Stock assessment of Blue Shark in the Indian Ocean

Rice J (2021) Stock assessment of Blue Shark in the Indian Ocean. In: IOTC- 17th Working Party on Ecosystems & Bycatch (Assessment). IOTC-2021-WPEB17(AS)-15_rev1, Online

This paper presents a stock assessment of blue shark in the Indian Ocean using Stock Synthesis (version The blue shark assessment model is an age structured (25 years), spatially aggregated (1 region) and two sex model. The catch, effort, and size composition of catch, are grouped into 8 fisheries covering the time period from 1950 through 2019. Six indices of abundance, all from longline fisheries were considered for this analysis. This assessment considered two alternative time series of total catch. The diagnostic case model is parameterized using indices of abundance from the Portugal (2000-2019), Reunion (2007-2019) and the Japanese late (1992-2019) series, along with estimates of catch generated via a generalized additive model. The estimated abundance trend is decreasing throughout the time frame of the model, and spawning stock abundance has decreased to approximately 1.21 times SSBMSY, (80% CI is 1.08-1.36). The fishing mortality has increased over the model time frame with F2019/FMSY= 0.81 (80% CI =0.66 to 0.96). Blue sharks are most often caught as bycatch in the Indian Ocean tuna fisheries, though some directed mixed species (sharks and tunas/billfish) fisheries do exist. Commercial reporting of landings has been minimal, as has information regarding the targeting and fate of blue sharks encountered in the fisheries. Useful data on catch and effort is mostly limited to recent years, and time series of historical catches have been estimated based on reported and observed catch rates, as well as observed ratios of blue shark to target species. This analysis was developed as an assessment model that included the Portuguese, EU-France (Reunion) and Japanese late CPUE series, with the estimates of total catch based on generalized additive model based GAM series as the diagnostic case, as it is referred to in the main text when presenting the model parametrization and diagnostics. The upcoming 17th meeting of the Indian Ocean Tuna Commission Working Party on Ecosystems and Bycatch (WPEB17) will recommend the final parameterization as a base case model for the provision of stock status. Initial analysis based on the sensitivity analysis done with SS3 indicated that the stock is not over fished nor experiencing overfishing. Changes made to this August 29 draft. 1) Three annexes have been added to the analysis, Annex 1 Additional model diagnostics, Annex 2, Control file specifications, and Annex 3. Stock assessment of Indian Ocean blue shark using JABBA. 2) The text has been updated to reflect the addition of these Annexes.