Individual Sharks Research Articles

Pacific nurse sharks Ginglymostoma unami exhibit yearly patterns of site fidelity to a tropical embayment subjected to seasonal upwelling

Seasonal upwelling in tropical coastal regions can create dynamic oceanographic conditions similar to temperate systems, which are known to influence the distribution and movement of marine species. Mobile marine species may seek warmer environments (i.e. exhibit behavioral thermoregulation) in response to stronger winds during the upwelling season, which cause cold, nutrient-rich waters to rise and cool shallower environments. Pacific nurse sharks Ginglymostoma unami are present in the upwelling regions of the Eastern Tropical Pacific coastline, serving as an ideal model species to study the effects of changing water temperatures on movement behavior. In this study, we analyzed 4 yr of passive acoustic telemetry data to quantify the movement, residency, and habitat use of G. unami in Santa Elena Bay, north Pacific coast of Costa Rica, located within the Papagayo upwelling system. A total of 28 sharks (15 males and 13 females) ranging from 85 to 256 cm total length were monitored inside the bay. The mean (±SD) residency index (i.e. number of days detected relative to the number of days monitored) of individual sharks was 37 ± 28%. Generalized additive mixed models revealed seasonal and diel patterns of G. unami use of Santa Elena Bay that were associated with water temperature and wind speed changes during the upwelling season. G. unami likely uses the warmer waters of Santa Elena Bay as a thermal refuge during the upwelling season to avoid colder adjacent waters. This research will serve as a baseline to understand the movement behavior of this species as climate change alters upwelling patterns.

First Evidence of Individual Sharks Involved in Multiple Predatory Bites on People

ABSTRACTIt is widely accepted that populations of terrestrial predators sometimes contain “problem individuals” that repeatedly attack humans, yet this phenomenon has never been demonstrated in sharks. Here, we present photographic and genetic evidence of individuals in populations of tiger Galeocerdo cuvier and oceanic whitetip Carcharhinus longimanus sharks that (1) demonstrated atypical behavior compared to the rest of the population, (2) engaged in repeated agonistic behavior directed toward humans, and (3) bit, or attempted to bite humans in probable foraging attempts. These case studies provide some of the first evidence for the existence of “problem individuals” among sharks. The percentage of fatalities due to the same shark individual are not known, so we recommend systematic swabbing of shark bite victims wounds to better understand the importance of this phenomenon and the possibility of identifying these animals. Environmentally conscientious management options for problem individuals range from prohibiting ocean activities (e.g., swimming and surfing) in their habitats to selectively removing the individual, although the latter would be challenging in the marine environment.

Assessing the potential of acoustic telemetry to underpin the regional management of basking sharks (Cetorhinus maximus)

Acoustic telemetry can provide valuable space-use data for a range of marine species. Yet the deployment of species-specific arrays over vast areas to gather data on highly migratory vertebrates poses formidable challenges, often rendering it impractical. To address this issue, we pioneered the use of acoustic telemetry on basking sharks (Cetorhinus maximus) to test the feasibility of using broadscale, multi-project acoustic receiver arrays to track the movements of this species of high conservation concern through the coastal waters of Ireland, Northern Ireland, and Scotland. Throughout 2021 and 2022, we tagged 35 basking sharks with acoustic transmitters off the west coast of Ireland; 27 of these were detected by 96 receiver stations throughout the study area (n = 9 arrays) with up to 216 detections of an individual shark (mean = 84, s.d. 65). On average, sharks spent ~ 1 day at each acoustic array, with discrete residency periods of up to nine days. Twenty-one sharks were detected at multiple arrays with evidence of inter-annual site fidelity, with the same individuals returning to the same locations in Ireland and Scotland over 2 years. Eight pairs of sharks were detected within 24 h of each other at consecutive arrays, suggesting some level of social coordination and synchronised movement. These findings demonstrate how multi-project acoustic telemetry can support international, cost-effective monitoring of basking sharks and other highly mobile species. Decision support tools such as these can consolidate cross-border management strategies, but to achieve this goal, collaborative efforts across jurisdictions are necessary to establish the required infrastructure and secure ongoing support.

