Neil Hammerschlag

Predatory behaviour of white sharks (Carcharodon carcharias) at Seal Island, South Africa

Between 1997 and 2003, there were 2088 natural predations by white sharks (Carcharodon carcharias) on Cape fur seals (Arctocephalus pusillus pusillus) and 121 strikes on towed seal-shaped decoys were documented from observation vessels at Seal Island, South Africa. White sharks at Seal Island appear to selectively target lone, incoming young of the year Cape fur seals at or near the surface. Most attacks lasted < I min and consisted of a single breach, with predatory success rate decreasing rapidly with increasing duration and number of subsequent breaches. A white shark predatory ethogram, composed of four phases and 20 behavioural units, is presented, including four varieties of initial strike and 11 subsequent behaviour units not previously defined in the literature. Behaviour units scored from 210 predatory attacks revealed that, for both successful and unsuccessful attacks, Polaris Breach was the most commonly employed initial strike, while Surface Lunge was the most frequent second event, closely followed by Lateral Snap. Examination of video footage, still images, and tooth impressions in decoys indicated that white sharks at Seal Island bite prey obliquely using their anterolateral teeth via a sudden lateral snap of the jaws and not perpendicularly with their anterior teeth, as previously supposed. Analysis of white shark upper tooth morphology and spacing suggest the reversed intermediate teeth of white sharks occur at the strongest part of the jaw and produce the largest wound. White shark predatory success at Seal Island is greatest (55%) within one hour of sunrise and decreases rapidly with increasing ambient light; the sharks cease active predation on seals when success rate drops to ±40%; this is the first evidence of cessation of foraging at unproductive times by any predatory fish. At Sea] Island, white shark predatory success is significantly lower at locations where frequency of predation is highest, suggesting that white sharks may launch suboptimal strikes in areas of greatest intraspecific competition; this is the first evidence of social influence on predation in any elasmo-branch. Idiosyncratic predatory behaviours and elevated success rates of known individual white sharks at Seal Island suggest some degree of trial-and-error learning. A hypothetical decision tree is proposed that models predatory behaviour of white sharks attacking Cape fur seals at the surface.

Global shark currency: the distribution, frequency, and economic value of shark ecotourism

Ecotourism represents a highly popularised activity which has exhibited global growth in recent years. In the present paper, we examine the distribution, frequency, and economic value of shark-based ecotourism operations worldwide. A total of 376 shark ecotour operations across 83 locations and 8 geographic regions were identified. Here we describe the global and regional scope of the industry; determine the species utilised in shark ecotourism activities; and examine the recreational usage values of sharks. Further, we conducted a case study of a shark tourism operation based in South Africa by analysing 12 years of demographical and economical data, revealing increasing trends in the total number of customers served and cost per trip over the sampling period. We also compare consumptive and non-consumptive values of shark resources and discuss the potential research and conservation implications of the industry to sharks worldwide.

Cyanobacterial Neurotoxin β-N-Methylamino-L-alanine (BMAA) in Shark Fins

Sharks are among the most threatened groups of marine species. Populations are declining globally to support the growing demand for shark fin soup. Sharks are known to bioaccumulate toxins that may pose health risks to consumers of shark products. The feeding habits of sharks are varied, including fish, mammals, crustaceans and plankton. The cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) has been detected in species of free-living marine cyanobacteria and may bioaccumulate in the marine food web. In this study, we sampled fin clips from seven different species of sharks in South Florida to survey the occurrence of BMAA using HPLC-FD and Triple Quadrupole LC/MS/MS methods. BMAA was detected in the fins of all species examined with concentrations ranging from 144 to 1836 ng/mg wet weight. Since BMAA has been linked to neurodegenerative diseases, these results may have important relevance to human health. We suggest that consumption of shark fins may increase the risk for human exposure to the cyanobacterial neurotoxin BMAA.

Ocean-wide tracking of pelagic sharks reveals extent of overlap with longline fishing hotspots

Significance Shark populations are declining worldwide because of overexploitation by fisheries with unknown consequences for ecosystems. Although the harvest of oceanic sharks remains largely unregulated, knowing precisely where they interact with fishing vessels will better aid their conservation. We satellite track six species of shark and two entire longline fishing vessel fleets across the North Atlantic over multiple years. Sharks actively select and aggregate in space-use “hotspots” characterized by thermal fronts and high productivity. However, longline fishing vessels also target these habitats and efficiently track shark movements seasonally, leading to an 80% spatial overlap. Areas of highest overlap between sharks and fishing vessels show persistence between years, suggesting current hotspots are at risk, and arguing for introduction of international catch limits. Overfishing is arguably the greatest ecological threat facing the oceans, yet catches of many highly migratory fishes including oceanic sharks remain largely unregulated with poor monitoring and data reporting. Oceanic shark conservation is hampered by basic knowledge gaps about where sharks aggregate across population ranges and precisely where they overlap with fishers. Using satellite tracking data from six shark species across the North Atlantic, we show that pelagic sharks occupy predictable habitat hotspots of high space use. Movement modeling showed sharks preferred habitats characterized by strong sea surface-temperature gradients (fronts) over other available habitats. However, simultaneous Global Positioning System (GPS) tracking of the entire Spanish and Portuguese longline-vessel fishing fleets show an 80% overlap of fished areas with hotspots, potentially increasing shark susceptibility to fishing exploitation. Regions of high overlap between oceanic tagged sharks and longliners included the North Atlantic Current/Labrador Current convergence zone and the Mid-Atlantic Ridge southwest of the Azores. In these main regions, and subareas within them, shark/vessel co-occurrence was spatially and temporally persistent between years, highlighting how broadly the fishing exploitation efficiently “tracks” oceanic sharks within their space-use hotspots year-round. Given this intense focus of longliners on shark hotspots, our study argues the need for international catch limits for pelagic sharks and identifies a future role of combining fine-scale fish and vessel telemetry to inform the ocean-scale management of fisheries.

