Preliminary Stock Assessment of northeastern Atlantic porbeagle (Lamna Nasus) using the Bayesian State-Space Surplus Production Model JABBA

Bayesian State-Space Surplus Production Models were fitted to Northeastern Atlantic porbeagle shark (Lamna nasus) catch and relative abundance indices using the ‘JABBA’ R package. This document presents details on the model diagnostics and stock status estimates for preliminary scenarios. A ref model was fitted to three indices reviewed by the ICES WKELASMO in 2022 and the full model also included a fourth historical index applied in 2009 stock assessment. The prior assumptions in the surplus production function were kept consistent with the ICES WKELASMO assessment presented in 2022. We evaluated model plausibility using four objective model diagnostics: (1) model convergence, (2) fits to the data, (3) consistency (e.g. retrospective patterns) and (4) prediction skill. Our results suggest that the full model represents the most plausible candidate model that incorporates all available relative indices of abundance. Results are consistent with the SPiCT model runs, indicating that the stock is currently overfished (0.45 B/BMSY) but no experiencing overfishing (0.01 F/FMSY). Additional sensitivity runs indicated that the full model was robust to alternative productivity and variance assumptions.