Shark Abundance Research Articles

Size distributions and sex ratios of sharks caught by Oman's artisanal fishery

Oman's fishery resources are exploited by artisanal and industrial fisheries, but the former accounts for almost 90% of landings. Within the artisanal fishery, sharks have traditionally been harvested for their flesh, but the development of a valuable export market for shark fin has led to increased utilisation of this resource, and anecdotal information suggests that shark abundance has decreased in recent years. Whereas management of the shark fishery is desirable, the biological and socio-economical data on which to base any management plan are lacking. The present study was undertaken to collect size frequency and sex ratio information from the shark species most commonly encountered in the artisanal landings, namely Carcharhinus falciformis, C. limbatus, C. macloti, C. sorrah, lago omanensis, Loxodon macrorhinus, Rhizoprionodon acutus and Sphyrna lewini. The occurrence of large female sharks in the landings, combined with a high proportion of juveniles of certain species, suggests that species-specific size restrictions should be considered in any management plan for Oman's shark fishery.

Assessing the distribution and relative abundance of wobbegong sharks (Orectolobidae) in New South Wales, Australia, using recreational scuba-divers

Wobbegongs are benthic sharks that are commercially targeted in New South Wales (NSW), Australia. Given a dramatic reduction of more than 50% in landed catch in a decade, there is a clear need to ensure that basic ecological data such as distribution and abundance are available for management use. Opportunistic sightings of wobbe- gongs collected by recreational scuba-divers were used to assess the distribution and relative abundance of wobbegongs in NSW. From July 2003 until January 2005, 304 dives were undertaken by recreational divers and 454 wobbegongs were reported. Larger numbers of wobbegongs were sighted in northern compared to southern NSW. Spotted and ornate wobbegongs were sighted in similar numbers, but species composition was highly variable across locations. Only a few juvenile and newborn spotted wobbegongs were sighted, whereas small ornate wobbegongs were mostly sighted north of central NSW. The latter were possibly the third, cryptic species, the dwarf ornate wobbegong. The paucity of sight- ings of small wobbegongs suggests that juveniles and newborns are inconspicuous to divers or that small wobbegongs are found in areas not visited by divers. Potential species and size segregation suggest that closing areas to fishing may enable populations to sustain current levels of commercial exploitation. The cost-effectiveness of using recre- ational scuba-divers to opportunistically collect distribution and relative abundance data was apparent from this study. However, the lack of spatial and temporal homogeneity in diving effort suggests that future studies should consider incorporating organized surveys and a facilitator, rather than using opportunistic records of sightings.

Seasonal cycles and long-term trends in abundance and species composition of sharks associated with cage diving ecotourism activities in Hawaii

SUMMARYShark cage diving is both popular and controversial, with proponents citing educational value and non-extractive use of natural resources and opponents raising concerns about public safety and ecological impacts. Logbook data collected 2004–2008 from two Oahu (Hawaii) shark cage diving operations were analysed to determine whether such voluntary records provide useful insights into shark ecology or ecotourism impacts. Operators correctly identified common shark species and documented gross seasonal cycles and long-term trends in abundance of Galapagos (Carcharhinus galapagensis), sandbar (Carcharhinus plumbeus) and tiger sharks (Galeorcerdo cuvier). Annual cycles in shark abundance may indicate seasonal migrations, whereas long-term trends suggest gradual exclusion of smaller sandbar sharks from cage diving sites. Numerically dominant (> 98%) Galapagos and sandbar sharks are rarely implicated in attacks on humans. Negligible impact on public safety is supported by other factors such as: (1) remoteness of the sites, (2) conditioning stimuli that are specific to the tour operations and different from inshore recreational stimuli and (3) no increase in shark attacks on the north coast of Oahu since cage diving started. Tracking studies are required to validate logbook data and to determine whether sharks associated with offshore cage diving travel into inshore areas used for in-water recreation.

