Explore the vast range of titles available.

Despite global declines of apex predatory sharks, evidence for ecosystem consequences remains limited and debated. This is likely a result of both the logistical difficulties of measuring such processes in marine systems and also due to shifting baselines, whereby the ecosystem changes have occurred prior to monitoring. Between 2000–2018, we conducted standardized monitoring of white shark ( Carcharodon carcharias ) abundance patterns (N = 6,333 shark sightings) and predatory activity (N = 8,076 attacks on seals) at Seal Island, a Cape fur seal ( Arctocephalus pusillus pusillus ) colony in False Bay, South Africa. Over the 18-year study, declines in white shark abundance and attack rates were documented between 2015–2018, with anomalous lows occurring in 2017 and 2018. This included prolonged periods of complete white shark absence from Seal Island. The disappearance of white sharks from Seal Island coincided with the unprecedented appearance of sevengill sharks ( Notorynchus cepedianus ; N = 120 sightings), an otherwise allopatric kelp-associated apex predator in False Bay. We also recorded a sevengill shark attacking a live seal in the absence of white sharks. These data provide empirical evidence for behavioral shifts in an allopatric marine predator following the decline and disappearance of white sharks from a foraging site. This study demonstrates the importance of historical data and long-term monitoring for disentangling ecological consequences of apex predator declines.

The nursery habitats of the broadnose sevengill shark Notorynchus cepedianus (Péron, 1807) remain largely unknown. The identification of these critical habitats is important for the conservation of these top-down regulator species and general coastal ecosystem dynamics. We investigated the role of the San Antonio cape (Argentina) as a nursery habitat for N. cepedianus and tested the hypothesis that physical conditions (turbidity, sea surface temperature) are more important than prey abundance in the habitat use by young-of-the-year (YOY). We used the presence, relative abundance, and size structure of YOY from a small-scale gillnet fishery (2008–2014) to assess the intra- and inter-annual persistence in habitat use. The results indicated intra- and inter-annual persistence in estuarine habitat use, and comparisons with previous studies suggested that YOYs were more commonly encountered in San Antonio cape than in other suitable areas. Finally, the YOYs’ use of the area may be explained by a combination of the spatial–temporal variation in turbidity conditions and food abundance. These results suggested San Antonio cape as a nursery habitat for N. cepedianus YOYs and highlighted the importance of understanding key biophysical effects that regulate the use, movement, and residency of N. cepedianus in their nursery habitats.

Killer whales (Orcinus orca) have been documented to prey on white sharks (Carcharodon carcharias), in some cases causing localised shark displacement and triggering ecological cascades. Notably, a series of such predation events have been reported from South Africa over the last decade, with killer whales specifically targeting sharks' liver. However, observations of these interactions are rare, and knowledge of their frequency across the world's oceans remains limited. In October 2023, a 4.7 m (total length) white shark carcass washed ashore in southeastern Australia, coinciding with reports from citizen scientists of killer whales hunting a large, unidentified prey item in the area. Visual inspection of the carcass revealed that the liver, digestive, and reproductive organs were missing, and the presence of four distinctive bite wounds, one of which was characteristic of killer whale liver extraction as seen in South Africa. Genomic analyses performed on swabs taken from the bite wounds confirmed the presence of killer whale DNA in the major bite area, while the other bites were embedded with genetic material from the scavenging broadnose sevengill shark (Notorynchus cepedianus). These results provide confirmed evidence of killer whale predation on white sharks in Australia and the likely selective consumption of the liver, suggesting predations of this nature are more globally prevalent than currently assumed. Using a combination of wildlife forensics and citizen science, we provide the first evidence of killer whale predation on Australian white sharks, targeting the liver. This is the first time a predation event of this nature has been recorded outside of South Africa, suggesting they are perhaps more globally prevalent than currently assumed.

