Smart, Jonathan J., Chin, Andrew, Tobin, Andrew J., White, William T., Kumasi, Brian, and Simpfendorfer, Colin A. (2017) Stochastic demographic analyses of the silvertip shark (Carcharhinus albimarginatus) and the common blacktip shark (Carcharhinus limbatus) from the Indo-Pacific. Fisheries Research, 191. pp. 95-107.

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Abstract

Information on how shark populations respond to fishing mortality (F) is critical to developing successful management and conservation strategies. However, data on catch, fishing effort and species abundance are often lacking for shark populations – preventing stock assessments from being conducted. Static demographic models circumvent this issue as they only require life history parameters. Age-structured Leslie Matrix models were developed and applied for silvertip shark Carcharhinus albimarginatus and common blacktip shark Carcharhinus limbatus sampled from heavily fished populations in the Indo-Pacific. Stochasticity was introduced to these models by varying vital rates though Monte Carlo simulations. Varying levels of F were introduced to the analyses to determine how both species respond to fishing pressure. Management scenarios were further developed to determine strategies that could facilitate sustainable harvesting. The demographic estimates demonstrated that without fishing both species would have increasing populations (λ = 1.06 yr⁻¹ for C. albimarginatus and 1.05 yr⁻¹ for C. limbatus) until density dependent effects occur. However, both populations would decline when low levels of F (>0.1 yr⁻¹) were applied to all age-classes. The matrix elasticities revealed that changes to fertility elements had little effect on λ, while changes in juvenile survival led to the largest changes. However, age-at-first-capture analysis suggests protecting the juvenile life stage of both species would be an ineffective management strategy as both species mature at old ages. An age-at-last-capture analysis suggests these species could be harvested while maintaining increasing populations through a gauntlet fishery. This required F to be restrained to individuals <100 cm TL while protecting the older age-classes to preserve the breeding stock. This strategy would allow up to 16% and 13% of this size class to be harvested for C. limbatus and C. albimarginatus, respectively, until density dependent effects begin to manifest. However, this strategy depends on the ability to successfully protect all other age-classes from fishing – a strategy that may not be pragmatic in developing nations where little regulation occurs.

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