Calculation of dietary niche characteristics using stable isotopes has become a popular approach to understand the functional role of taxa across food webs. An underlying assumption of this approach ...is that stable isotopes accurately reflect the dietary breadth of a species over a temporal duration defined by tissue-specific isotopic turnover rates. In theory, dietary niche estimates derived from fast turnover rate tissues (e.g., blood plasma and liver) may augment stomach content-derived estimates more agreeably than slower turnover rate tissues (e.g., muscle or fin). We tested this hypothesis by comparing commonly used dietary niche estimates derived from stomach contents (nicheSCA: Levins’, Shannon–Wiener’s, and Smith’s), with those estimated using stable isotopes nicheSIA: standard ellipse area (SEA), convex hull total area (TA), theta (θ), and ellipse eccentricity (E) of liver and muscle tissue. Model species were three large-bodied sharks: white (Carcharodon carcharias), dusky (Carcharhinus obscurus), and scalloped hammerhead (Sphyrna lewini). Within-technique comparisons for nicheSCA and nicheSIA metrics (i.e., SEA vs. TA) were often correlated; however, we did not observe any statistically significant correlations between nicheSCA and liver/muscle tissue nicheSIA (i.e., Levins’ vs. SEA). We conclude that nicheSCA and nicheSIA do not provide comparable estimates of dietary niche, at least for the three predator species examined. This fundamental discrepancy highlights technique-specific limitations to estimating organismal dietary niche and identifies a need for the use of clearly defined niche metrics, i.e., the standardized use and reporting of the term isotopic niche as proposed by Newsome et al. (Front Ecol Environ 5:429–436, 2007). Finally, further investigation into the factors underpinning nicheSIA is required to better contextualize this popular ecological metric when compared to nicheSCA.
A recent Letter to the Editor by Gennari et al. (2024) contends that methodological issues and data uncertainties may be obscuring declines in abundance of the white shark population in South Africa ...in the analyses of Bowlby et al. (2023). We have addressed their critiques using scientifically accepted analytical understanding to demonstrate why our ecological conclusions remain unchanged: (1) the relative abundance of white sharks has not exhibited systematic increases or declines at a regional level since protection in 1991, and (2) observed data on human-shark incidents are consistent with the hypothesis that white sharks have partially redistributed along the South African coastline. Future long-term, standardized monitoring throughout South Africa would be expected to substantially reduce uncertainty about the population trend and status of white sharks. Ultimately, the lack of abundance increase following protection remains concerning and necessitates continued conservation efforts.
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•Non-traditional monitoring data allow evaluation of abundance and distribution trends.•White sharks’ regional status remains stable despite localized decline.•Human-shark incidents ...demonstrate a shift eastwards from traditional aggregation sites.•Future monitoring would benefit from improved standardization to track population status.•Understanding ecological drivers and mortality sources is crucial.
Unprecedented levels of change in ocean ecosystems bring an ever-increasing need for re-analyses of existing data to explore pressing conservation questions. Substantial declines in white shark (Carcharodon carcharias) presence at two primary aggregation sites have raised concerns about the species’ status throughout South Africa. Using the most comprehensive suite of abundance indices compiled to date, we evaluated temporal trends and the strength of evidence for regional redistribution. Individual indices from all primary aggregation sites in South Africa were highly variable. The overall temporal trend from a log-linear Generalized Additive Model was relatively flat, indicating largely unchanged abundance throughout South Africa since protection in 1991. However, reports of human-shark incidents showed a general shift from the Western to the Eastern Cape. Correlations among individual abundance indices demonstrated that movements were not as simple as animals leaving one site to inhabit another. Further research is needed to explore the effect of movement on monitoring data. Our results reaffirm the need for better standardization in data collection methods to generate abundance indices and to develop long-term monitoring programs on the Eastern Cape. Ideally, environmental or operational factors affecting abundance indices should also be explored in future status assessments at a regional level. Our results provide a baseline for future work, directing research to understand drivers of localized and regional changes and focusing management on reducing anthropogenic sources of mortality within their Southwest Indian Ocean range.
