Using multi-species surplus production models to estimate ecosystem-level maximum sustainable yields

Abstract

We used time series of biomass and catches for the major commercial groundfish stocks in the Eastern Bering Sea/Aleutian Islands region and in the Gulf of Alaska to compute annual surplus production for 1977–2004. Annual surplus production and average annual biomass were aggregated across stocks within each region to examine the relationship between total surplus production and total aggregated biomass. We fit two surplus production models (Graham-Schaefer and Pella-Tomlinson) to the observed relationships to estimate maximum multi-species surplus production (equivalent to maximum sustainable yield) of the groundfish complexes in these ecosystems. Maximum multi-species surplus production was estimated to be approximately 2.5 × 10 6 t in the Bering Sea/Aleutian Islands and 330 × 10 3 t in the Gulf of Alaska. These point estimates were smaller than the sum of single-species MSY proxies from recent stock assessments, and estimates for the Gulf of Alaska were much smaller than earlier estimates that were used to specify optimum yield ranges for the Gulf of Alaska groundfish complex. Therefore, optimum yield ranges for the Gulf of Alaska may need to be re-assessed to reflect more recent conditions. We further estimated the effects of environmental variability on annual surplus production using correlations and generalized surplus production models with environmental covariates. Results suggest that surplus production in the Bering Sea may be lower during conditions associated with the positive phase of the Pacific Decadal Oscillation, which is characterized by warmer temperatures, reduced ice, and reduced wind mixing over the shelf. Similarly, surplus production on the Gulf of Alaska shelf was lower during years with warm bottom temperatures coupled with low salinities in late winter. If maximum multi-species surplus production varies as a result of environmental variability, optimum yield ranges for the groundfish complexes in these ecosystems should take such fluctuations into account.