The effectiveness of the semi‐automated Identifin software to accurately fast‐track dorsal fin photographic identifications of sharks

Abstract For some marine species, photo‐identification (photo‐ID) data can be used to generate population estimates critical for conservation efforts. For the white shark (Carcharadon carcharias), different photo‐ID methods have been proposed: (1) the traditional manual matching method based on non‐organized databases, (2) the notches code method based on organized databases and (3) the semi‐automatic software, Identifin. However, the effectiveness of these different methods and the applicability to other shark species have not been adequately explored. Eleven inexperienced and two experienced investigators used the three different photo‐ID methods to record the accuracy and time taken to identify individual white sharks from photo‐ID images. Both the traditional and notches code methods allowed for the accurate matching of individuals to the databases provided. For all investigators, the notches code method used significantly less time (14 min ± 0.14 min) than the traditional matching method (19 min ± 1.34 min). The semi‐automated method, Identifin, further shortened the identification time to 2 min (± 0.10 min) and provided a significantly higher matching accuracy (96% correct matches) compared with traditional manual matching (78% correct matches) and the notches code method (70% correct matches). The use of Identifin was tested to identify bronze whaler sharks (Carcharhinus brachyurus), a species with finer notches patterns when compared with white sharks. Identifin showed less efficiency in matching individual photographs to the database, but when very‐high‐quality (VHQ) photographs were available, matching improved substantially. Identifin software shows great potential in assisting researchers to accurately identify individual sharks in large databases, and this can contribute significantly to the conservation of shark species. The software and automated process need to be adapted to be functional on a variety of different shark species, but it proves to be an effective tool in managing large photo‐ID databases.

Individual residency behaviours and seasonal long-distance movements in acoustically tagged Caribbean reef sharks in the Cayman Islands.

Understanding how reef-associated sharks use coastal waters through their ontogeny is important for their effective conservation and management. This study used the horizontal movements of acoustically tagged Caribbean reef sharks (Carcharhinus perezi) to examine their use of coastal space around the Cayman Islands between 2009 and 2019. A total of 39 (59.1%) tagged sharks (male = 22, female = 17, immature = 18, mature = 21) were detected on the islands wide network of acoustic receivers. The detection data were used to calculate values of Residency Index (RI), Site-Fidelity Index (SFI) and minimum linear displacement (MLD), as well as for network analysis of individual shark movements to test for differences between demographics, seasons, and diel periods. Sharks were detected for up to 1,598 days post-tagging and some individuals showed resident behaviour but the majority of tagged individuals appear to have been one-off or only occasional transient visitors to the area. Generally, individuals showed strong site-fidelity to different areas displaying linear home ranges of < 20 km. The evidence indicates that there was no pattern of diel behaviour. Tagged sharks generally showed increased movements within and between islands during the summer (April-September), which may be related to breeding activity. Some individuals even made occasional excursions across 110 km of open water > 2,000 m deep between Grand Cayman and Little Cayman. One mature female shark showed a displacement of 148.21 km, the greatest distance reported for this species. The data shows that the distances over which some sharks moved, greatly exceeded the extent of any one of the islands' marine protected areas indicating that this species may be more mobile and dispersive than previously thought. This study provides support for the blanket protection to all sharks throughout Cayman waters, which was incorporated within the National Conservation Act in 2015.

Examining individual behavioural variation in wild adult bull sharks (Carcharhinus leucas) suggests divergent personalities

Abstract Although many animal species demonstrate individual personalities, studying these traits in wild sharks has proven challenging. Past research focused mainly on captive or juvenile sharks. Our ethological study of 31 wild adult bull sharks (Carcharhinus leucas) utilized an artificial provisioning site, amassing 2813 observations over 651 dives from October 2015 to January 2018 (27 months). Behavioural traits, including boldness-shyness and aggressiveness-placidity, were assessed using an ad hoc ethogram and an influencing factors table. This innovative approach not only allowed us to characterize individual shark behaviours but also to quantify their changes over time. Our findings suggest that adult bull sharks likely possess distinct personalities, spanning from extreme shyness to pronounced boldness, with varying levels of plasticity among individuals. Further exploration of shark personalities holds promise for advancing our comprehension of human–shark interactions and refining the management of potential aggressive behaviours exhibited by large shark species toward humans.