The cosmopolitan lamniform shark
Carcharias taurus
(commonly known as the ragged-tooth, grey nurse or sand tiger shark) is threatened by overexploitation in parts of its range. Return migrations of ...females to specific nursery areas suggest that females exhibit reproductive philopatry, a behaviour that over time might lead to genetically isolated subpopulations over various spatial scales. To investigate genetic evidence for reproductive philopatry, genetic data from mitochondrial and microsatellite markers were generated for 104 young-of-the-year and juvenile sharks. Comparing the smallest versus the largest young sharks revealed a pattern of size-related differentiation between nurseries that was only found in the smaller size class. This not only confirms reproductive philopatry of their mothers, but is also in line with previous observations of larger juvenile sharks increasing their migration range and moving between sites. Our results highlight the need to target young-of-the-year sharks when investigating reproductive philopatry to exclude roaming individuals that obscure size-related signals of genetic differentiation. Given the species’ high susceptibility to overexploitation, the evidence for reproductive philopatry is of direct importance to the management and conservation of
C. taurus
worldwide. As many nursery areas as possible should be protected to ensure that the number of locally resident juveniles and the pool of the returning females remain stable in the long term. This may warrant protected areas, or time-area closures, prohibiting exploitation in the nursery areas during pupping season.
Although shark teeth are abundant in the fossil record, their bodies are rarely preserved. Thus, our understanding of the anatomy of the extinct
Otodus megalodon
remains rudimentary. We used an ...exceptionally well-preserved fossil to create the first three-dimensional model of the body of this giant shark and used it to infer its movement and feeding ecology. We estimate that an adult
O. megalodon
could cruise at faster absolute speeds than any shark species today and fully consume prey the size of modern apex predators. A dietary preference for large prey potentially enabled
O. megalodon
to minimize competition and provided a constant source of energy to fuel prolonged migrations without further feeding. Together, our results suggest that
O. megalodon
played an important ecological role as a transoceanic superpredator. Hence, its extinction likely had large impacts on global nutrient transfer and trophic food webs.
Otodus megalodon
3D model suggests unprecedented swimming and prey intake abilities, and potential global ecological impacts.
Andreotti et al. (2016; Mar Ecol Prog Ser 552:241–253) estimate an abundance (N) of 438 white sharks Carcharodon carcharias and a contemporary effective population size (CNₑ) of 333 individuals along ...the South African coast. N was estimated by using a mark-recapture analysis of photographic identification records from a single aggregation site (Gansbaai). CNₑ was calculated based on the levels of pairwise linkage disequilibrium of genetic material collected from 4 aggregation sites across approximately 965 km of South African coastline. However, due to the complex stock structure of white sharks and the model assumptions made by Andreotti et al. (2016), the conclusions drawn cannot be supported by their methods and data.
White sharks, Carcharodon carcharias, are often described as elusive, with little information available due to the logistical difficulties of studying large marine predators that make long-distance ...migrations across ocean basins. Increased understanding of aggregation patterns, combined with recent advances in technology have, however, facilitated a new breadth of studies revealing fresh insights into the biology and ecology of white sharks. Although we may no longer be able to refer to the white shark as a little-known, elusive species, there remain numerous key questions that warrant investigation and research focus. Although white sharks have separate populations, they seemingly share similar biological and ecological traits across their global distribution. Yet, white shark’s behaviour and migratory patterns can widely differ, which makes formalising similarities across its distribution challenging. Prioritisation of research questions is important to maximise limited resources because white sharks are naturally low in abundance and play important regulatory roles in the ecosystem. Here, we consulted 43 white shark experts to identify these issues. The questions listed and developed here provide a global road map for future research on white sharks to advance progress towards key goals that are informed by the needs of the research community and resource managers.
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