2019 Assessment of the Yellowfin Sole Stock in the Bering Sea and Aleutian Islands
The Yellowfin Sole (Limanda aspera) is one of the most abundant flatfish species in the eastern Bering Sea (EBS) and currently is the target of the largest flatfish fishery in the world. They inhabit the EBS shelf and are considered one stock.
In 2019 a new model was introduced based on the 2018 Model 18.1 that retains female natural mortality fixed at 0.12 while allowing the model to estimate male natural mortality. This model is referred to as Model 18.2. Model 18.1 (and Model 18.1a) included the survey mean bottom temperature across stations < 100m as a covariate on survey catchability, as in previous years, but added survey start date as an additional covariate within the model, based on a recent study by Nichol et al. (2018). Model 18.2 retains these features.
An unexpected 32 percent decrease in the NMFS eastern Bering Sea survey biomass was observed in 2018. In 2019 the survey biomass was 6 percent higher than in 2018 at 2,006,510 t. Spawning biomass estimated by Model 18.2 remained high at 1.94 * BMSY . Therefore, Yellowfin Sole continues to qualify for management under Tier 1a. Similar to recent years, the 1978-2013 age-1 recruitments and the corresponding spawning biomass estimates were used to fit the stock recruitment curve and determine the Tier 1 harvest recommendations.
This assessment updates last year’s assessment with results and management quantities that are higher than the 2018 assessment. This is due to a higher 2019 survey biomass point estimate, 6 percent higher than the 1 2018 estimate. Secondarily, the model estimated male natural mortality slightly higher than female natural mortality, 0.135, which increased biomass estimates.
Catch as of October 28, 2019 was 109,620 t. Over the past 5 years (2014–2018), 92.4 percent of the catch has taken place by this date. Therefore, the full year’s estimate of catch in 2019 was 118,642 t. Future catch for the next 10 years, 2020–2029 was estimated as the mean of the past 10 years catch, 137,230 t.
Yellowfin Sole continue to be above BMSY and the annual harvest remains below the ABC level. Management quantities are given in the following table for the 2018 base model (Model 18.1a) and the 2019 preferred model (Model 18.2). The projected estimate of total biomass for 2020 was higher by 17 percent from the 2018 assessment of 2,331,500 t, to 2,726,370 t. The model projection of spawning biomass for 2020, assuming catch for 2019 as described above, was 1,051,050 t, 132 percent of the projected 2020 spawning biomass from the 2018 assessment of 796,600 t. The 2020 and 2021 ABCs using FABC from this assessment model were higher than the 2018 ABC of 249,100 t; 296,060 t and 296,793 t. The 2020 and 2021 OFLs estimated in this assessment were 321,794 t and 322,591 t.