The availability of prey in an environment does not ensure that a predator will consume it: prey must also be detected, captured, and successfully handled. The morphology of the predator and prey imposes limitations on prey selection due to biomechanical constraints, making some prey functionally inaccessible. Morphological factors, including but not limited to tooth shape, body size, and mouth gape, therefore impose constraints on predator trophic niches. We assessed how two important components of trophic morphology (tooth shape and body length) may influence prey selectivity and trophic niche in two large‐bodied sympatric sharks with contrasting foraging strategies. The first species captures prey using spear‐shaped, grasping teeth (grey nurse/sand tiger/raggedtooth shark, Carcharias taurus), while the second has multi‐cuspid cutting teeth used to serrate larger prey (sevengill shark, Notorynchus cepedianus). Stomach content analysis and isotopic values of δ13C, δ15N, and δ34S from muscle and liver were used to characterize isotopic niche and prey selection. As gape‐limited grey nurse sharks grew, their consumption of teleosts decreased inversely to chondrichthyans. By contrast, non‐gape limited sevengill sharks consumed teleosts and chondrichthyans in similar proportions, along with marine mammals, but with no clear relationship to body size. As body length increased, both species consumed prey from higher trophic levels (higher δ15N values), but sevengill sharks accessed prey at relatively higher trophic levels. Values of δ13C and δ34S remained relatively unchanged with body length presumably because mouth gape and dentition do not limit access to pelagic or benthic food webs. Although many other morphological factors, such as swim performance, biomechanics, or behavior, could also drive the results we observed, it is clear that morphological characteristics play an important role in prey selection and may be the primary mechanism facilitating resource partitioning in large sympatric predators. Their inclusion in ecological studies will help understand prey choice and how it shapes trophodynamics in marine ecosystems. RESUMEN La disponibilidad de presas en un ambiente no garantiza su consumo por parte de un depredador; la presa también debe ser detectada, capturada y manipulada con éxito. La morfología tanto del depredador como de la presa impone restricciones biomecánicas que pueden hacer que algunas presas sean funcionalmente inaccesibles y, por tanto, afectar la selección de presas. Factores morfológicos, tales como la forma de los dientes, el tamaño corporal, la apertura bucal, entre otros, imponen restricciones a los nichos tróficos de los depredadores. En este estudio se evaluó cómo dos importantes componentes de la morfología trófica (forma de los dientes y longitud corporal) pueden influir en la selectividad de presas y en el nicho trófico de dos especies simpátricas de tiburones de gran tamaño, con estrategias alimentarias contrastantes. La primera especie, el tiburón sarda (Carcharias taurus), presenta dientes largos, estrechos y puntiagudos, adaptados para capturar y tragar presas enteras; mientras que la segunda, el tiburón pintarroja (Notorynchus cepedianus), posee dientes multicúspides cortantes, especializados en despedazar presas de mayor tamaño. Se utilizaron análisis de contenido estomacal y valores isotópicos de δ13C, δ15N y δ34S en músculo e hígado para caracterizar el nicho trófico y la selección de presas en ambas especies. Conforme el tiburón sarda crece, limitado por el tamaño de su apertura bucal, su consumo de teleósteos disminuye en proporción inversa al de condrictios. En contraste, el tiburón pintarroja, al no tener limitaciones asociadas al tamaño de su abertura bucal, consume teleósteos y condrictios en proporciones similares, además de mamíferos marinos, aunque sin una relación clara con su tamaño corporal. En ambas especies, el aumento de la longitud corporal se asocia con el consumo de presas de niveles tróficos más altos (reflejado en mayores valores de δ15N), aunque el tiburón pintarroja accede a presas de niveles tróficos relativamente superiores. Los valores de δ13C y δ34S se mantienen relativamente invariables con la longitud corporal, presumiblemente porque el tamaño de la apertura bucal y la dentición no limitan el acceso a redes alimenticias pelágicas o bentónicas. Si bien otros factores morfológicos, como aquellos que afectan el rendimiento del nado, la biomecánica o el comportamiento, también podrían influir en los resultados observados, está claro que ciertas características morfológicas desempeñan un papel importante en la selección de presas y pueden constituir el principal mecanismo que facilita la repartición de recursos en grandes predadores simpátricos. La inclusión de estos caracteres en estudios ecológicos ayuda a comprender la selección de presas y cómo ello moldea la trofodinámica en los ecosistemas marinos. Predator morphology imposes limitations on prey selection due to biomechanical constraints, making some prey functionally inaccessible and thereby constraining predator trophic niches. We assessed how two key components of trophic morphology—tooth shape and body size—affect prey selectivity and trophic niche in two sympatric sharks with contrasting foraging strategies: the gape‐limited grey nurse shark (Carcharias taurus) and the non‐gape‐limited sevengill shark (Notorynchus cepedianus). Our findings demonstrate that predator morphology plays a significant role in prey selection and may be the primary mechanism facilitating resource partitioning in sympatric species and influencing trophodynamics in marine ecosystems.

