A BIOMASS-BASED ASSESSMENT MODEL FOR NORTHERN ANCHOVY, ENGRAULIS-MORDAX

We developed a relatively simple and parsimonious (SMPAR) biomass dyna mics model for estimating abundance of northern anchovy, Engraulis mor dax, off southern California and Baja California, Mexico, during the 1 963 to 1991 fishing seasons. The SMPAR model was a compromise between simple surplus production and complex age-structured models. It was de signed to give more precise biomass estimates for management of northe rn anchovy for which there are no age-composition data and only noisy abundance index data. We evaluated consistent bias in biomass and recr uitment estimates, bias in recruitment estimates due to log transforma tion, and retrospective bias. Simple corrections based on bootstrap pr ocedures were used to remove consistent bias and log transformation bi as. Retrospective bias was not a significant problem. Results indicate that the SMPAR model estimates stock biomass more reliably than recru itment because abundance indices for northern anchovy contain little i nformation about interannual recruitment variability. Asymptotic varia nce estimates calculated by inverting the Hessian matrix averaged 20% smaller than variances calculated by bootstrapping. Outliers in abunda nce data were the biggest source of uncertainty in biomass estimates. Simulation results indicate that our approach could be useful in a var iety of situations.