Physical factors influencing the distribution of a top predator in a subtropical oligotrophic estuary

We used longline fishing to determine the effects of distance from the ocean, season, and short‐term variation in abiotic conditions on the abundance of juvenile bull sharks (Carcharhinus leucas) in an estuary of the Florida Everglades, U.S.A. Logistic regression revealed that young‐of‐the‐year sharks were concentrated at a protected site 20 km upstream and were present in greater abundance when dissolved oxygen (DO) levels were high. For older juvenile sharks (age 1+), DO levels had the greatest influence on catch probabilities followed by distance from the ocean; they were most likely to be caught at sites with >3.5 mg L−1 DO and on the main branch of the river 20 km upstream. Salinity had a relatively small effect on catch rates and there were no seasonal shifts in shark distribution. Our results highlight the importance of considering DO as a possible driver of top predator distributions in estuaries, even in the absence of hypoxia. In Everglades estuaries hydrological drivers that affect DO levels (e.g., groundwater discharge, modification of primary productivity through nutrient fluxes) will be important in determining shark distributions, and the effects of planned ecosystem restoration efforts on bull sharks will not simply be mediated by changing salinity regimes and the location of the oligohaline zone. More generally, variation in DO levels could structure the nature and spatiotemporal pattern of top predator effects in the coastal Everglades, and other tropical and subtropical estuaries, because of interspecific variation in reliance on DO within the top predator guild.

Aerial survey as a tool to estimate whale shark abundance trends

Aerial surveys have been used to estimate population abundance of both terrestrial and marine species; in the marine environment this has largely been used for air-breathing species that spend time regularly at the surface. Whale sharks spend a large proportion of their time close to the surface and so are amenable to aerial survey techniques. This study presents the results of six years of synoptic aerial belt-surveys done nearly daily during the peak whale shark season around the island of Mahe, Seychelles. A total of 580 survey flights were flown providing 699.7 hours of survey record. A seasonal peak of shark sightings per hour was recorded in September or October in most years with the maximum on a single survey of 28.4 h - 1 in October 2006. The aerial survey data were used to generate an estimate of relative population abundance indicating that highest mean annual relative population estimate was also in 2006, with an estimate of 38, while the lowest mean estimate was 11 in 2004. These estimates were then compared to weekly capture-mark-recapture estimates of abundance based on unique individual identification data. The results indicate that the use of aerial survey data alone may give an acceptable indication of instantaneous relative population abundance but further refinement is necessary to estimate absolute regional abundance.

Estimates of juvenile and adult raggedtooth shark (Carcharias taurus) abundance along the east coast of South Africa

A Cormack–Jolly–Seber model was developed to estimate abundance, survival, and probability of capture for juvenile (<1.8 m total length, TL) and adult (>1.8 m TL) raggedtooth sharks (Carcharias taurus) along the east coast of South Africa. Model estimates were adjusted to account for tag loss, nonreporting, and after release mortality. The model was constructed using mark–recapture data from the Oceanographic Research Institute and Port Elizabeth Museum cooperative tagging programs from 1984 to 2004. The adjusted estimate for juvenile survival was 0.56 and that for adult survival was 0.89. The adjusted estimate of probability of capture for juvenile sharks ranged from 0.06 to 0.17, while that for adult sharks was from 0.02 to 0.04. The mean annual abundance of juvenile sharks was 6800 (coefficient of variation, CV = 13%) and adult sharks 16 700 (CV = 9%). The accumulated effect of tag loss, nonreporting, and after release mortality were to reduce the overall estimate of juvenile and adult abundance by approximately 50%. The adjusted estimate of population size for both juvenile and adult sharks over the last decade appears to have remained constant (P > 0.05).

