Gulf of Alaska | Fishes and Fish Habitats MASTER’S ORAL PRESENTATION Evaluation of harvest control rules for Prince William Sound Pacific herring Presenter: Joshua Zahner , jzahner@uw.edu, University of Washington
In the spring of 1992, the Prince William Sound (PWS) Pacific herring ( Clupea pallasii ) stock suffered a large-scale collapse, which has resulted in a prolonged closure of the historical fishery. However, recent trends in estimated biomass have indicated that the stock is slowly recovering and that a reopening of the fishery may be possible in the coming years. This project seeks to evaluate several different harvest control rules that could be used to set future catch limits for a reopened fishery using a management strategy evaluation (MSE) simulation framework. The MSE is composed of an age-structured population dynamics model and a Bayesian-style assessment model which are used to simulate the long-term effects of a given harvest strategy on the population. Control rules include eight different threshold scaling rules, one rule that considers the evenness of the population age structure, another that considers the most- recent three-year gradient in biomass, and a final rule that is based only on the biomass of fish larger than 130 grams in weight. We performed five hundred fifty-year simulations for each of the twelve control rules, which were evaluated based on realized catch, catch variation, biomass depletion, and probability of fishery closure. Simulations indicate that the eight threshold rules behave similarly: with catches being highest and biomass depletion being most severe, when the maximum allowable harvest rate is high, or when the range of biomass over which fishing is allowed is wide. The evenness rule offers a small amount of additional protection to the stock when biomass is near the lower limit threshold for fishing by decreasing the allowable harvest rate when the stock is dominated by a single cohort of fish. Meanwhile, the gradient rule promotes higher catches at the cost of high interannual catch variability and lower final biomass levels. The 40-20 threshold rule that is currently in place for the stock performed intermediately well across all metrics. While these results do not indicate an optimal control rule for the management of the PWS Pacific herring stock, they do highlight the inherent trade-off between catch and stock status, even when more advanced control rules are utilized for setting target harvest rates or catch levels.
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