First estimates of population size and home range of Caribbean reef and nurse sharks using photo-identification and BRUVS

The assessment of parameters population size and individual home range is important for effective conservation management of sharks. This study uses the novel application of photo identification (photo-ID) to BRUVS footage as a non-invasive alternative to tagging in order to generate individual capture histories. These were used in mark-recapture models to estimate effective population sizes and to determine home ranges. In the Cayman Islands a total of 499 shark sightings of six coastal shark species were recorded on BRUVS from 2015 - 2018, but re-sighting rates were only sufficient for the determination of population parameters for two species - Caribbean reef shark (Carcharhinus perezi) and nurse shark (Ginglymostoma cirratum). The calculated super-population sizes for Caribbean reef shark (180 ± 37 SE) and nurse shark (336 ± 61 SE) were greater than the estimates for each species based on a closed-population model (Caribbean reef shark: 128 ± 40 SE, nurse shark: 249 ± 48 SE), though both measures indicated that there were about twice as many nurse sharks (1.3 - 1.8 sharks/km2) as Caribbean reef sharks (0.7 – 1 shark/km2) within the study area. The demographic compositions included numerous immature individuals, indicating that breeding of both species takes place within the study area of 188 km2. Most recognizable individuals of both species showed linear home ranges of &amp;lt;20 km, but a few individuals were observed to have moved longer distances (Caribbean reef shark: 125.37 km, nurse shark: 156.07 km). The data indicate that the home ranges and long-distance movements of individual sharks observed within the islands’ marine protected areas (MPAs) often extend to areas beyond the MPA’s boundary, potentially exposing them to fishing activities. This study provides the first estimates of population size for Caribbean reef and nurse sharks in the Cayman Islands and the first estimate of a Caribbean reef shark population globally.

Mercury and selenium concentrations and their toxicological implications in silky sharks Carcharhinus falciformis (Elasmobranchii: Chondrichthyes) in the northwestern Indian Ocean

Mercury (Hg) and selenium (Se) were assessed in silky sharks, Carcharhinus falciformis, a carnivorous apex predator and an important resource in the Indian Ocean. Muscle from the caudal peduncle was collected from individual sharks and analyzed using ICP-MS. Results showed that 40% of the specimens exceeded the allowed limits (1.00μg/g ww) of Hg. The mean Hg concentration (1.39 ±1.44μg/g ww) was also higher than the limits. The mean Se concentration in shark muscle was 0.324 ±1.44μg/g ww. The mean Se/Hg molar ratio was 2.23 (range: 0.17–24.5), with 30% of individuals exceeding 1.0, showing a protective effect of Se against Hg. However, for individual sharks’ mean Se health benefit value (Se-HBV) was −3.83. The risks of consuming silky sharks by male and female adults were also calculated. Results showed that the estimated weekly intake (EWI) was 2.9 to 3.5 times higher than the provisional tolerable weekly intake (PTWI). Consumption risks based on hazard quotient (HQ), estimated daily intakes (EDI), and EWI exceeded the threshold for fish consumption by adults, posing immediate health concerns.

Widespread diversity deficits of coral reef sharks and rays.

A global survey of coral reefs reveals that overfishing is driving resident shark species toward extinction, causing diversity deficits in reef elasmobranch (shark and ray) assemblages. Our species-level analysis revealed global declines of 60 to 73% for five common resident reef shark species and that individual shark species were not detected at 34 to 47% of surveyed reefs. As reefs become more shark-depleted, rays begin to dominate assemblages. Shark-dominated assemblages persist in wealthy nations with strong governance and in highly protected areas, whereas poverty, weak governance, and a lack of shark management are associated with depauperate assemblages mainly composed of rays. Without action to address these diversity deficits, loss of ecological function and ecosystem services will increasingly affect human communities.

Novel Method for the Extraction of DNA from Lamniformes Tooth and Denticle Enamel Suitable for PCR

Extraction of high-quality genomic DNA from Lamniformes tooth fragment and dermal denticle enamel is discussed as a method of identifying individual sharks. We describe a procedure that permits isolation of genomic DNA of satisfactory size and quality for PCR analysis, as well as for most routine cloning applications. This method should allow for the non-invasive collection of genomic samples from Lamniformes.