Shark Attacks in Dakar and the Cap Vert Peninsula, Senegal: Low Incidence despite High Occurrence of Potentially Dangerous Species

BackgroundThe International Shark Attack File mentions only four unprovoked shark attacks on the coast of West Africa during the period 1828–2004, an area where high concentrations of sharks and 17 species potentially dangerous to man have been observed. To investigate if the frequency of shark attacks could be really low and not just under-reported and whether there are potentially sharks that might attack in the area, a study was carried out in Dakar and the Cap Vert peninsula, Senegal.Methodology/Principal FindingsPersonnel of health facilities, administrative services, traditional authorities and groups of fishermen from the region of Dakar were interviewed about the occurrence of shark attacks, and visual censuses were conducted along the coastline to investigate shark communities associated with the coasts of Dakar and the Cap Vert peninsula. Six attacks were documented for the period 1947–2005, including two fatal ones attributed to the tiger shark Galeocerdo cuvieri. All attacks concerned fishermen and only one occurred after 1970. Sharks were observed year round along the coastline in waters 3–15 m depth. Two species potentially dangerous for man, the nurse shark Ginglymostoma cirratum and the blacktip shark Carcharhinus limbatus, represented together 94% of 1,071 sharks enumerated during 1,459 hours of observations. Threatening behaviour from sharks was noted in 12 encounters (1.1%), including 8 encounters with C. limbatus, one with Galeocerdo cuvieri and 3 with unidentified sharks.Conclusions/SignificanceThese findings suggest that the frequency of shark attacks on the coast of West Africa is underestimated. However, they also indicate that the risk is very low despite the abundance of sharks. In Dakar area, most encounters along the coastline with potentially dangerous species do not result in an attack. Compared to other causes of water related deaths, the incidence of shark attack appears negligible, at least one thousand fold lower.

Spatial and temporal variation in shark communities of the lower Florida Keys and evidence for historical population declines

Sharks are top predators in many marine ecosystems. Despite recent concerns over declines in shark populations, studies of shark communities in coastal habitats are limited. We used drumlines and longlines to determine shark community composition and habitat affinities in the Florida Keys, USA. Community composition varied among habitats. Catch rates of smaller sharks were highest in protected shallow waters, while large sharks were more abundant in deep channels. Overall probabilities of catching large sharks on drumlines did not vary with water temperature, while catches of small sharks on longlines increased with increasing water temperature. Individual species differed in their responsiveness to variation in water temperatures and habitat. Bait type affected catch rates of some species, suggesting that fishing methods should be considered explicitly in studies describing shark communities or temporal trends in abundance. Catch rates of large-bodied sharks were higher in a remote and protected location compared with similar habitats near inhabited Keys. Also, historical accounts of a shark fishery in the study area during the 1920s suggest substantial declines in large shark abundance and shifts in community composition. By implication, ecosystem impacts of changes in the large shark community may be dramatic and likely occurred before adequate baselines were established.

Gillnet mesh selectivity of the sandbar shark (Carcharhinus plumbeus): implications for fisheries management

Abstract McAuley, R. B., Simpfendorfer, C. A., and Wright, I. W. 2007. Gillnet mesh selectivity of the sandbar shark (Carcharhinus plumbeus): implications for fisheries management. – ICES Journal of Marine Science, 64. Gillnet mesh selectivity parameters for the sandbar shark (Carcharhinus plumbeus) were estimated from catches taken by an experimental net of six panels of mesh, varying in size from 10.2 to 25.4 cm. The length selectivity of each mesh size was described by five different models. According to model deviance values, the four models based on the SELECT method of estimation provided better fits to the data than the gamma model previously applied to sharks. Lengths at maximum selectivity were estimated to be between 5.3 and 7.0×stretched mesh size. The breadth of the selectivity curves was greater than have been reported for most species of shark. Lognormal and normal curve forms yielded the lowest model deviance and were judged to provide the best fits to the data. Peak selectivity of the commercially utilized mesh sizes was generally estimated to be greater than the observed modal length class of the commercial C. plumbeus catch. This suggests that a relatively high abundance of smaller sharks in the study area offsets gear selectivity effects in determining the size composition of commercial catches. These results have important implications for the recovery of this overexploited stock and also for managing international gillnet fisheries for the species.

Fear factor: do dugongs (Dugong dugon) trade food for safety from tiger sharks (Galeocerdo cuvier)?

