Habitat suitability of Rhincodon typus in three localities of the Gulf of California: Environmental drivers of seasonal aggregations

The whale shark is an endangered species that usually feeds in coastal areas of highly productive seas such as the Gulf of California, Mexico. This study aims to describe the effect of sea surface temperature, chlorophyll a, bathymetry and slope on the habitat suitability of whale sharks in three important aggregation sites of the Gulf of California. A total of 2396 records of occurrence of whale sharks were obtained from international databases and scientific literature between 1996 and 2018. These records were used for the creation of a species distribution model using MaxEnt for each of the three aggregation sites. The concentration of chlorophyll a explained 71% of the habitat suitability, followed by bathymetry and slope with a combined 17%, and sea surface temperature constituting 10% of the model. Habitat suitability was related to areas where nontargeted fisheries may impact whale sharks through bycatch, entanglement and ship strikes. The implications for the conservation of whale sharks should be considered for management decisions in terms of marine protected areas, fishing refugees or bans, and other regulations regarding fisheries activities.     

Reef sharks exhibit site-fidelity and higher relative abundance in marine reserves on the Mesoamerican Barrier Reef

Carcharhinid sharks can make up a large fraction of the top predators inhabiting tropical marine ecosystems and have declined in many regions due to intense fishing pressure. There is some support for the hypothesis that carcharhinid species that complete their life-cycle within coral reef ecosystems, hereafter referred to as “reef sharks”, are more abundant inside no-take marine reserves due to a reduction in fishing pressure (i.e., they benefit from marine reserves). Key predictions of this hypothesis are that (a) individual reef sharks exhibit high site-fidelity to these protected areas and (b) their relative abundance will generally be higher in these areas compared to fished reefs. To test this hypothesis for the first time in Caribbean coral reef ecosystems we combined acoustic monitoring and baited remote underwater video (BRUV) surveys to measure reef shark site-fidelity and relative abundance, respectively. We focused on the Caribbean reef shark (Carcharhinus perezi), the most common reef shark in the Western Atlantic, at Glover''s Reef Marine Reserve (GRMR), Belize. Acoustically tagged sharks (N = 34) were detected throughout the year at this location and exhibited strong site-fidelity. Shark presence or absence on 200 BRUVs deployed at GRMR and three other sites (another reserve site and two fished reefs) showed that the factor “marine reserve” had a significant positive effect on reef shark presence. We rejected environmental factors or site-environment interactions as predominant drivers of this pattern. These results are consistent with the hypothesis that marine reserves can benefit reef shark populations and we suggest new hypotheses to determine the underlying mechanism(s) involved: reduced fishing mortality or enhanced prey availability.     

Predicting current and future global distributions of whale sharks

The Vulnerable (IUCN) whale shark spans warm and temperate waters around the globe. However, their present‐day and possible future global distribution has never been predicted. Using 30 years (1980–2010) of whale shark observations recorded by tuna purse‐seiners fishing in the Atlantic, Indian and Pacific Oceans, we applied generalized linear mixed‐effects models to test the hypothesis that similar environmental covariates predict whale shark occurrence in all major ocean basins. We derived global predictors from satellite images for chlorophyll a and sea surface temperature, and bathymetric charts for depth, bottom slope and distance to shore. We randomly generated pseudo‐absences within the area covered by the fisheries, and included fishing effort as an offset to account for potential sampling bias. We predicted sea surface temperatures for 2070 using an ensemble of five global circulation models under a no climate‐policy reference scenario, and used these to predict changes in distribution. The full model (excluding standard deviation of sea surface temperature) had the highest relative statistical support (wAIC_c = 0.99) and explained ca. 60% of the deviance. Habitat suitability was mainly driven by spatial variation in bathymetry and sea surface temperature among oceans, although these effects differed slightly among oceans. Predicted changes in sea surface temperature resulted in a slight shift of suitable habitat towards the poles in both the Atlantic and Indian Oceans (ca. 5°N and 3–8°S, respectively) accompanied by an overall range contraction (2.5–7.4% and 1.1–6.3%, respectively). Predicted changes in the Pacific Ocean were small. Assuming that whale shark environmental requirements and human disturbances (i.e. no stabilization of greenhouse gas emissions) remain similar, we show that warming sea surface temperatures might promote a net retreat from current aggregation areas and an overall redistribution of the species.     