Ecological risk assessment and its application to elasmobranch conservation and management.

Ecological risk assessments (ERAs) are employed to quantify and predict the vulnerability of a particular species, stock or population to a specific stressor, e.g. pollution, harvesting, climate change, by-catch. Data generated from ERAs are used to identify and prioritize species for implementation of effective conservation and management strategies. At this time, ERAs are of particular importance to elasmobranchs, given the ecological importance and documented global population declines of some elasmobranch species. Here, ERAs as a tool for elasmobranch conservation and management are reviewed and a theoretical roadmap provided for future studies. To achieve these goals, a brief history of ERAs and approaches used within them (in the context of elasmobranchs) are given, and a comprehensive review conducted of all ERA studies associated with elasmobranchs published between 1998 and 2011. The hazards assessed, species evaluated and methodological approaches taken are recorded. Chronological and geographical patterns suggest that this tool has grown in popularity as a commercial fishery management instrument, while also signalling a recent precautionary approach to elasmobranch management in commercial fisheries globally. The analysis demonstrates that the predominant parameters incorporated in previous ERAs are largely based on life-history characteristics, and sharks have received the majority of attention; batoids (including skates) have received less attention. Recreational fishing and habitat degradation are discussed as hazards which warrant future investigation through ERA. Lastly, suggestions are made for incorporating descriptive ecological data to aid in the continued development and evolution of this management tool as it applies to future elasmobranch conservation.

Stable isotope analysis as a tool for elasmobranch conservation research: a primer for non-specialists

Many elasmobranch species are undergoing population declines on a global scale; however, implementation of effective conservation and management strategies is hindered, to a large extent, by a lack of sufficient data on diet, life historyandbehaviour.Thisworkisaprimerofhowstableisotopeanalysiscanbeusedasacost-effective,relativelysimple tool for examining resource use patterns (e.g. diet, habitat) and generating relevant data in support of science-based elasmobranch conservation and management. Specifically, isotopes can resolve the feeding niches of elasmobranchs, detect ontogenetic trophic shifts and calculate relative diet breadth of different species. Stable isotope analysis can also be employed to investigate the extent of anthropogenic impacts on diet or to infer the source of toxins affecting elasmobranchs. Additionally, this tool can be used to study migration patterns and habitat usage. Depending on the tissueanalysed,thistechniquecanalsobenon-lethalandminimallyinvasive.Thelimitationsofstableisotopeanalysisare discussed, and recommendations for future work are presented. Additional keywords: dietary analysis, ecotoxicology, food web analysis, individual diet specialisation, migration. Received 24 October 2011, accepted 30 April 2012, published online 2 July 2012

A Comparison of Spatial and Movement Patterns between Sympatric Predators: Bull Sharks (Carcharhinus leucas) and Atlantic Tarpon (Megalops atlanticus)

Background 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. Methodology/Principal Findings 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? Conclusions/Significance 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.

Evolutionary theory as a tool for predicting extinction risk

Timely and proactive wildlife conservation requires strategies for determining which species are most at the greatest threat of extinction. Here, we suggest that evolutionary theory, particularly the concept of specialization, can be a useful tool to inform such assessments and may greatly aid in our ability to predict the vulnerabilities of species to anthropogenic impacts.

Marine predator–prey contests: Ambush and speed versus vigilance and agility

Abstract Differences in relative strengths and weaknesses between predators and prey under tactical contexts result in complex and dynamic contests between them. These contests are often brief and difficult to observe in marine systems. Here, we employ basic principles of underwater optics and physics to provide a conceptual understanding of mechanisms underlying predator–prey interactions between white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus pusillus pusillus) that have been previously described at Seal Island in False Bay, South Africa.

Osmoregulation in elasmobranchs: a review for fish biologists, behaviourists and ecologists

This article provides a broad review of osmoregulation in elasmobranchs for non-specialists, focusing on recent advances. Marine and euryhaline elasmobranchs in seawater regulate urea and other body fluid solutes (trimethylamine oxide (TMAO), Na+, Cl−) such that they remain hyper-osmotic to their environment. Salt secretions of the rectal gland and excretions in the urine compensate for continuous inward diffusion of environmental salts. Freshwater and euryhaline elasmobranchs in fresh water synthesise less urea and retain less urea and other body fluid solutes compared to marine elasmobranchs and thus have relatively lower osmolarity. Electrolyte uptake at the gills and kidney reabsorption of salts maintain acid–base balance and ionic consistency. The role of the gills, kidney, liver and rectal gland in elasmobranch osmoregulation is reviewed. The ontogeny of osmoregulatory systems in elasmobranchs and the contribution of drinking and eating processes in maintaining osmotic consistency are discussed. Recommendations for future research are presented.