Predators can influence plants indirectly by altering spatial patterns of herbivory, so studies assessing the relationship between perceived predation risk and habitat use by herbivores may improve our understanding of community organization. In marine systems, the effects of predation danger on space use by large herbivores have received little attention, despite the possibility that predator-mediated alterations in patterns of grazing by these animals influence benthic community structure. We evaluated the relationship between habitat use by foraging dugongs (Dugong dugon) and the threat of tiger shark predation in an Australian embayment (Shark Bay) between 1997 and 2004. Dugong densities were quantified in shallow (putatively dangerous) and deep (putatively safe) habitats (seven survey zones allocated to each habitat), and predation hazard was indexed using catch rates of tiger sharks (Galeocerdo cuvier); seagrass volume provided a measure of food biomass within each zone. Overall, dugongs selected shallow habitats, where their food is concentrated. Foragers used shallow and deep habitats in proportion to food availability (input matching) when large tiger sharks were scarce and overused deep habitats when sharks were common. Furthermore, strong synchrony existed between daily measures of shark abundance and the extent to which deep habitats were overused. Thus, dugongs appear to adaptively manage their risk of death by allocating time to safe but impoverished foraging patches in proportion to the likelihood of encountering predators in profitable but more dangerous areas. This apparent food-safety trade-off has important implications for seagrass community structure in Shark Bay, as it may result in marked temporal variability in grazing pressure.

The Sandbar Shark Summer Nursery within Bays and Lagoons of the Eastern Shore of Virginia

AbstractWe sought to examine the spatial use of coastal bays and lagoons associated with the Eastern Shore of Virginia (i.e., the Atlantic shoreline north of the mouth of the Chesapeake Bay) as nursery habitat for sandbar sharks Carcharhinus plumbeus. Generalized linear mixed models were used to examine the relationship between physical factors and the abundance of neonate and juvenile sandbar sharks within the study area. Neonates, small juveniles, and large juveniles were present throughout the sampling area in relatively high abundance throughout the months of June‐September. Abundances of neonates and small juveniles were correlated with sampling distance from the inlet; larger fractions of these groups were captured as distance from the inlet increased. Large‐juvenile abundance was not significantly correlated with any physical factor. The catch rates of neonate and juvenile sandbar sharks within this area were comparable with those observed in nearby Chesapeake Bay, although a larger number of juveniles greater than 100 cm total length were caught within the Eastern Shore lagoons. The bays and lagoons we examined function as an important primary and secondary nursery ground for this species and fit the criteria to be included in future management measures as a habitat area of particular concern.

Can measures of prey availability improve our ability to predict the abundance of large marine predators?

Apex marine predators can structure marine communities, so factors underlying their abundance are of broad interest. However, such data are almost completely lacking for large sharks. We assessed the relationship between tiger shark abundance, water temperature, and the availability of a variety of known prey over 5 years in Western Australia. Abundance of sharks in four size categories and the density of prey (cormorants, dugongs, sea snakes, sea turtles) were indexed using daily catch rates and transects, respectively. Across all sizes, thermal conditions were a determinant of abundance, with numerical peaks coinciding with periods of high water temperature. However, for sharks exceeding 300 cm total length, the inclusion of dugong density significantly improved temperature-based models, suggesting that use of particular areas by large tiger sharks is influenced by availability of this sirenian. We conclude that large marine predator population models may benefit from the inclusion of measures of prey availability, but only if such measures consider prey types separately and account for ontogenetic shifts in the diet of the predator in question.

