Quantitative ecological risk assessment of shortfin mako shark (isurus oxyrinchus): proposed model and application example

Abstract

Most of the world's catches of sharks are incidentally taken by tuna fishing gear, constituting bycatches that increase the extinction risk of several species of shark. This not only alters ecosystem functions by removing top predators, but may also hinder the industry production itself due to cutoff measures set by authorities. This paper focuses on the female population abundance of a very important bycatch species, the shortfin mako shark (Isurus oxyrinchus), and proposes a stochastic model for quantitative risk assessment under varying harvest regimes and control measures. The proposed model can be applied to any shortfin mako shark population by changing initial conditions and harvest parameters. The flexibility of the model makes it practicable to simulate hundreds of scenarios, analyse and compare the most relevant results such as the risk of extinction caused by a given harvest regime, median time to extinction, expected minimum biomass, risk of low harvest, and risk reduction caused by control measure; these outcomes are useful for rational decisions under uncertainty. We present an application example as a means to demonstrate how the proposed model can be used to drive management decisions for sustainable tuna harvest.