Resource Ecology & Fisheries Management (REFM) Division

Status of Stocks & Multispecies Assessment Program

Groundfish Stock Assessments for 2006:  Gulf of Alaska (GOA)

In the GOA, full assessments were presented for 17 stocks and assemblages and an appendix on the status of forage fish was also prepared. The sum of the recommended ABCs for 2006 is 501,366 t which represents an 8% decrease from the 2005 total. The largest component of the decrease was from arrowtooth flounder (which dropped 18% from the 2005 ABC), followed by flathead sole (down 16%), and pollock (down 6%). These declines in ABCs were offset by increases in Pacific cod (+19%), deepwater flatfish (+27%) and Atka mackerel.

Pacific ocean perch and shortraker and rougheye rockfish ABC levels increased slightly (~5%) while pelagic shelf rockfish increased substantively (+19%). Northern rockfish ABCs were the same as last year as further refinements to the model were requested prior to increasing the level as preliminary results indicate. Brief summaries of each GOA species or species group follows.
 

GOA Pollock:  The 2005 summer bottom trawl survey pollock biomass estimate was down 11% from the 2003 level while the winter Shelikof Strait EIT survey indicated a 2% increase from the 2004 estimate. The model estimate of spawning biomass in 2006 is 193,092 t, which is 35% of unfished spawning biomass and below B_40% (224,000 t). Estimates of stock status in 2006 are similar to 2005 and are consistent with survey trend estimates. Projections incorporating uncertainty in the stock assessment indicate that there is very low probability that the pollock stock will drop below critical levels in the near future.

The ecosystem considerations section for pollock was also substantially updated. This analysis confirmed that pollock are an important prey species in the GOA, especially for Pacific halibut, Steller sea lions, arrowtooth flounder, and Pacific cod. Analyses of food habits collections from 1990 through 2005 combined with estimates of predator consumption rates suggest that consumption of pollock by arrowtooth flounder is substantial, and consumption by arrowtooth, halibut, and cod combined accounted for the majority of pollock consumption in the ecosystem.

While arrowtooth flounder cause the highest proportion of pollock mortality, they are less dependent on pollock than halibut and sea lions because pollock represent a relatively small proportion of the arrowtooth diet. Therefore, while a pollock decline might influence biomass trends for halibut, it is unlikely to do so for arrowtooth. Model simulations indicate that increased pollock mortality had stronger effects on halibut (and the halibut fishery) and sea lions than on arrowtooth flounder or other predators. Currently under way, the management strategy evaluation (MSE) analyses linked with ecosystem modeling may provide advice on whether additional adjustments to ABC recommendations are needed.


GOA Pacific Cod:  The 2005 summer bottom trawl survey Pacific cod biomass estimate was up about 4% from the 2003 level to 308,000 t. The model estimate of spawning biomass in 2006 is 116,600 t, which is above the B_40% (106,800 t). The assessment was done using new software that allowed a number of enhancements over previous analyses. More significantly, revised estimates of maturity-at-age were used based on recent studies completed by James Stark (RACE Division). These new estimates resulted in substantially higher estimates of spawning biomass compared to previous years. Hence, the recommended ABC level is up 19% from last year to 68,859 t.


GOA/BSAI Sablefish:  The longline survey abundance index decreased 3% from 2004 level but is about 6% higher than the value for 2000. Spawning biomass is currently about 38% of the estimated unfished level and is projected to decrease to 34% by 2008 given current estimates of recruitment. The Council-recommended 2006 ABC was level compared to 2005 at 21,000 t. However, due to apparent changes in the relative densities of sablefish between areas, the ABC allocated to the GOA dropped by 7% while the ABC for the EBS and Aleutian Islands regions increased by 25% and 18%, respectively.


GOA Flatfish:  The 2005 survey estimate of arrowtooth flounder biomass (for the western and central GOA) was 1.65 million t compared to the 1984-2005 average of 1.4 million t. This compares with a value of 479,000 t for Pacific halibut and 661,000 t for all other flatfish species in this region. However, compared with the 2003 survey estimates the 2005 values are lower. For arrowtooth flounder the assessment model results in a 2006 ABC recommendation that drops by 18% to a value of 177,844 t (compared to a 2005 catch level of only 18,000 t).

The other flatfish management groups and 2006 ABCs were deep water flatfish-8,665 t (compared to 2005 catch of 404 t); shallow water flatfish-51,450 t (compared to 2005 catch of 4,600 t); flathead sole-37,820 t (compared to 2005 catch level of 2,400 t); and rex sole-9,200 t (compared to 2005 catch level of 2,200 t).

Enhancements to this year’s group of flatfish assessments included greater consideration of ecosystem effects within each section and the adoption of a full age-structured model for Dover sole (included in the “deep water flatfish” group).

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AFSC Quarterly
Research Reports Oct-Dec 2005