Time-dependent Parameterization of Natural Mortality, Maturity, and Fishing Selectivity

Time-dependent Parameterization of Natural Mortality, Maturity, and Fishing Selectivity

A Case Study Using the Pacific Decadal Oscillation for Pacific Herring (Clupea pallasii) in Sitka Sound, Alaska

Peter-John F. Hulson, Sara E. Miller, Sherri C. Dressel, Terrance J. Quinn II, and Kray Van Kirk

Time-dependent Parameterization of Natural Mortality, Maturity, and Fishing SelectivityThis is part of Impacts of a Changing Environment on the Dynamics of High-latitude Fish and Fisheries
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Description

Integrated age-structured assessment models often estimate parameters such as natural mortality, maturity, and fishing selectivity as constant over time or across age classes. If parameters are time-dependent but modeled as time-invariant, bias can occur. The goal of this study was to determine whether a statistical catch-at-age assessment model to assess the Pacific herring (Clupea pallasii) population in Sitka Sound, Alaska, is improved by using time-dependent parameters. Model scenarios included: (1) estimating natural mortality, maturity, and fishing selectivity parameters as time-invariant; (2) estimating natural mortality, maturity, and fishing selectivity parameters as time-dependent using time-blocks associated with patterns in sea-surface temperature as measured by the Pacific Decadal Oscillation (PDO) index; and (3) estimating natural mortality, maturity, and fishing selectivity as time-dependent using the PDO index as a covariate. Models incorporating the PDO index as environmental information, whether through time-blocks or as a covariate, resulted in better model fits to available data compared to the model in which these parameters were time-invariant. The time-block parameterization resulted in the preferred time-dependent parameterization overall. A significant correlation between estimated natural mortality and the mean PDO index within time-blocks resulted, in which larger natural mortality was associated with positive PDO time-blocks and smaller natural mortality was associated with negative PDO time-blocks, indicating that natural mortality for herring is higher in warmer waters. From retrospective analyses, these results were also robust over time.

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