An integrated mark-recapture and genetic approach to estimate the population size of white sharks in South Africa

Andreotti, Sara; Rutzen, Michael; Walt, Stéfan van der; Heyden, Sophie von der; Henriques, Romina; Meÿer, Michael A.; Oosthuizen, Herman; Matthee, Conrad A.

doi:10.3354/meps11744

Cited by 40 publications

(38 citation statements)

References 86 publications

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“…These spatiotemporal patterns of white shark abundance have also been found through region‐wide survey data of white sharks through standardized boat‐based baited surveys as part of a project to estimate population sizes of white sharks in South Africa (Andreotti , Andreotti et al. ).…”

Section: Methodsmentioning

confidence: 62%

“…, Andreotti et al. ). At these locations, there also exists clear seasonal variation in predation risk, as sharks only actively hunt Cape fur seals during winter months (Hammerschlag et al.…”

Section: Introductionmentioning

confidence: 98%

“…Here, the population of Cape fur seals in the region is segregated into discrete colonies that inhabit different inshore islands; but the seals from the different island colonies move offshore to feed where they share the same general foraging grounds (Rand 1959, 1967, Oosthuizen 1991. White sharks only actively target certain seal colonies, creating a natural spatial variation in seal colonies to threat of attack (Bonfil et al 2005, Dudley 2012, De Vos et al 2015a, Andreotti et al 2016. At these locations, there also exists clear seasonal variation in predation risk, as sharks only actively hunt Cape fur A D B C FIG.…”

Section: Introductionmentioning

confidence: 99%

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Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals

Hammerschlag

Meÿer²,

Seakamela³

et al. 2017

Ecology

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Predators can impact ecosystems through consumptive or risk effects on prey. Physiologically, risk effects can be mediated by energetic mechanisms or stress responses. The predation-stress hypothesis predicts that risk induces stress in prey, which can affect survival and reproduction. However, empirical support for this hypothesis is both mixed and limited, and the conditions that cause predation risk to induce stress responses in some cases, but not others, remain unclear. Unusually clear-cut variation in exposure of Cape fur seals (Arctocephalus pusillus pusillus) to predation risk from white sharks (Carcharodon carcharias) in the waters of Southwestern Africa provides an opportunity to test the predation-stress hypothesis in the wild. Here, we measured fecal glucocorticoid concentrations (fGCM) from Cape fur seals at six discrete islands colonies exposed to spatiotemporal variation in predation risk from white sharks over a period of three years. We found highly elevated fGCM concentrations in seals at colonies exposed to high levels of unpredictable and relatively uncontrollable risk of shark attack, but not at colonies where seals were either not exposed to shark predation or could proactively mitigate their risk through antipredatory behavior. Differences in measured fGCM levels were consistent with patterns of risk at the site and seasonal level, for both seal adults and juveniles. Seal fGCM levels were not correlated with colony population size, density, and geographic location. Investigation at a high risk site (False Bay) also revealed strong correlations between fGCM levels and temporal variation in shark attack rates, but not with shark relative abundance. Our results suggest that predation risk will induce a stress response when risk cannot be predicted and/or proactively mitigated by behavioral responses.

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Section: Methodsmentioning

confidence: 62%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals

Hammerschlag

Meÿer²,

Seakamela³

et al. 2017

Ecology

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“…Likewise, Andreotti et al . () showed that CMR estimates (438, 95% c.i . = 353–522) of adult white sharks Carcharodon carcharias (L. 1758) on the coast of South Africa affectively matched N e estimates (333, 95% c.i .…”

Section: Problems With Estimates Of Genetic Effective Population Sizementioning

confidence: 99%

“…As time goes by, studies are appearing where the ratio between estimates of N e and census size (adult abundance, N ) approach unity, many of which involve elasmobranch species (Portnoy et al , ; Dudgeon & Ovenden, ; Andreotti et al , ) and ratios like this appear to be backed up by theoretical studies (Waples et al , ). This contrasts to previous studies that report orders of magnitude difference between the two parameters (Palstra & Ruzzante, ).…”

Section: Introductionmentioning

confidence: 99%

Can estimates of genetic effective population size contribute to fisheries stock assessments?

Ovenden

Leigh²,

Blower

et al. 2016

Journal of Fish Biology

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Sustainable exploitation of fisheries populations is challenging to achieve when the size of the population prior to exploitation and the actual numbers removed over time and across fishing zones are not clearly known. Quantitative fisheries' modeling is able to address this problem, but accurate and reliable model outcomes depend on high quality input data. Much of this information is obtained through the operation of the fishery under consideration, but while this seems appropriate, biases may occur. For example, poorly quantified changes in fishing methods that increase catch rates can erroneously suggest that the overall population size is increasing. Hence, the incorporation of estimates of abundance derived from independent data sources is preferable. We review and evaluate a fisheries-independent method of indexing population size; inferring adult abundance from estimates of the genetic effective size of a population (N ). Recent studies of elasmobranch species have shown correspondence between N and ecologically determined estimates of the population size (N). Simulation studies have flagged the possibility that the range of N /N ratios across species may be more restricted than previously thought, and also show that declines in N track declines in the abundance of model fisheries species. These key developments bring this new technology closer to implementation in fisheries science, particularly for data-poor fisheries or species of conservation interest.

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

Maroen,

Rowlinson,

Matthee

et al. 2024

Aquatic Conservation

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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.

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An integrated mark-recapture and genetic approach to estimate the population size of white sharks in South Africa

Cited by 40 publications

References 86 publications

Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals

Physiological stress responses to natural variation in predation risk: evidence from white sharks and seals

Can estimates of genetic effective population size contribute to fisheries stock assessments?

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

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