2015 Assessment of the Walleye Pollock Stock in the Gulf of Alaska
The age-structured assessment model is similar to the model used for the 2014 assessment and was developed using AD Model Builder (a C++ software language extension and automatic differentiation library). The only changes made to the model were those necessary to include the summer acoustic
survey in the assessment, and to estimate a power coefficient for the age-1 winter acoustic survey index catchability
The base model projection of female spawning biomass in 2016 is 321,626 t, which is 42.9% of unfished spawning biomass (based on average post-1977 recruitment) and above B40% (300,000 t), thereby placing Gulf of Alaska pollock in sub-tier “a” of Tier 3. In last few assessments, the magnitude of the 2012 year class was a major issue when deciding which ABCs and OFLs to recommend. New information about 2012 year class came from the 2015 Shelikof Strait survey, the 2015 NMFS bottom trawl survey, and the 2015 summer acoustic survey. All of this new information indicates that this year class is still very abundant. The 2015 Shelikof Strait acoustic survey estimate of age-3 pollock is 1.64 billion, which is the largest age-3 estimate in time series. Therefore we have continued the approach of using the 2012 year class abundance as estimated to project ABCs and OFLs.
The new survey data for 2015 included the Shelikof Strait acoustic survey, the summer acoustic survey, and the NMFS bottom trawl surveys, all of which remain at relatively high levels. There was a large and unexplained decline in pollock biomass in the 2015 ADFG survey (58% decline), which is a concern, especially since this time series has been the most stable used in the assessment. Since this low observation is included in the model, the estimated ABCs and OFLs somewhat factor in this concern. The estimated abundance of mature fish is projected to peak in 2017, and then decline as the strong 2012 year class passes through the population.
The author’s 2016 ABC recommendation for pollock in the Gulf of Alaska west of 140° W lon. (W/C/WYK regions) is 254,310 t, which is an increase of 33% from the 2015 ABC. This recommendation is based on a more conservative alternative to the maximum permissible FABC introduced in the 2001 SAFE applied to the base model. In 2017, the ABC for an adjusted F40% harvest rate is 250,544 t. The OFL in 2016 is 322,858 t, and the OFL in 2017 if the recommended ABC is taken in 2016 is 289,937 t.
For pollock in southeast Alaska (Southeast Outside region), the ABC recommendation for both 2016 and 2017 is 9,920 t (see Appendix A) and the OFL recommendation for both 2016 and 2017 is 13,226 t. These recommendations are based on a Tier 5 assessment using the estimated biomass in 2016 and 2017 from a random effects model fit to the 1990-2015 bottom trawl survey biomass estimates in Southeast Alaska.