Effects of the Pleistocene on the mitochondrial population genetic structure and demographic history of the silky shark (<Emphasis Type="Italic">Carcharhinus falciformis</Emphasis>) in the western Atlantic Ocean

The silky shark, Carcharhinus falciformis, is a large-bodied, oceanic-coastal, epipelagic species found worldwide in tropical and subtropical waters. Despite its commercial importance, concerns about overexploitation, and likely ecological significance of this shark as an upper trophic-level predator, understanding of its population dynamics remains unclear for large parts of its distribution. We investigated the genetic diversity, population structure and demographic history of the silky shark along the western Atlantic Ocean based on the use of 707 bp of the mitochondrial DNA control region (mtCR). A total of 211 silky sharks were sampled, originating from five areas along the western Atlantic Ocean. The mitochondrial sequences revealed 40 haplotypes, with overall haplotype and nucleotide diversities of 0.88 (± 0.012) and 0.005 (± 0.003), respectively. The overall population structure was significantly different among the five western Atlantic Ocean regions. Phylogenetic analysis of mtCR sequences from globally sourced silky shark samples revealed two lineages, comprising a western Atlantic lineage and western Atlantic—Indo-Pacific lineage that diverged during the Pleistocene Epoch. In general, tests for the demographic history of silky sharks supported a population expansion for both the global sample set and the two lineages. Although our results showed that silky sharks have high genetic diversity, the current high level of overexploitation of this species requires long-term, scientifically informed management efforts. We recommend that fishery management and conservation plans be done separately for the two western Atlantic matrilineal populations revealed here.     

To go or not to go with the flow: Environmental influences on whale shark movement patterns

Seven whale sharks were tracked using satellite-linked tags from Ningaloo Reef, off northern Western Australia, following tagging in April and June 2002 and April-May 2005. We investigated how the movements of those whale shark tracks were influenced by geostrophic surface currents during sequential one-week periods by using a passive diffusion model parameterised with observed starting locations of the sharks and weekly maps of surface current velocity and direction (derived from altimetry). We compared the outputs from the passive diffusion model and maps of chlorophyll-a concentration (SeaWiFs/MODIS) and with the actual tracks of the sharks using GIS and generalized linear mixed-effects models (GLMM). The GLMM indicated very little support for passive diffusion with sea-surface ocean currents influencing whale shark distributions in the north eastern Indian Ocean. Moreover, the sharks' movements correlated only weakly with the spatial distribution of sea-surface chlorophyll-a concentrations. The seven whale sharks had average swimming speeds comparable with those recorded in other satellite tracking studies of this species. Swimming speeds of the seven sharks were similar to those reported in previous studies and up to three times greater than the maximum sea-surface current velocities that the sharks encountered while traversing into lower southerly latitudes (moving northward towards the equator). Our results indicate that whale sharks departing from Ningaloo travel actively and independently of near-surface currents where they spend most of their time despite additional metabolic costs of this behaviour.     

Structure and Dynamics of the Shark Assemblage off Recife,Northeastern Brazil

Understanding the ecological factors that regulate elasmobranch abundance in nearshore waters is essential to effectively manage coastal ecosystems and promote conservation. However, little is known about elasmobranch populations in the western South Atlantic Ocean. An 8-year, standardized longline and drumline survey conducted in nearshore waters off Recife, northeastern Brazil, allowed us to describe the shark assemblage and to monitor abundance dynamics using zero-inflated generalized additive models. This region is mostly used by several carcharhinids and one ginglymostomid, but sphyrnids are also present. Blacknose sharks, Carcharhinus acronotus, were mostly mature individuals and declined in abundance throughout the survey, contrasting with nurse sharks, Ginglymostoma cirratum, which proliferated possibly due to this species being prohibited from all harvest since 2004 in this region. Tiger sharks, Galeocerdo cuvier, were mostly juveniles smaller than 200 cm and seem to use nearshore waters off Recife between January and September. No long-term trend in tiger shark abundance was discernible. Spatial distribution was similar in true coastal species (i.e. blacknose and nurse sharks) whereas tiger sharks were most abundant at the middle continental shelf. The sea surface temperature, tidal amplitude, wind direction, water turbidity, and pluviosity were all selected to predict shark abundance off Recife. Interspecific variability in abundance dynamics across spatiotemporal and environmental gradients suggest that the ecological processes regulating shark abundance are generally independent between species, which could add complexity to multi-species fisheries management frameworks. Yet, further research is warranted to ascertain trends at population levels in the South Atlantic Ocean.     

