Investigation into the effects of catch time series estimations on stock assessment of Silky shark (Carcharhinus falciformis) in the Indian Ocean | IOTC

Reported catches of sharks to the IOTC are likely highly inaccurate due to insufficient species-specific reporting. This creates difficulties in conducting quantitative stock assessments in the region. Silky sharks are one of the most abundant shark species caught in Indian Ocean and are ranked as one of the most vulnerable species in the region, putting their populations at high risk of being overfished. Following the preliminary stock assessment of Indian Ocean silky sharks in 2018 (uncertain status), this study proposes novel estimated catch time series for silky sharks and investigates their effects on stock assessment results using the data-limited CMSY model. It was found that estimated values of resilience (r) and F-based statistics were considerably different when using different r categories. The two reconstructions varied most in k, MSY and BMSY, while FMSY, B/k, B/BMSY and F remained relatively constant across all reconstructions within the same r categories. F/FMSY, however, remained consistent across all scenarios. Overall, the input resilience category has a larger influence on the output of the model than the estimated catch time series. All 4 assessments showed that overfishing (F/FMSY > 1) is occurring and that the stock is more likely to be overfished when a low resilience category is used, compared to very low. Therefore, adopting a precautionary approach and introducing specific management measures for silky sharks in the Indian Ocean is highly encouraged.