2018 Assessment of the Walleye Pollock Stock in the Gulf of Alaska
The base model projection of female spawning biomass in 2019 is 345,352 t, which is 62.4% of unfished spawning biomass (based on average post-1977 recruitment) and above B40% (221,000 t), thereby placing GOA pollock in sub-tier “a” of Tier 3. New survey data in 2018 are highly contrasting, with the 2018 Shelikof Strait acoustic survey indicating high biomass, and the ADFG trawl survey indicating relatively low biomass (though increased from the previous two years). The risk matrix table recommended by the SSC was used to determine whether to recommend an ABC lower than the maximum permissible. The table is applied by evaluating the severity of three types of considerations that could be used to support a scientific recommendation to reduce the ABC from the maximum permissible. We identified substantially increased concerns for the stock assessment, the population dynamics of pollock, and environmental/ecosystem factors that are likely to affect pollock.
Assessment considerations: In the last several years, there have been strongly contrasting trends in the survey abundance indices, with bottom trawl indices showing a steep decline, while acoustic surveys showing record highs. The model is unable to fit strongly contrasting trends, which has resulted in very poor model fits to the most recent survey indices. This increases the uncertainty of the assessment.
Population dynamics considerations: The age structure of pollock in the Gulf of Alaska has been being strongly perturbed by an unusual sequence of events. The first event was the very strong recruitment of the 2012 year class. Recruitment since then has been very weak until 2017, where there is evidence of an average year class based on acoustic surveys conducted in winter of 2018. The age-diversity of pollock has dropped rapidly, and both the fishery and population are now completely dominated by a single large year class. The 2012 year class has showed reduced growth, early maturation, and apparent reduced natural mortality.
Environmental/Ecosystem considerations: Limited information indicates age-0 pollock may have been relatively abundant in summer of 2018, but conditions do not appear to be favorable for winter survival with the recent onset of a marine heatwave in the GOA, and forecasted warm temperatures through winter of 2018/19. If the 2018 year class turns out to be weak, this would likely lead to downward trend in adult pollock biomass, since the 2017 year class is the first since 2012 that is estimated to be of average size. There are mixed signals regarding current foraging conditions for largely planktivorous adult pollock. Increases in large copepods and euphausiids suggest improved foraging conditions this past year. In contrast, planktivorous parakeet auklets nesting in the Semidi Islands had poor reproductive success in summer 2018, suggesting a lack of forage for pollock.