Quantifying Shark Distribution Patterns and Species-Habitat Associations: Implications of Marine Park Zoning

Quantifying shark distribution patterns and species-specific habitat associations in response to geographic and environmental drivers is critical to assessing risk of exposure to fishing, habitat degradation, and the effects of climate change. The present study examined shark distribution patterns, species-habitat associations, and marine reserve use with baited remote underwater video stations (BRUVS) along the entire Great Barrier Reef Marine Park (GBRMP) over a ten year period. Overall, 21 species of sharks from five families and two orders were recorded. Grey reef Carcharhinus amblyrhynchos, silvertip C. albimarginatus, tiger Galeocerdo cuvier, and sliteye Loxodon macrorhinus sharks were the most abundant species (>64% of shark abundances). Multivariate regression trees showed that hard coral cover produced the primary split separating shark assemblages. Four indicator species had consistently higher abundances and contributed to explaining most of the differences in shark assemblages: C. amblyrhynchos, C. albimarginatus, G. cuvier, and whitetip reef Triaenodon obesus sharks. Relative distance along the GBRMP had the greatest influence on shark occurrence and species richness, which increased at both ends of the sampling range (southern and northern sites) relative to intermediate latitudes. Hard coral cover and distance across the shelf were also important predictors of shark distribution. The relative abundance of sharks was significantly higher in non-fished sites, highlighting the conservation value and benefits of the GBRMP zoning. However, our results also showed that hard coral cover had a large effect on the abundance of reef-associated shark species, indicating that coral reef health may be important for the success of marine protected areas. Therefore, understanding shark distribution patterns, species-habitat associations, and the drivers responsible for those patterns is essential for developing sound management and conservation approaches.     

Global pattern of phylogenetic species composition of shark and its conservation priority

The diversity of marine communities is in striking contrast with the diversity of terrestrial communities. In all oceans, species richness is low in tropical areas and high at latitudes between 20 and 40°. While species richness is a primary metric used in conservation and management strategies, it is important to take into account the complex phylogenetic patterns of species compositions within communities. We measured the phylogenetic skew and diversity of shark communities throughout the world. We found that shark communities in tropical seas were highly phylogenetically skewed, whereas temperate sea communities had phylogenetically diversified species compositions. Interestingly, although geographically distant from one another, tropical sea communities were all highly skewed toward requiem sharks (Carcharhinidae), hammerhead sharks (Sphyrnidae), and whale sharks (Rhincodon typus). Worldwide, the greatest phylogenetic evenness in terms of clades was found in the North Sea and coastal regions of countries in temperate zones, such as the United Kingdom, Ireland, southern Australia, and Chile. This study is the first to examine patterns of phylogenetic diversity of shark communities on a global scale. Our findings suggest that when establishing conservation activities, it is important to take full account of phylogenetic patterns of species composition and not solely use species richness as a target. Protecting areas of high phylogenetic diversity in sharks, which were identified in this study, could form a broader strategy for protecting other threatened marine species.     