Integrating isotopic and nutritional niches reveals multiple dimensions of individual diet specialisation in a marine apex predator.

Dietary specialisations are important determinants of ecological structure, particularly in species with high per-capita trophic influence like marine apex predators. These species are, however, among the most challenging in which to establish spatiotemporally integrated diets. We introduce a novel integration of stable isotopes with a multidimensional nutritional niche framework that addresses the challenges of establishing spatiotemporally integrated nutritional niches in wild populations, and apply the framework to explore individual diet specialisation in a marine apex predator, the white shark Carcharodon carcharias. Sequential tooth files were sampled from juvenile white sharks to establish individual isotopic (δ-space; δ13 C, δ15 N, δ34 S) niche specialisation. Bayesian mixing models were then used to reveal individual-level prey (p-space) specialisation, and further combined with nutritional geometry models to quantify the nutritional (N-space) dimensions of individual specialisation, and their relationships to prey use. Isotopic and mixing model analyses indicated juvenile white sharks as individual specialists within a broader, generalist, population niche. Individual sharks differed in their consumption of several important mesopredator species, which suggested among-individual variance in trophic roles in either pelagic or benthic food webs. However, variation in nutrient intakes was small and not consistently correlated with differences in prey use, suggesting white sharks as nutritional specialists and that individuals could use functionally and nutritionally different prey as complementary means to achieve a common nutritional goal. We identify how degrees of individual specialisation can differ between niche spaces (δ-, p- or N-space), the physiological and ecological implications of this, and argue that integrating nutrition can provide stronger, mechanistic links between diet specialisation and its intrinsic (fitness/performance) and extrinsic (ecological) outcomes. Our time-integrated framework is adaptable for examining the nutritional consequences and drivers of food use variation at the individual, population or species level.

Elasmobranchs of the western Arabian Gulf: Diversity, status, and implications for conservation

In spite of the ecological services provided by elasmobranchs, their diversity and populations are significantly declining even before appropriate assessments are conducted. This paper presents information on elasmobranch diversity in the Saudi waters of the Arabian Gulf based on fishery-independent and dependent surveys. A total of 369 individual sharks and batoids were collected from 119 out of 228 trawl stations surveyed between 2013 and 2016. Gymnura poecilura and Carcharhinus dussumieri were the most dominant batoid and shark species, respectively. The catch per unit area indicated the waters around Jana Island as a hotspot of elasmobranchs. A total of 135 surveys at the landing sites and fish markets from 2016 to 2020 showed that 88% of elasmobranchs (out of 4,055 individuals recorded) were caught by gill nets. Sharks were the most abundant (> 80 %) with three dominant species: Carcharhinus sorrah, C. humani, and C. limbatus. In total, 47 species of elasmobranchs (24 sharks and 23 batoids) belonging to 16 families and 5 orders were recorded from a possible 58 total species predicted by species richness extrapolators (Chao 1). High values of Margalef richness (> 2) and Shannon–Wiener index (3–4) suggested rich diversity of elasmobranchs in the study area with homogeneous distribution over the years and seasons as shown by cluster and similarity profile analysis. Of the 47 species recorded, six species were Critically Endangered regionally, six Endangered, and seven species Vulnerable according to the IUCN Red List of Threatened Species, necessitating proper management and conservation measures.

Acoustic detection of the Greenland shark (Somniosus microcephalus) using multifrequency split beam echosounder in Svalbard waters

The conservation status of the long-lived Greenland shark (Somniosus microcephalus) is unknown, and methods for non-invasively estimating local or regional abundances are wanted. Using a multifrequency split beam echosounder during long line fishery for Greenland sharks, we demonstrate how individual sharks can be identified and separated with high accuracy from the acoustic backscattering at three frequencies. From a slowly drifting vessel, hooked and free-swimming targets with similar target characteristics were identified on echograms along the bottom and in the water column over a depth of ∼ 400 m. A discriminate analysis using target frequency response, target strength and echo pulse stretching was used to identify shark targets from the only possible competitor, with respect to target strength, being large Atlantic cod (Gadus morhua). Isolation and tracking of single targets allowed us to calculate the swimming speeds ranging between 0.16 and 0.84 m s−1 (mean ± SD, 0.47 ± 0.18 m s−1) from 15 free-swimming sharks over distances from 11 and 50 m in time periods of 28 to 122 s per track. Our findings demonstrate the first acoustic detection of Greenland sharks, and thus provide a new non-invasive monitoring method applicable in otherwise difficult-to-access arctic and deep-sea waters.