Fil: Trobbiani, Gastón Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina

The broadnose sevengill shark, Notorynchus cepedianus, a common coastal species in the eastern North Pacific, was sampled during routine capture and tagging operations conducted from 2005-2012. One hundred and thirty three biopsy samples were taken during these research operations in Willapa Bay, Washington and in San Francisco Bay, California. Genotypic data from seven polymorphic microsatellites (derived from the related sixgill shark, Hexanchus griseus) were used to describe N. cepedianus genetic diversity, population structure and relatedness. Diversity within N. cepedianus was found to be low to moderate with an average observed heterozygosity of 0.41, expected heterozygosity of 0.53, and an average of 5.1 alleles per microsatellite locus. There was no evidence of a recent population bottleneck based on genetic data. Analyses of genetic differences between the two sampled estuaries suggest two distinct populations with some genetic mixing of sharks sampled during 2005-2006. Relatedness within sampled populations was high, with percent relatedness among sharks caught in the same area indicating 42.30% first-order relative relationships (full or half siblings). Estuary-specific familial relationships suggest that management of N. cepedianus on the U.S. West Coast should incorporate stock-specific management goals to conserve this ecologically important predator.

During the reproductive season, sea turtles use a restricted area in the vicinity of their nesting beaches, making them vulnerable to predation. At Raine Island (Australia), the highest density green turtle Chelonia mydas rookery in the world, tiger sharks Galeocerdo cuvier have been observed to feed on green turtles, and it has been suggested that they may specialise on such air-breathing prey. However there is little information with which to examine this hypothesis. We compared the spatial and temporal components of movement behaviour of these two potentially interacting species in order to provide insight into the predator-prey relationship. Specifically, we tested the hypothesis that tiger shark movements are more concentrated at Raine Island during the green turtle nesting season than outside the turtle nesting season when turtles are not concentrated at Raine Island. Turtles showed area-restricted search behaviour around Raine Island for ∼3-4 months during the nesting period (November-February). This was followed by direct movement (transit) to putative foraging grounds mostly in the Torres Straight where they switched to area-restricted search mode again, and remained resident for the remainder of the deployment (53-304 days). In contrast, tiger sharks displayed high spatial and temporal variation in movement behaviour which was not closely linked to the movement behaviour of green turtles or recognised turtle foraging grounds. On average, tiger sharks were concentrated around Raine Island throughout the year. While information on diet is required to determine whether tiger sharks are turtle specialists our results support the hypothesis that they target this predictable and plentiful prey during turtle nesting season, but they might not focus on this less predictable food source outside the nesting season.