Ocean‐scale prediction of whale shark distribution

Aim Predicting distribution patterns of whale sharks (Rhincodon typus, Smith 1828) in the open ocean remains elusive owing to few pelagic records. We developed multivariate distribution models of seasonally variant whale shark distributions derived from tuna purse‐seine fishery data. We tested the hypotheses that whale sharks use a narrow temperature range, are more abundant in productive waters and select sites closer to continents than the open ocean. Location Indian Ocean. Methods We compared a 17‐year time series of observations of whale sharks associated with tuna purse‐seine sets with chlorophyll a concentration and sea surface temperature data extracted from satellite images. Different sets of pseudo‐absences based on random distributions, distance to shark locations and tuna catch were generated to account for spatiotemporal variation in sampling effort and probability of detection. We applied generalized linear, spatial mixed‐effects and Maximum Entropy models to predict seasonal variation in habitat suitability and produced maps of distribution. Results The saturated generalized linear models including bathymetric slope, depth, distance to shore, the quadratic of mean sea surface temperature, sea surface temperature variance and chlorophyll a had the highest relative statistical support, with the highest percent deviance explained when using random pseudo‐absences with fixed effect‐only models and the tuna pseudo‐absences with mixed‐effects models (e.g. 58% and 26% in autumn, respectively). Maximum Entropy results suggested that whale sharks responded mainly to variation in depth, chlorophyll a and temperature in all seasons. Bathymetric slope had only a minor influence on the presence. Main conclusions Whale shark habitat suitability in the Indian Ocean is mainly correlated with spatial variation in sea surface temperature. The relative influence of this predictor provides a basis for predicting habitat suitability in the open ocean, possibly giving insights into the migratory behaviour of the world’s largest fish. Our results also provide a baseline for temperature‐dependent predictions of distributional changes in the future.     

Refuges and risks: Evaluating the benefits of an expanded MPA network for mobile apex predators

Aim

Concurrently, assessing the effectiveness of marine protected areas and evaluating the degree of risk from humans to key species provide valuable information that can be integrated into conservation management planning. Tiger sharks (Galeocerdo cuvier) are a wide‐ranging ecologically important species subject to various threats. The aim of this study was to identify “hotspots” of tiger shark habitat use in relation to protected areas and potential risks from fishing.

Location

Southwest Indian Ocean, east coast of South Africa and Mozambique.

Methods

Satellite tags were fitted to 26 tiger sharks. A subset of 19 sharks with an average period at liberty of 197 (SD = 110) days were analysed using hotspot analysis to identify areas of core habitat use. The spatial and temporal overlap of significant hotspots with current and planned marine protected areas as well as risks from fishing and culling was then calculated.

Results

There was a 5.97% spatial overlap between tiger shark hotspots and marine protected areas, which would increase significantly (p < .05) to 24.36% with the expansion of planned protected areas in South Africa and could be as high as 41.43% if Mozambique similarly expanded neighbouring protected area boundaries. Tiger sharks remained largely coastal, but only showed a spatial overlap of 5.12% with shark culling nets in South Africa. Only three sharks undertook open ocean migrations during which they were more likely to interact with longline fisheries in the region.

Main conclusions

This study demonstrates how spatial information can be used to assess the overlap between marine protected areas and the core habitats of top marine predators and highlights how congruent transnational conservation management can improve the effectiveness of protected areas. Core habitat use of marine apex predators may also be indicative of productive habitats, and therefore, predators such as tiger sharks could act as surrogate species for identifying key habitats to prioritize for conservation planning.

     

Shark bycatch and depredation in the U.S. Atlantic pelagic longline fishery

The non-target bycatch of sharks in pelagic longline (PLL) fisheries represents a potential source of compromise to shark populations worldwide. Moreover, shark bycatch and depredation (damage inflicted on gear, bait, and catch) complicates management of sharks and other species, and can undermine the operations and financial interests of the pelagic longline industry. Thus, deducing means to reduce shark interactions is in the best interest of multiple stakeholder groups. Prior to doing so, however, the extent, cause and effect of these interactions must be better understood. In this review we address or conduct the following in relation to the U.S. Atlantic, Gulf of Mexico and Caribbean PLL fishery: (1) U.S. management governing shark interactions in the Atlantic; (2) the primary species encountered and historical shark catch data associated with PLL fishing in the Atlantic; (3) a historical comparison of area-specific shark species catch records between the two primary sources of shark catch data in this fishery; (4) the conditions and dynamics that dictate shark interactions in this fishery, and potential means to reduce these interactions, and; (5) a synthesis of the estimated impacts of this fishery on shark populations relative to other fisheries in the Atlantic. As has been found in other PLL fisheries, the blue shark (Prionace glauca) is clearly the shark species most commonly encountered in this fishery in the Atlantic, and receives the majority of attention in this review. U.S. management areas with high relative shark species diversities had a greater divergence in historical shark species percent-compositions between data sources (Pelagic Observer Program versus mandatory pelagic Logbook databases); this complicates the ability to conclude which species are most impacted by PLL fishing in those areas. The current fishing effort by the U.S. PLL fleet is small compared to that of PLL fishing targeting sharks in the Atlantic by non-U.S. fleets, and therefore poses a comparatively lower threat to the stability of Atlantic shark populations. However, incidental shark encounters are inevitable in U.S. Atlantic PLL fishing operations. Thus, it is in the best interest of all stakeholders in the Atlantic to better understand the extent and conditions governing these interactions, and to explore methods to reduce both their occurrence and those aspects leading to higher rates of incidental shark mortality.     