Microplastics in Demersal Sharks From the Southeast Indian Coastal Region

Microplastic (MPs) contamination has emerged as a serious worldwide issue. Human activity, commercial enterprises, and fishing are concentrated around the seashore, causing high levels of MPs contamination in coastal and marine organisms. When it comes to their vulnerability to MPs ingestion, sharks are least studied organism. The objective of this study is to investigate MPs accumulation in sharks collected from the Southeast Indian coastal zone (Bay of Bengal). We present evidence of MPs ingestion in demersal sharks caught by the trawlers during trawling operations in marine waters beyond a depth of 80 m in the Southeast India coast. Shark samples were also checked for any gender or size differences in contaminant loading. Gill and gut (digestive tract) were examined in 40 sharks and 82.5% of samples contained at least one MP particle. The average number of MP particles was found to be 4.67 items per individual shark; the gastrointestinal tract showed more MPs than the gills. The majority of the MPs were blue and pale white followed by black and transparent particles with diameters ranging from 0.5 to 2 mm. The fibre fragments were prevalent in the intestines of the shark. Fourier Transform Infrared (FT-IR) spectroscopy revealed that the bulk of polymers were polypropylene (PP), polyacrylamides (PA), and polyethylene (PE). MPs contamination poses an unknown level of harm to shark species. The present study revealed the first scientific data of MPs and associated fibre ingestion in shark species in their habitat in the Bay of Bengal.

Evaluation of the shark fisheries along the Coastline of Ghana, West Africa

The decline in traditionally valuable fish stocks, coupled with an increased demand for shark fins and meat has caused many fishermen to target sharks. However, there is limited information on the fishing practices, gears, and catch distribution for the shark fisheries. This study used a semi-structured interview to characterize the shark fisheries and techniques used to catch sharks as well as the catch statistics and trade in fifteen coastal communities along the Eastern, Central, and Western coasts of Ghana. Out of the 470 fishers interviewed, 46% were specialized shark fishers while the remaining 54% landed sharks as bycatch. Specialized shark fishers captured between 7–10 individual sharks per fishing trip while by-catch shark fishers captured between 1–5 sharks. The shark species harvested include the blue shark, thresher shark, common and sand tiger, bull shark, short and longfin mako shark, hammerhead, and milk shark. Fishers along the eastern coast mostly capture sharks as bycatch, while those along the central and western coast of Ghana target sharks with specialized fishing gears. The catch data obtained from the fishers indicated higher shark landings and was linked to the development of longlines and drift gill nets with hooks attached to the footrope that target sharks, and are usually deployed along the western coastlines of Ghana. Nearly all the shark species captured in the study communities were listed by the IUCN as either “Endangered”, “Critically Endangered” or “Vulnerable”. Among fishers who target sharks, 74% stated that shark populations were declining and were spending more effort to capture the fish. Specialized shark fishers indicated a poor perception and attitude towards shark conservation along the entire coastline of the country and are not in support of shark conservation. It is recommended that the capture and trade in these species should be closely monitored to prevent the extinction of vulnerable populations.

