South Atlantic blue shark stock assessment 1971-2021 using stock synthesis

This document describes the development of the South Atlantic blue shark (Prionace glauca) stock assessment using Stock Synthesis. The model runs from 1971 to 2021 and was fit to length composition by sex for eight fleets and four abundance indices. Life-history parameters were sexspecific with values based on the ICCAT Shark Working Group recommendations. Steepness and natural mortality at age were fixed at independently estimated values. Model improvements during the stock assessment meeting included changes in the life history parameters, time-blocks for CPUE time series and the implementation of a two-stage data weighting approach. Diagnostics for the final reference case model demonstrated fast and stable convergence, good retrospectives, and a robust solution across different starting values The estimated spawning output time series for the final reference case indicate a decreasing stock from the late 1980s to the early 2010s, a slight recovery until 2017, and a new slight decrease until 2021. Fishing mortality increased significantly from the late 1980s, reaching its all-time high in the early 2010s, decreasing fast until mid-2010s and increasing again in the last seven years.