Living on the edge: dugongs prefer to forage in microhabitats that allow escape from rather than avoidance of predators

When threatened with predation, foraging prey can cease feeding and seek refuge or shift to feeding sites (microhabitats) offering increased safety. Predator-induced microhabitat shifts by large herbivores are of interest to ecologists because spatial patterns of foraging by these animals shape plant communities. The influence of predation risk on microhabitat use by large herbivores in marine systems remains poorly understood. We explored the relationship between microhabitat use by dugongs, Dugong dugon, and tiger shark, Galeocerdo cuvier, predation risk in an Australian embayment over 3 years. Use by foraging dugongs of two seagrass microhabitats, edge (lower-quality seagrass, swift escape from sharks) and interior (higher-quality seagrass, fewer escape options), was monitored in seven survey zones. We indexed predation danger using catch rates of tiger sharks that were greater than 3.0 m in total length. The degree of dissimilarity between forager densities in edge and interior microhabitats was a function of tiger shark abundance: foragers underused edge (safe) microhabitat when sharks were scarce, overused it when sharks were common, and responded to daily changes in shark abundance in a threat-sensitive fashion, showing the greatest preference for edges when shark abundance was highest. We conclude that dugongs manage their probability of death by allocating more time to safe but lower-quality feeding microhabitats when the likelihood of encountering sharks is elevated. Dugong grazing can influence seagrass biomass and patch composition, so tiger sharks probably affect the microhabitat structure of seagrass meadows, and ultimately their benthic communities, indirectly by altering the way that dugongs use feeding patches.

Foundations of the method of dating global fluctuations of abundance of oceanic sharks on the basis of the amount of fossil teeth in bottom sediments

A method for dating fluctuations in abundance of Elasmobranchii based on three parameters: density of fossil teeth on the ocean floor, density of modern species, and the rate of teeth replacement in modern species is proposed. As a model event, the substitution by the blue shark Prionace glauca of sharks of the genus Isurus that dominated from the Pliocene to the Late Holocene was used. It was determined that, at various values of the aforementioned parameters, the blue shark occupied a dominant position in the eastern Pacific between 302 and 3940 years ago. It is suggested that, in case of a large amount of material in geological collections using this method, a great number of paleoreconstructions of the development and changes in the oceanic biota may be carried out.

Circle Hooks for Pacific Longliners: Not a Panacea for Marlin and Shark Bycatch, but Part of the Solution

AbstractBlue marlin Makaira nigricans, striped marlin Tetrapturus audax, and pelagic sharks (e.g., blue shark Prionace glauca) are commonly caught as bycatch by longline fisheries in the central North Pacific Ocean. Recently, concern has increased about depletion of these species. Modifications in longline gear may offer one solution. Here, we test the use of circle hooks, rather than the conventional tuna‐style hooks, on longlines using an ecosystem model of the central North Pacific Ocean. The simulations considered span a range of reasonable circle hook catchability and survival rates for released fish. The results suggest that if circle hooks have higher catchability than the currently used tuna‐style hooks, switching to circle hooks depletes marlin biomass by 25–40% and shark biomass by 15–35% over 30 years. However, these depletions do not occur if circle hook catchability is equal to or lower than that of tuna‐style hooks. When the effects of catch‐and‐release requirements for marlins and sharks were also considered, we found that regardless of assumptions about circle hook catchability and survival rates, a combined policy of using circle hooks and releasing sharks and marlins leads to net increases in marlin and shark abundance. The simulations show a trade‐off between the abundance of marlins and sharks and their prey items, yellowfin tuna Thunnus albacares and small blue sharks. There is also evidence of trophic trade‐offs between yellowfin tuna and small blue sharks and their prey, small scombrids (Auxis spp.) and mahi mahi Coryphaena hippurus. The results illustrate the importance of understanding catchability and survival rates for circle hooks compared with those for tuna‐style hooks and encourage further research in this area.