Population genetic structure of Earth's largest fish, the whale shark (Rhincodon typus)

Large pelagic vertebrates pose special conservation challenges because their movements generally exceed the boundaries of any single jurisdiction. To assess the population structure of whale sharks (Rhincodon typus), we sequenced complete mitochondrial DNA control regions from individuals collected across a global distribution. We observed 51 single site polymorphisms and 8 regions with indels comprising 44 haplotypes in 70 individuals, with high haplotype (h = 0.974 +/- 0.008) and nucleotide diversity (pi = 0.011 +/- 0.006). The control region has the largest length variation yet reported for an elasmobranch (1143-1332 bp). Phylogenetic analyses reveal no geographical clustering of lineages and the most common haplotype was distributed globally. The absence of population structure across the Indian and Pacific basins indicates that oceanic expanses and land barriers in Southeast Asia are not impediments to whale shark dispersal. We did, however, find significant haplotype frequency differences (AMOVA, Phi(ST) = 0.107, P < 0.001) principally between the Atlantic and Indo-Pacific populations. In contrast to other recent surveys of globally distributed sharks, we find much less population subdivision and no evidence for cryptic evolutionary partitions. Discovery of the mating and pupping areas of whale sharks is key to further population genetic studies. The global pattern of shared haplotypes in whale sharks provides a compelling argument for development of broad international approaches for management and conservation of Earth's largest fish.     

Telemetry and Satellite Tracking of Whale Sharks, Rhincodon Typus, in the Sea of Cortez, Mexico, and the North Pacific Ocean

We used satellite-linked radio telemetry to document the geographic and vertical movements and thermal habitats of whale sharks in the Sea of Cortez and as they migrated into the north Pacific Ocean. Of 17 sharks tagged between 1994 and 1996, six dispersed widely in the Sea of Cortez during 12–39 days of tracking. Four others left the Sea of Cortez and ranged extensively in the north Pacific Ocean. Indeed, one whale shark migrated to the western north Pacific Ocean, covering over 13000km in 37 months of tracking. The sharks generally occupied areas where sea surface water temperatures were between 28 and 32°C, though several ranged to depths of 240m or deeper where water temperature reached 10°C or colder. Whale sharks may segregate by size and sex, and their movement patterns appear to be related to oceanographic features, such as sea mounts and boundary currents, where primary productivity may be enhanced. These results have important implications for the global conservation of the world's largest yet least known fish. We think that satellite telemetry is a exceptionally promising tool for learning more about the ecology of whale sharks, especially when combined with conventional methods of telemetry and molecular biology.authorship arranged alphabetically     

Global diversity hotspots and conservation priorities for sharks

Sharks are one of the most threatened groups of marine animals, as high exploitation rates coupled with low resilience to fishing pressure have resulted in population declines worldwide. Designing conservation strategies for this group depends on basic knowledge of the geographic distribution and diversity of known species. So far, this information has been fragmented and incomplete. Here, we have synthesized the first global shark diversity pattern from a new database of published sources, including all 507 species described at present, and have identified hotspots of shark species richness, functional diversity and endemicity from these data. We have evaluated the congruence of these diversity measures and demonstrate their potential use in setting priority areas for shark conservation. Our results show that shark diversity across all species peaks on the continental shelves and at mid-latitudes (30-40 degrees N and S). Global hotspots of species richness, functional diversity and endemicity were found off Japan, Taiwan, the East and West coasts of Australia, Southeast Africa, Southeast Brazil and Southeast USA. Moreover, some areas with low to moderate species richness such as Southern Australia, Angola, North Chile and Western Continental Europe stood out as places of high functional diversity. Finally, species affected by shark finning showed different patterns of diversity, with peaks closer to the Equator and a more oceanic distribution overall. Our results show that the global pattern of shark diversity is uniquely different from land, and other well-studied marine taxa, and may provide guidance for spatial approaches to shark conservation. However, similar to terrestrial ecosystems, protected areas based on hotspots of diversity and endemism alone would provide insufficient means for safeguarding the diverse functional roles that sharks play in marine ecosystems.     