Sensational shark skin and dazzling dermal denticles

Shark skin has a unique structure which has served as a source of inspiration for engineers interested in reducing drag in water or preventing parasites from becoming attached. The skin is covered in tooth-like structures called dermal denticles, made of an enamel-type tissue and dentine on the outermost portion and containing a central cavity filled with pulp. Their general shape is similar to a pointy canine tooth where the base is wider than the edge and is thought to reduce the amount of drag pulling on the animal when moving through water. Interestingly, previous work suggests a wide range of denticle shapes on the skin around the gills of dogfish, where the denticles closer to the head are rounded and smooth compared with the ridged and sharp denticles toward the tail. But does this diversity in denticle shape near the gills also occur in other shark species? And what purpose might these different shapes serve?To study these questions, Molly Gabler-Smith and George Lauder from Harvard University, USA, Dylan Wainwright from Yale University, USA, and colleagues from Harvard University compared the shape and size of the denticles around the gills of 13 shark species with various lifestyles. They aimed to better understand whether the shape of these denticles differs between species and whether these differences might help water to flow over the gills.The researchers collected the sharks from fishing surveys and museums, ranging from the thresher shark, known for its massive tail, enabling it to jump out of water like a dolphin, to the spiny dogfish, which are smaller and known to live in deep sea beds. They then took 3D measurements of denticle length and width and used electron and light microscopy to look at differences in denticle shape across the skin around the gills of individual sharks and between different species.Comparing the structures, it turned out that the denticle shape varied across different areas of the skin surrounding the gills in all of the species, with the short broad denticles arranged toward the front of the shark and longer, pointier denticles occurring toward to the tail of the shark. In fact, the denticles found closer to the tail had pronounced ridges on their surfaces and three spiky spines, in contrast to the smoother almond-shaped denticles found towards the head. Some species, such as the chain catshark, had extreme changes in denticle shape over a distance of a few millimeters. They also found that the shark species that continuously swim to breathe had larger denticles toward the head compared with shark species that breathe by opening and closing their mouths to pump water over the gills.The extreme differences in shape between the denticles found towards the head versus those toward the tail may serve to reduce friction and prevent damage to the skin when the gills flap against each other while breathing. The dramatic shape difference may also reduce drag from water moving through the gills, which could prevent water from flowing in the opposite direction, stopping the shark from getting enough oxygen from the water. This study gives us a foundation for future research on how denticle shape can help sharks to breathe and if there are differences in how water moves over the denticles surrounding the gills of active or passively breathing species. Understanding the great diversity and function of denticles in shark skin can help us to learn how they evolved and even allow us to better imitate their amazing ability to reduce drag in water.

Size structure of broadnose sevengill sharks (Notorynchus cepedianus) in Sawdust Bay, Rakiura/Stewart Island, estimated using underwater stereo-photogrammetry

ABSTRACT The broadnose sevengill shark (Notorynchus cepedianus) is a large marine predator found in temperate coastal marine habitats. Despite being commonly encountered, details of its ecology are limited. To partially address this, we quantified the size and sex structure of N. cepedianus in Paterson Inlet, Stewart Island, New Zealand. A stereo-camera baited underwater video system (BRUV) was deployed in a shallow coastal embayment to capture underwater video of N. cepedianus for stereo photogrammetric measurement between June 2016 and November 2017. Individual sharks were sexed via observation of claspers. Three-dimensional underwater calibration of the photogrammetric system, using a precisely measured cube, indicated a mean error of <1%. Estimated total lengths ranged from 149.0 to 276.5 cm (n = 78 individuals, mean = 211.4, S.E. = 2.97). The observed sex ratio was heavily biased, with females making up 95% of the population in winter 2016 and 73% in summer 2017. This study provides new data on N. cepedianus in southern New Zealand as well a non-invasive, low-cost methodology that can be applied to other study areas.

Individual haplotyping of whale sharks from seawater environmental DNA.

Population genetic data can provide valuable information on the demography of a species. For rare and elusive marine megafauna, samples for generating the data are traditionally obtained from tissue biopsies, which can be logistically difficult and expensive to collect and require invasive sampling techniques. Analysis of environmental DNA (eDNA) offers an alternative, minimally invasive approach to provide important genetic information. Although eDNA approaches have been studied extensively for species detection and biodiversity monitoring in metabarcoding studies, the potential for the technique to address population-level questions remains largely unexplored. Here, we applied "eDNA haplotyping" to obtain estimates of the intraspecific genetic diversity of a whale shark (Rhincodon typus) aggregation at Ningaloo reef, Australia. Over 2weeks, we collected seawater samples directly behind individual sharks prior to taking a tissue biopsy sample from the same animal. Our data showed a 100% match between mtDNA sequences recovered in the eDNA and tissue sample for all 28 individuals sampled. In the seawater samples, >97% of all reads were assigned to six dominant haplotypes, and a clear dominant signal (~99% of sample reads) was recovered in each sample. Our study demonstrates accurate individual-level haplotyping from seawater eDNA. When DNA from one individual clearly dominates each eDNA sample, it provides many of the same opportunities for population genetic analyses as a tissue sample, potentially removing the need for tissue sampling. Our results show that eDNA approaches for population-level analyses have the potential to supply critical demographic data for the conservation and management of marine megafauna.

