Life history characteristics, management strategies, and environmental and economic factors that contribute to the vulnerability of rockfish stocks off Alaska

Abstract

This study explored the extent to which variations in biological characteristics, environmental and economic factors, and management strategies have affected the tendency for rockfish to become overfished. The analysis used data on 5 species of rockfish that account for more than 95% of commercial catch of rockfish in the Gulf of Alaska (GOA) and Bering Sea and Aleutian Island (BSAI) management regions. These species are: Shortraker Rockfish (Sebastes borealis), Pacific Ocean Perch (Sebastes alutus), Northern Rockfish (Sebastes polyspinis), Dusky Rockfish (Sebastes variabilis), and Shortspine Thornyhead (Sebastolobus alascanus). Fishery management models often treat BMSY, the biomass level that maximizes sustainable yield, as a critical reference point; whenever the biomass of a federally managed fish or shellfish stock is estimated at less than 0.5×BMSY, the stock is declared "overfished" and managers are required to develop a recovery plan that will restore stock abundance above BMSY within about one generation length. Because estimates of BMSY are unavailable for some GOA and BSAI rockfish stocks included in this analysis and because we were interested in developing a model that could be applied to data-poor stocks, we explored two proxies for BMSY. The mean of past estimates of exploitable biomass (avgExpB) was used as a proxy for BMSY for the better-studied stocks. The mean of past catch (avgC) was used as a proxy for BMSY for data-poor stocks. These values were used to scale time series estimates of exploitable biomass (ExpBt) or catch (Ct). A systems estimation approach, seemingly unrelated regression (SUR), was used to estimate parameters of linear and nonlinear models that included available numerical and categorical variables (biological, management, environmental, and economic factors) thought to contribute to increases or decreases in ExpBt / avgExpB or Ct / avgC. Goodness-of-fit statistics and tests of individual coefficients and groupings of coefficients were used to guide model refinement. The modeling approach worked well for better-studied stocks but not for data-poor stocks. The preferred 5-stock model (Pacific Ocean Perch in the GOA and BSAI, Northern Rockfish in the GOA and BSAI, and Dusky Rockfish in the GOA) had an excellent fit to the overall system (R² = 0.922, P << 10⁻⁶) and statistically significant coefficient estimates of the variables included. The model indicated that the past values of ExpBt / avgExpB can be accounted for through time and across stocks by nonlinear variation in: spawning biomass, intrinsic growth rates (k), maximum age, exploitation rates, habitat preferences, Pacific Decadal Oscillation, and ex-vessel price. Because some of these factors are subject to management control and others are predictable, it should be possible to take account of anticipated changes in these factors when setting harvest targets and harvest limits, selecting spatial management strategies, or considering changes to harvest control rules or fisheries governance systems.