Multi-Year Impacts of Ecotourism on Whale Shark (Rhincodon typus) Visitation at Ningaloo Reef,Western Australia

In-water viewing of sharks by tourists has become a popular and lucrative industry. There is some concern that interactions with tourists with ecotourism operations might harm sharks through disruption of behaviours. Here, we analysed five years of whale shark (Rhincodon typus) encounter data by an ecotourism industry at Ningaloo Reef, Western Australia, to assess the impact of ecotourism interactions on shark visitation, within the context of the biological and physical oceanography of the region. Our data base consisted of 2823 encounter records for 951 individual whale sharks collected by ecotourism operators between 2007 and 2011. We found that total encounters per whale shark and encounters per boat trip increased through time. On average, whale sharks re-encountered in subsequent years were encountered earlier, stayed longer and tended to be encountered more often within a season than sharks that were only encountered in a single year. Sequential comparisons between years did not show any patterns consistent with disturbance and the rate of departure of whale sharks from the aggregation was negatively correlated to the number of operator trips. Overall, our analysis of this multi-year data base found no evidence that interactions with tourists affected the likelihood of whale shark re-encounters and that instead, physical and biological environmental factors had a far greater influence on whale shark visitation rates. Our approach provides a template for assessing the effects of ecotourism interactions and environmental factors on the visitation patterns of marine megafauna over multiple years.     

Population structure,connectivity, and demographic history of an apex marine predator,the bull shark Carcharhinus leucas

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 microsatellite 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 mitochondrial DNA. Integrating information from both types of markers and using Bayesian computation with a random forest procedure (ABC‐RF), this discordance was found to be due to a complete lack of contemporary gene flow. High genetic connectivity was found both within the Western Indian Ocean and within the Western Pacific. In conclusion, these results suggest important structuring of bull shark populations globally with important gene flow occurring along coastlines, highlighting the need for management and conservation plans on regional scales rather than oceanic basin scale.     

Ancient Divergence in the Trans-Oceanic Deep-Sea Shark Centroscymnus crepidater

Unravelling the genetic structure and phylogeographic patterns of deep-sea sharks is particularly challenging given the inherent difficulty in obtaining samples. The deep-sea shark Centroscymnus crepidater is a medium-sized benthopelagic species that exhibits a circumglobal distribution occurring both in the Atlantic and Indo-Pacific Oceans. Contrary to the wealth of phylogeographic studies focused on coastal sharks, the genetic structure of bathyal species remains largely unexplored. We used a fragment of the mitochondrial DNA control region, and microsatellite data, to examine genetic structure in C. crepidater collected from the Atlantic Ocean, Tasman Sea, and southern Pacific Ocean (Chatham Rise). Two deeply divergent (3.1%) mtDNA clades were recovered, with one clade including both Atlantic and Pacific specimens, and the other composed of Atlantic samples with a single specimen from the Pacific (Chatham Rise). Bayesian analyses estimated this splitting in the Miocene at about 15 million years ago. The ancestral C. crepidater lineage was probably widely distributed in the Atlantic and Indo-Pacific Oceans. The oceanic cooling observed during the Miocene due to an Antarctic glaciation and the Tethys closure caused changes in environmental conditions that presumably restricted gene flow between basins. Fluctuations in food resources in the Southern Ocean might have promoted the dispersal of C. crepidater throughout the northern Atlantic where habitat conditions were more suitable during the Miocene. The significant genetic structure revealed by microsatellite data suggests the existence of present-day barriers to gene flow between the Atlantic and Pacific populations most likely due to the influence of the Agulhas Current retroflection on prey movements.     

