2018 Assessment of the Sablefish Stock in Alaska
The longline survey abundance index increased 9% from 2017 to 2018 following a 14% increase in 2017 from 2016. The lowest point of the time series was 2015. The fishery catch-rate/abundance index stayed level from 2016 to 2017 and is at the time series low (the 2018 data are not available yet). Spawning biomass is projected to increase rapidly from 2019 to 2022, and then stabilize.
Sablefish are managed under Tier 3 of NPFMC harvest rules. Reference points are calculated using recruitments from 1977-2014. The updated point estimate of B40%, is 116,738 t. Since projected female spawning biomass (combined areas) for 2019 is 96,687 t (83% of B40%, or B33%), sablefish is in sub-tier “b” of Tier 3. The updated point estimates of F40%, and F35% from this assessment are 0.099, and 0.117, respectively, but Tier 3b uses the control rule to adjust these values downward. Thus, the maximum permissible value of FABC under Tier 3b is 0.081, which translates into a 2019 ABC (combined areas) of 28,171 t. The adjusted OFL fishing mortality rate is 0.096 which translates into a 2019 OFL (combined areas) of 33,141 t. Model projections indicate that this stock is not subject to overfishing, overfished, nor approaching an overfished condition.
Instead of maximum permissible ABC, we are recommending the 2019 ABC to be equal to the 2018 ABC, which translates to a 45% downward adjustment from max ABC. The final 2019 ABC of 15,068 t is 1% higher than the 2018 ABC because of updated whale depredation adjustments that are slightly smaller. The maximum permissible ABC for 2019 is 10% higher than the 2018 maximum permissible ABC of 25,583 t. The 2017 assessment projected a 41% increase in ABC for 2019 from 2018. The author recommended ABCs for 2019 and 2020 are lower than maximum permissible ABC for several important reasons that are examined in the new SSC-endorsed risk-matrix approach for ABC reductions. First, the 2014 year class is estimated to be 2 times higher than any other year class observed in the current recruitment regime (1977 – 2014). Tier 3 stocks have no explicit method to incorporate the uncertainty of this extremely large year class into harvest recommendations. While there are clearly positive signs of strong incoming recruitment, there are concerns regarding the lack of older fish and spawning biomass, the uncertainty surrounding the estimate of the strength of the 2014 year class, and the uncertainty about the environmental conditions that may affect the success of the 2014 year class in the future. These concerns warrant additional caution when recommending the 2019 and 2020 ABCs. It is unlikely that the 2014 year class will be average or below average, but projecting catches under the assumption that it is 7.5x average introduces risk given the uncertainty associated with this estimate. Only one large year class since 1999 has been observed, and there are only two observations of age compositions to support the magnitude of the 2014 year class. Our caution in 2017 seems justified as the estimate of the 2014 year class has decreased 30% since last year’s estimate. The cause of this decrease could be simply imprecision in the age compositions for the first year it was seen, or something real like an increase in natural mortality. Future surveys will help determine the magnitude of the 2014 year class and will help detect additional incoming large year classes other than the 2014 year class; there are indications that subsequent year classes may also be above average.