Relative Abundance of Pelagic Sharks in the Western North Atlantic Ocean, Including the Gulf of Mexico and Caribbean Sea

Little information exists on the status of pelagic shark populations in the Atlantic Ocean, especially in the Gulf of Mexico and Caribbean Sea. We derived indices of relative abundance for pelagic sharks based on mandatory logbooks and observer reports from a scientific observer program of the United States (US) pelagic longline fleet. Time series data from the pelagic longline logbook program (1986-2005) and the pelagic longline observer program (1992-2005) were standardized with Generalized Linear Model (GLM) procedures. Declines in relative abundance for the 6 pelagic shark species or genera examined in the logbook data analysis ranged from 43% for mako sharks, Isurus spp., to 88% for blue sharks, Prionace glauca, whereas declines in relative abundance obtained from the observer data analysis were less accentuated than those in the logbook data analysis, with the trend being positive for night sharks, Carcharhinus signatus, and thresher sharks, Alopias spp. There was no significant change in the fork length at capture over the time period considered for blue sharks, shortfin makos Isurus oxyrinchus, or night sharks. The trends obtained must be viewed cautiously given recognized shortcomings, especially of the logbook dataset, and the highly migratory nature of pelagic sharks, which requires a more comprehensive evaluation of trends throughout their range.

Influence of teleost abundance on the distribution and abundance of sharks in Florida Bay, USA

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 of teleosts 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.

Does tiger shark predation risk influence foraging habitat use by bottlenose dolphins at multiple spatial scales?

Prey availability and predation risk are important determinants of habitat use, but their importance may vary across spatial scales. In many marine systems, predator and prey distributions covary at large spatial scales, but do no coincide at small spatial scales. We investigated the influences of prey abundance and tiger shark (Galeocerdo cuvier) predation risk on Indian Ocean bottlenose dolphin (Tursiops aduncus) habitat use across multiple spatial scales, in Shark Bay, Western Australia. Dolphins were distributed between deep and shallow habitats and across microhabitats within patches approximately proportional to prey density when shark abundance was low. When shark abundance was high, foraging dolphins greatly reduced their use of dangerous, but productive, shallow patches relative to safer deep ones. Also, dolphins reduced their use of interior portions of shallow patches relative to their edges, which have higher predator density but lower intrinsic risk (i.e. a higher probability of escape in an encounter situation). These results suggest that predation risk and prey availability influence dolphin habitat use at multiple spatial scales, but intrinsic habitat risk, and not just predator encounter rate, is important in shaping dolphin space use decisions. Therefore, studies of habitat use at multiple spatial scales can benefit from integrating data on prey availability and the subcomponents of predation risk.

Marine Parks Need Sharks?

In their report “Fishing, trophic cascades, and the process of grazing on coral reefs” (6 Jan., p. 98), P. J. Mumby et al. showed that increased grouper biomass within the Exuma Cays Land and Sea Park (ECLSP) did not acutely impair the grazing capacity of parrotfish populations (a major prey item), alleviating concerns that marine reserves may cause trophic cascades that will impede the recovery of imperiled Caribbean coral reefs. Although Mumby et al. suggest that this finding was primarily due to coincident beneficial reserve effects for parrotfish, we suggest that the abundance of sharks in the ECLSP, due to limited shark fisheries in the Bahamas (1), may have been an important contributing factor.

Effects of recreational and commercial fishing on blue sharks (Prionace glauca) in Atlantic Canada, with inferences on the North Atlantic population

The nominal catch of blue sharks (Prionace glauca) reported for the Canadian Atlantic grossly underestimates the annual catch mortality of about 1000 tonnes (t), making blue sharks the most frequently caught large shark in Canadian waters. Although blue sharks accounted for 99% of all sharks landed at recreational shark fishing tournaments, tournament catches accounted for only 3% of total fishing mortality. Standardized catch rate indices suggested a decline in blue shark abundance of about 5%–6%·year–1 since 1995. An increased mortality rate in recent years was suggested by a decline in the median size of blue sharks in the commercial catch. Two independent calculations suggest that North Atlantic catches exceeded 100 000 t, with catch mortalities ranging between 26 000 and 37 000 t. Because tagging studies indicated that blue sharks are highly migratory with a single population in the North Atlantic, the Canadian contribution to overall population mortality accounts for only 2% of the total. The fact that blue shark populations are relatively productive and resilient may help explain their persistence in the face of high international catch mortality and a decline in relative abundance.