Using Multiple Indices for Biomass and Apportionment Estimation of Alaska Groundfish Stocks

Fish stocks in the Bering Sea, Aleutian Islands, and Gulf of Alaska are managed by the North Pacific Fishery Management Council through a six-tier system. These tiers are defined by the data availability and complexity of assessment method. In particular, Tiers 4 and 5 of this system use the area-swept biomass from bottom trawl surveys performed by the Alaska Fisheries Science Center in a random effects model to estimate total biomass from which management quantities of Acceptable Biological Catch and the Overfishing Limit are determined. In this study, we present a method in which biomass indices from a longline survey are also incorporated into the random effects model using the Gulf of Alaska shortraker rockfish and shortspine thornyhead stocks as examples of the development of this method. Overall, we find that estimates of biomass, as well as apportionment of biomass among sub-regions of the Gulf of Alaska, become more stable over time when random effects models incorporate longline survey biomass indices in addition to bottom trawl survey biomass estimates. We also show that this method can be used to evaluate spatial estimation of the catchability coefficient between the two surveys, as well as relative weighting of the longline survey index. This method may also be useful for several other Tier 5 species, as well as sub-region apportionment of biomass for Tier 3 stocks that are captured by both surveys. 2021 NMFS (National Marine Fisheries Service) AFSC (Alaska Fisheries Science Center) Submitted https://doi.org/10.25923/by6g-4s98 Public Domain 1861