Effects of Food Provisioning on the Daily Ration and Dive Site Use of Great Hammerhead Sharks, Sphyrna mokarran

Wildlife provisioning is popular, economically valuable, and a rapidly growing part of marine tourism, with great potential to benefit conservation. However, it remains controversial due to limited understanding of its implications on the behavior and ecology of target species. In this study, we modeled how various abiotic and biotic factors influenced great hammerhead sharks’ (Sphyrna mokarran) use of a recreational dive site in Bimini, the Bahamas, where shark-feeding has been conducted since 2012. Further, we calculated bioenergetic models to estimate their daily ration and examined if individual sharks fulfilled their daily energetic requirements from food uptake during dives. Between December 2016 and May 2017, we collected data during 104 provisioning dives in collaboration with a local dive operator. Twenty-eight individual great hammerhead sharks were observed, 11 were philopatric (i.e., identified at the dive site in previous years), and 17 were new (i.e., identified at the dive site for the first time during this study) individuals. On average, four sharks were observed daily, occasionally up to nine individuals, with some individuals spending more than 2 h attending each dive, consuming up to 4.75 kg of provisioned food per dive and returning repeatedly throughout the study period. When we grouped sharks based on their previous experience of the dive site (i.e., philopatric vs. new sharks), we found significantly higher attendance indices, i.e., the number of attended dives divided by the total number dives, and longer presence times during dives in philopatric sharks and different responses toward the number of boats and conspecifics between the two groups. Overall, great hammerhead sharks increased their bait uptake during longer dives and when more boats were present at the dive site. Finally, nine of 12 provisioned great hammerhead sharks were regularly able to fuel their daily energetic requirements from provisioned food alone, with two sharks doing so on 77.8% of all dives. Our study provides insights into how large-bodied marine predators react toward wildlife tourism associated provisioning and allows further discussion about daily energy uptake during provisioning dives, its potential impacts on the ecological role of the target species and associated management measures.

Telemetry‐validated nitrogen stable isotope clocks identify ocean‐to‐estuarine habitat shifts in mobile organisms

Abstract Throughout their life history, many animals transition among heterogeneous environments to facilitate behaviours such as reproduction, foraging and predator avoidance. The dynamic environmental and biological conditions experienced by mobile species are integrated in the chemical composition of their tissues, providing retrospective insight into movement. Here, we present a unique application of nitrogen stable isotope clocks (‘isotopic clocks’), which integrate tissue turnover rates, consumer stable isotope ratios and habitat‐specific isotope baselines to predict time‐since‐immigration and the timing of habitat shifts in a migratory species. Nitrogen stable isotope values of blood plasma collected from juvenile sand tiger sharks Carcharias taurus, a species known to undertake seasonal movements between ocean and estuarine environments, were used to derive estimates of time‐since‐immigration and the timing of seasonal habitat shifts undertaken by this species. Nitrogen isotopic clocks estimated for 65 juvenile sand tiger sharks sampled across 6 years indicated that individual sharks predominantly arrived to estuarine habitats between June and July, with some individuals arriving as early as mid‐May. These estimates were validated by comparing isotope‐derived estuarine arrival times with those from acoustically tracked individuals. The median estuarine arrival day estimates from our isotopic approach aligned with estimates from acoustic telemetry for each sampling population. Sensitivity analyses indicated that isotopically inferred time‐since‐immigration and estuarine arrival estimates were robust to variation in isotopic turnover rate and diet tissue discrimination factors under multiple modelling scenarios. This suggests that parameterization of the nitrogen isotopic clock provides reliable estimates of time‐since‐immigration and day of arrival into new habitats if isotopic variation exists between origin and new locations. Our study presents a unique application of telemetry‐validated isotope clocks to derive retrospective estimates of time‐since‐immigration and timing of habitat shifts for animals that seasonally traverse heterogeneous environments. This approach can be readily applied across many temporal and spatial scales, and to other species and ecosystems, to facilitate rapid assessment of changes in animal habitat use and broader ecosystem structure.