The vital parameter data for 62 stocks, covering 38 species, collected from the literature, including parameters of age, growth, and reproduction, were log-transformed and analyzed using multivariate analyses. Three groups were identified and empirical equations were developed for each to describe the relationships between the predicted finite rates of population increase (λ') and the vital parameters, maximum age (Tmax), age at maturity (Tm), annual fecundity (f/Rc)), size at birth (Lb), size at maturity (Lm), and asymptotic length (L∞). Group (1) included species with slow growth rates (0.034 yr(-1) < k < 0.103 yr(-1)) and extended longevity (26 yr < Tmax < 81 yr), e.g., shortfin mako Isurus oxyrinchus, dusky shark Carcharhinus obscurus, etc.; Group (2) included species with fast growth rates (0.103 yr(-1) < k < 0.358 yr(-1)) and short longevity (9 yr < Tmax < 26 yr), e.g., starspotted smoothhound Mustelus manazo, gray smoothhound M. californicus, etc.; Group (3) included late maturing species (Lm/L∞ ≧ 0.75) with moderate longevity (Tmax < 29 yr), e.g., pelagic thresher Alopias pelagicus, sevengill shark Notorynchus cepedianus. The empirical equation for all data pooled was also developed. The λ' values estimated by these empirical equations showed good agreement with those calculated using conventional demographic analysis. The predictability was further validated by an independent data set of three species. The empirical equations developed in this study not only reduce the uncertainties in estimation but also account for the difference in life history among groups. This method therefore provides an efficient and effective approach to the implementation of precautionary shark management measures.

Cosmopolitan marine pelagic species display variable patterns of population connectivity among the world’s major oceans. While this information is crucial for informing management, information is lacking for many ecologically important species, including apex predators. In this study we examine patterns of genetic structure in the broadnose sevengill shark, Notorynchus cepedianus across its global distribution. We estimate patterns of connectivity among broadnose sevengill shark populations from three major oceanic regions (South Atlantic, Oceania and Eastern Pacific) by contrasting mitochondrial and nuclear DNA haplotype frequencies. We also produced time calibrated Bayesian Inference phylogenetic reconstructions to analyses global phylogeographic patterns and estimate divergence times among distinctive shark lineages. Our results demonstrate significant genetic differentiation among oceanic regions (ΦST = 0.9789, P < 0.0001) and a lack of genetic structuring within regions (ΦST = − 0.007; P = 0.479). Time calibrated Bayesian Inference phylogenetic reconstructions indicate that the observed patterns of genetic structure among oceanic regions are historical, with regional populations estimated to have diverged from a common ancestor during the early to mid-Pleistocene. Our results indicate significant genetic structuring and a lack of gene flow among broadnose sevengill shark populations from the South Atlantic, Oceania and Eastern Pacific regions. Evidence of deep lineage divergences coinciding with the early to mid-Pleistocene suggests historical glacial cycling has contributed to the vicariant divergence of broadnose sevengill shark populations from different ocean basins. These finding will help inform global management of broadnose sevengill shark populations, and provides new insights into historical and contemporary evolutionary processes shaping populations of this ecologically important apex predator.

Diving with sharks, often in combination with food baiting/provisioning, has become an important product of today's recreational dive industry. Whereas the effects baiting/provisioning has on the behaviour and abundance of individual shark species are starting to become known, there is an almost complete lack of equivalent data from multi-species shark diving sites. In this study, changes in species composition and relative abundances were determined at the Shark Reef Marine Reserve, a multi-species shark feeding site in Fiji. Using direct observation sampling methods, eight species of sharks (bull shark Carcharhinus leucas, grey reef shark Carcharhinus amblyrhynchos, whitetip reef shark Triaenodon obesus, blacktip reef shark Carcharhinus melanopterus, tawny nurse shark Nebrius ferrugineus, silvertip shark Carcharhinus albimarginatus, sicklefin lemon shark Negaprion acutidens, and tiger shark Galeocerdo cuvier) displayed inter-annual site fidelity between 2003 and 2012. Encounter rates and/or relative abundances of some species changed over time, overall resulting in more individuals (mostly C. leucas) of fewer species being encountered on average on shark feeding dives at the end of the study period. Differences in shark community composition between the years 2004-2006 and 2007-2012 were evident, mostly because N. ferrugineus, C. albimarginatus and N. acutidens were much more abundant in 2004-2006 and very rare in the period of 2007-2012. Two explanations are offered for the observed changes in relative abundances over time, namely inter-specific interactions and operator-specific feeding protocols. Both, possibly in combination, are suggested to be important determinants of species composition and encounter rates, and relative abundances at this shark provisioning site in Fiji. This study, which includes the most species from a spatially confined shark provisioning site to date, suggests that long-term provisioning may result in competitive exclusion among shark species.