Challenges to estimating maturity in stock assessment: a case study of Pacific herring in Prince William Sound, AK

Tuesday April, 21st at 2:00 PM (PST)

John T. Trochta & Trevor A. Branch

Mature proportions at age or length, known as the maturity ogive, are important parameters in stock assessment. Information on the maturity ogive is consequential for fisheries management because it determines the final spawning biomass estimated by models and used for decision making (e.g. via harvest control rules). However, obtaining an accurate maturity ogive is particularly challenging for fish populations such as Pacific herring, in which the surveyed aggregations are not representative of the true population. For Pacific herring in Prince William Sound, Alaska, there is evidence not all fish are available to sampling of pre-spawning and spawning aggregations which are otherwise assumed to represent all spawning fish in the age-structured assessment (ASA). We investigated these and other maturity-related issues in the Prince William Sound herring ASA by conducting a sensitivity analysis with the ASA. We make different assumptions about the true maturity ogive and availability of herring to surveys to develop a suite of 11 models that bound the range of effects from mis-specifying maturity in the ASA. These effects are represented by key model outputs we compare across models. We also use Bayesian model selection to rank the most likely models. In this talk, I describe survey and modeling challenges regarding maturity, methods, and results from this analysis, preliminarily concluding the herring ASA is robust to different assumptions on maturity.