2018 Assessment of the Thornyhead Stock Complex in the Gulf of Alaska
Rockfish have historically been assessed on a biennial stock assessment schedule to coincide with the availability of new trawl survey data (odd years). In 2017, the Alaska Fisheries Science Center (AFSC) participated in a stock assessment prioritization process. It was recommended that the Gulf of Alaska (GOA) thornyhead complex remain on a biennial stock assessment schedule with a full stock assessment produced in even years and no stock assessment produced in odd years. However, we performed a partial stock assessment in 2017 because the allowable biological catch (ABC) has been exceeded in the past in the western GOA, and because the biomass estimates provided by the GOA trawl surveys have at times displayed extreme variability between surveys. We followed the recommendation of the Science and Statistical Committee (SSC) and the GOA Groundfish Plan Team that “partial assessments for Tiers 4-5 should be an expanded version of the current off-year executive summaries, including catch/biomass ratios for all species in addition to re-running the random effects model” (SSC minutes – February 2017).
For this on-cycle year, we incorporate Relative Population Weights (RPWs) from the 1992 - 2018 longline surveys, and update auxiliary data sources.
This stock is classified as a Tier 5 stock. We continue to use a random effects (RE) model fit to survey data to estimate exploitable biomass and determine the recommended ABC, but we present a new method of combining the AFSC longline survey Relative Population Weight (RPW) index (1992 - 2018) with the AFSC bottom trawl survey biomass index (1984 – 2017) within the random effects model. The RE model was fit to the time series of trawl survey biomass values and estimates of uncertainty by region and depth strata and regional RPW indices from the AFSC longline survey (with associated estimates of uncertainty). These regional biomass estimates from the RE model were then summed to obtain Gulfwide biomass.