Initial results for north and south Atlantic shortfin mako (Isurus oxyrinchus) stock assessments using the Bayesian surplus production model JABBA and the catch-resilience method CMSY

Also published as IOTC-2020-WPEB16-INF09.

A Bayesian State-Space Surplus Production Model, JABBA, was fitted to the North and South Atlantic shortfin mako shark (Isurus oxyrinchus) catch and CPUE data. Results for base-case fits indicate the MSY estimate for North Atlantic was 1134.1 tons and 1130.5 tons for the South Atlantic. Stock status trajectories showed a typical anti-clockwise pattern for the North Atlantic, moving from underexploited through a period of unsustainable fishing, leading to a 99% probability of being overexploited in 2015. In contrast, the South Atlantic stock reveals a clockwise pattern moving from underexploited to a recovery state as result of decreasing biomass under sustainable fishing, which is seemingly implausible. Model diagnostics corroborated that these results were likely implausible. Results from the catch only method, CMSY, were similar to the JABBA results for North Atlantic shortfin mako shark, but strong discrepancies between CMSY and JABBA results were found for the South Atlantic. The latter can be attributed to the apparent contradiction between the observation process (i.e. CPUE) and process equation, which is informed by the catch and resilience (r) information.