A Preliminary Stock Assessment for the Shortfin Mako Shark in the Indian Ocean Using Data-Limited Approaches
Despite its importance as a by-catch species and high biological vulnerability, there are currently no quantitative stock assessments for the shortfin mako, Isurus oxyrinchus, in the Indian Ocean. A quantitative stock assessment has been planned by the IOTC-WPEB for 2020. The aim of this paper is to provide a preliminary stock assessment and status for this stock, namely by providing 1) a catch series reconstruction between 2917-2015, 2) standardized CPUEs for EU longline fleets (Spain and Portugal), 3) estimation of a prior distribution for intrinsic growth rate (r) from demographic models, and 4) provide preliminary stock status using data-limited methods. Both a catch-only model (CMSY) and a Bayesian Schaefer production model were tested. The preliminary results indicate that the exploitation rate exceeded msy levels since the 1990s, with current F estimated to be well above Fmsy (F2015/Fmsy = 2.57). The relative biomass (B/Bmsy) is predicted to have had a continuously decreasing trend over time, and is estimated to be currently (B2015/Bmsy) close to 1. This indicates that currently the shortfin mako in the Indian Ocean is subject to overfishing but not overfished; however with trajectories showing consistent trends towards the overfished and subject to overfishing status. A reduction to fishing mortality levels observed during the early 1990s (around 3,000t) would likely be sustainable. However, and given the levels of uncertainty in the estimations, using the lower 95% confidence limit of MSY (1,570t) could serve as a more conservative guidance for total allowable catches. Due to the preliminary nature of this work and considerable amount of uncertainty in the estimations, management advice at this stage is not clear. However, this work it could serve as a baseline for more robust and complete analysis in the 2020 SMA IOTC stock assessment.