An age-structured assessment model for chinook salmon (Oncorhynchus tshawytscha)

Abstract

Age-structured assessment models are rarely used for estimating the abundance of exploited salmon stocks. We developed such a model for a chinook salmon (Oncorhynchus tshawytscha) population in the Copper River, Alaska. Information consisted of catch-age data from three fisheries (commercial, recreational, and subsistence) and two sources of auxiliary data (escapement index and spawner–recruit relationship). Model parameters included brood-year returns, proportions of a brood year returning at age and year, annual exploitation rates, gear selectivity, spawner–recruit parameters, and a calibration parameter for the escapement index. Results suggested that population parameter estimates with high precision and low bias were produced by an approach that considered measurement error in the pooled catch-age data from all three fisheries and brood-year return proportions that varied over time. A sensitivity analysis revealed that brood-year return, catch, and escapement index estimates were insensitive to large changes in data weightings. The absence of strong deviations in the retrospective patterns of the brood-year returns suggested that there were no serious model misspecifications. The model integrated all sources of available information, accounted for uncertainty, and provided estimates of optimal escapement and its associated exploitation level. We believe that the model has broad application for use in assessments of chinook salmon systems.