Defining distribution and habitat use of west‐central Florida’s coastal sharks through a research and education program

Identifying critical habitat for highly mobile species such as sharks is difficult, but essential for effective management and conservation. In regions where baseline data are lacking, non‐traditional data sources have the potential to increase observational capacity for species distribution and habitat studies. In this study, a research and education organization conducted a 5‐year (2013–2018) survey of shark populations in the coastal waters of west‐central Florida, an area where a diverse shark assemblage has been observed but no formal population analyses have been conducted. The objectives of this study were to use boosted regression tree (BRT) modeling to quantify environmental factors impacting the distribution of the shark assemblage, create species distribution maps from the model outputs, and identify spatially explicit hot spots of high shark abundance. A total of 1036 sharks were captured, encompassing eleven species. Abundance hot spots for four species and for immature sharks (collectively) were most often located in areas designated as “No Internal Combustion Engine” zones and seagrass bottom cover, suggesting these environments may be fostering more diverse and abundant populations. The BRT models were fitted for immature sharks and five species where n > 100: the nurse shark (Ginglymostoma cirratum), blacktip shark (Carcharhinus limbatus), blacknose shark (C. acronotus), Atlantic sharpnose shark (Rhizoprionodon terraenovae), and bonnethead (Sphyrna tiburo). Capture data were paired with environmental variables: depth (m), sea surface temperature (°C), surface, middle, and bottom salinity (psu), dissolved oxygen (mg/L), and bottom type (seagrass, artificial reef, or sand). Depth, temperature, and bottom type were most frequently identified as predictors with the greatest marginal effect on shark distribution, underscoring the importance of nearshore seagrass and barrier island habitats to the shark assemblage in this region. This approach demonstrates the potential contribution of unconventional science to effective management and conservation of coastal sharks.     

Spatial dynamics and expanded vertical niche of blue sharks in oceanographic fronts reveal habitat targets for conservation

Dramatic population declines among species of pelagic shark as a result of overfishing have been reported, with some species now at a fraction of their historical biomass. Advanced telemetry techniques enable tracking of spatial dynamics and behaviour, providing fundamental information on habitat preferences of threatened species to aid conservation. We tracked movements of the highest pelagic fisheries by-catch species, the blue shark Prionace glauca, in the North-east Atlantic using pop-off satellite-linked archival tags to determine the degree of space use linked to habitat and to examine vertical niche. Overall, blue sharks moved south-west of tagging sites (English Channel; southern Portugal), exhibiting pronounced site fidelity correlated with localized productive frontal areas, with estimated space-use patterns being significantly different from that of random walks. Tracked female sharks displayed behavioural variability in diel depth preferences, both within and between individuals. Diel depth use ranged from normal DVM (nDVM; dawn descent, dusk ascent), to reverse DVM (rDVM; dawn ascent, dusk descent), to behavioural patterns where no diel differences were apparent. Results showed that blue sharks occupy some of the most productive marine zones for extended periods and structure diel activity patterns across multiple spatio-temporal scales in response to particular habitat types. In so doing, sharks occupied an extraordinarily broad vertical depth range for their size (1.0-2.0 m fork length), from the surface into the bathypelagic realm (max. dive depth, 1160 m). The space-use patterns of blue sharks indicated they spend much of the time in areas where pelagic longlining activities are often highest, and in depth zones where these fisheries particularly target other species, which could account for the rapid declines recently reported for blue sharks in many parts of the world's oceans. Our results provide habitat targets for blue shark conservation that may also be relevant to other pelagic species.     

White Sharks (Carcharodon carcharias) Scavenging on Whales and Its Potential Role in Further Shaping the Ecology of an Apex Predator

Scavenging, a result of a temporary pulse of resources, occurs in virtually all ecosystems containing carnivores, and is an important energy transfer pathway that can impact ecosystem structure and function, and this ecological significance has largely been considered from a terrestrial standpoint; however, little is known about the role of scavenging in shaping the behavioral ecology of marine species, specifically apex predators. Here we present findings from multiple opportunistic observations of white sharks scavenging on whale carcasses in False Bay, South Africa. Observations of white sharks scavenging over successive days provided evidence of strategic and selective scavenging by this species. Moreover, extended daily observations permitted recordings of unique social, aggregative, and feeding behaviors. We further compare these data against observations of natural predation by sharks on seals in the study area. We discuss these data in relation to environmental conditions, shark social interactions, migration patterns, whale biology, and behaviorally-mediated trophic cascades. While the appearance of a whale carcass is largely a stochastic event, we propose that white shark scavenging on whales may represent an underestimated, yet significant component to the overall foraging ecology of this species, especially as individuals attain sexual maturity.