qui peut être considérée comme inexploitée. Cela expliquerait la forte proportion, 77 %, des espèces à longue espérance de vie de la biomasse totale de cette zone. La gestion des pêcheries artisanales dans l’ensemble du Pacifique Sud, où les données sont rarement disponibles, implique la prise en compte de ces observations.
Article
Adult bull sharks Carcharhinus leucas were monitored with electronic tags to investigate horizontal and vertical movements in the Atlantic and Pacific Oceans. In both locations, C. leucas showed some fidelity to specific coastal areas with only limited horizontal movements away from the tagging sites after tag attachment. Fish tagged in the Bahamas were detected mostly in the upper 20 m of the water column in water 25-26° C, whereas C. leucas tagged in Fiji spent most of their time below 20 m in water usually >26° C. The results highlight the importance of coastal inshore habitats for this species.
Article
1. Apex predators are often assumed to be dietary generalists and, by feeding on prey from multiple basal nutrient sources, serve to couple discrete food webs. But there is increasing evidence that individual level dietary specialization may be common in many species, and this has not been investigated for many marine apex predators. 2. Because of their position at or near the top of many marine food webs, and the possibility that they can affect populations of their prey and induce trophic cascades, it is important to understand patterns of dietary specialization in shark populations. 3. Stable isotope values from body tissues with different turnover rates were used to quantify patterns of individual specialization in two species of 'generalist' sharks (bull sharks, Carcharhinus leucas, and tiger sharks, Galeocerdo cuvier). 4. Despite wide population-level isotopic niche breadths in both species, isotopic values of individual tiger sharks varied across tissues with different turnover rates. The population niche breadth was explained mostly by variation within individuals suggesting tiger sharks are true generalists. In contrast, isotope values of individual bull sharks were stable through time and their wide population level niche breadth was explained by variation among specialist individuals. 5. Relative resource abundance and spatial variation in food-predation risk tradeoffs may explain the differences in patterns of specialization between shark species. 6. The differences in individual dietary specialization between tiger sharks and bull sharks results in different functional roles in coupling or compartmentalizing distinct food webs. 7. Individual specialization may be an important feature of trophic dynamics of highly mobile marine top predators and should be explicitly considered in studies of marine food webs and the ecological role of top predators.
Article
Habitat use, movement and residency of bull sharks Carcharhinus leucas were determined using satellite pop-up archival transmitting (PAT) tags throughout coastal areas in the U.S., Gulf of Mexico and waters off the south-east U.S. From 2005 to 2007, 18 fish (mean size = 164 cm fork length, L(F)) were tagged over all seasons. Fish retained tags for up to 85 days (median = 30 days). Based on geolocation data from initial tagging location to pop-off location, C. leucas generally travelled c. 5-6 km day(-1) and travelled an average of 143.6 km. Overall, mean proportions of time at depth revealed C. leucas spent the majority of their time in waters <20 m. They exhibited significant differences among depths but were not found at a particular depth regardless of diurnal period. Most fish occupied temperatures c. 32 degrees C with individuals found mostly between 26 and 33 degrees C. Geolocation data for C. leucas were generally poor and varied considerably but tracks for two individuals revealed long distance movements. One fish travelled from the south-east coast of the U.S. to coastal Texas near Galveston while another moved up the east coast of the U.S. to South Carolina. Data on C. leucas movements indicated that they are found primarily in shallower waters and tend to remain in the same location over long periods. While some individuals made large-scale movements over open ocean areas, the results emphasize the importance of the coastal zone for this species as potential essential habitat, particularly in areas of high freshwater inflow.
Article
The case of a fatal, unprovoked shark attack is reported and analyzed. The incident took place on the 30th of September 2007, in the lagoon of Luengoni Bay, Lifou Island (Loyalty Islands, New Caledonia). A young French woman who was snorkeling was severely bitten on the right thigh and died of hemorrhage. An analysis based in particular on the size and color of the shark, the characteristics of the wounds, and the behavior of the shark before and after the bite suggests that the aggressor was a great white shark, Carcharodon carcharias.
Article
Plasma and erythrocyte solute properties were examined in freshwater (FW) acclimated juvenile Carcharhinus leucas following acute transfer to 75% seawater (SW), and 100% SW. Blood samples were taken at 0, 12 and 96 h following transfer to 75% SW and 24 and 72 h after transfer to 100% SW. A control group in FW was subjected to the same sampling regime. Upon transfer of C. leucas to 75% and 100% SW, plasma Na+, Cl-, K+, Mg2+, Ca2+, urea and TMAO concentrations all increased significantly but disproportionately. Plasma Na+ and Cl- increased immediately, followed by an increase in plasma urea. Erythrocyte urea and TMAO concentrations increased significantly following transfer to 75% and 100% SW; however, changes in erythrocyte inorganic ion concentrations were insignificant. Haematocrit, haemoglobin and mean cell haematocrit did not differ significantly after transfer to seawater; however, plasma water was slightly reduced after 24 and 72 h in 100% SW. Red blood cell (RBC) water content was elevated 24 h after transfer to 100% SW but returned to FW levels after 72 h. These results demonstrate that the transfer from 75% to 100% SW presents C. leucas with a greater osmoregulatory challenge than transfer from FW to 75% SW, despite the larger concentration gradient in the latter. In summary, C. leucas tolerate rapid and significant increases in salinity by rapidly increasing plasma osmolality to be hyperosmotic to the environment whilst maintaining a tight regulation of their intracellular fluid environment.