Status of Stocks & Multispecies Assessment Program
Groundfish Stock Assessments for 2009: Bering Sea/Aleutian Islands (BSAI)
The sum of the ABCs for 2009, as recommended by the Scientific and Statistical Committee, is just over about 2.4 million t, about 10% lower than the sum of the 2008 ABC values. This drop was driven by the reduction in ABC from EBS pollock (1.0 million t in 2008 compared to 815,000 t in 2009). Brief summaries of each BSAI species or species group follows.
EBS Pollock: Analyses of survey and fishery data show a decline in the walleye pollock biomass, which has resulted in the Council recommending a cut to the EBS pollock catch limit for 2009. The EBS pollock biomass was above average in the 1990s and apparently peaked in 2003, but the decline since then has contributed to lower catch limits for 2009.
Walleye pollock in the EBS typically have been monitored with regular annual bottom trawl and biennial acoustic surveys. The 2008 effort, however, marked the third consecutive year that both surveys were conducted. The results of the 2008 bottom trawl survey suggested pollock abundance was in line with previous analyses, but the acoustic survey showed lower levels of abundance than expected.
Combining these results and recent fishery observer information in an integrated analysis indicate a lower catch limit of 815,000 t for 2009, an 18.5% reduction from the 2008 catch level. The Council reviewed and discussed the assessment prepared at its December meeting and agreed with the need for lower pollock catch limits for 2009.
Although the assessment indicates that the pollock biomass is low, there is some optimism about the future of the stock. The prognosis for 2010 is for improved stock levels because the 2006 year class appears to be above-average. The 2009 surveys will play a critical role in monitoring and in subsequent management decisions.
While survey data plays a critical role in advising fisheries management, by autumn 2010 the fieldwork portion of the Bering Sea Integrated Research Program (BSIERP) will have been completed. This project will help to tie environmental and ecosystem data more fully into fishery management practices.
AI Pollock: In summer 2008 a panel of three experts outside of NMFS was convened to review the assessment approaches used for AI Atka mackerel and pollock. They developed a series of reports which detailed recommendations for improvements.
These reports were presented to the Plan Teams and Council during September and October and subsequently were incorporated into the assessments for the December analyses. Relative to last year's assessment, the numerous revisions to this year's model resulted in a major change in the estimated trajectory of the stock relative to biomass reference points.
In last year's assessment, the stock was estimated to have been well above B40% for the entire time series. In contrast, this year's assessment estimates that spawning biomass to be about 75% of the B40% level.
The slight increases in spawning biomass since 1999 have resulted more from a decrease in harvest rather than from good recruitment. However, it should be noted that the average recruitment for this stock is estimated to be almost twice the median level. Spawning biomass for 2009 is projected to be 85,500 t.
BSAI Pacific Cod: The assessment authors evaluated a large array of model configurations and presented these to the Council and Plan Teams during meetings in September-October and November-December.
The models all indicated a series of poor year classes from 2001 to 2005. However, the 2006 year class appears to be nearly 2 times higher than the average recruitment. The Council-selected model resulted in an ABC (and TAC, not counting state-waters allocation) of 182,000 t.
The spawning biomass is projected to continue a slow decline from 2009 to 2010 before the strong 2006 year class would boost the female spawning biomass from 363,000 t in 2010 to 401,000 t in 2012.
BSAI Flatfish: Some significant model changes for flatfish species occurred in 2008 based on requests for evaluations from the Council. The yellowfin sole and northern rock sole assessment models were revised and converted to account for sexual dimorphism. This change made better use of the available data and reduced the number of assumptions about the population ecology of these stocks.
Results were similar to previous model configurations and combined flatfish, but the total flatfish biomass was 13% lower compared to last year's totals. This was largely due to model revisions to arrowtooth flounder and others rather than due to declining biomass trends.
As with last year, the Council accepted that estimates of Fmsy, and the associated uncertainties were adequately estimated (based on fitting a stock-recruitment relationship within the integrated assessment model) for yellowfin sole and northern rock sole.
The ABC for yellowfin sole was estimated at 210,000 t - a decrease of 15% from the 2008 ABC level. The arrowtooth flounder biomass remains at high levels and shows a slight increase in trend. Northern rock sole 2009 ABC dropped by a few percent to 296,000 t, and the stock condition continues to show an increase. The flathead sole ABC for 2009 was estimated at 71,400 t, and the Alaska plaice abundance increased resulting in an ABC increase of 20% over the 2008 value to 232,000 t for 2009. "Other flatfish" ABC level was also up 10% to 23,100 t.
BSAI Greenland Turbot: Surveys suggested a slight decline from previous year's results but still indicated that recent recruitment conditions have improved. Since an EBS trawl survey of the slope region (the main habitat region for this stock in the U.S. EEZ) was conducted, the assessment information has improved compared to 2007.
Previously, the ABC was kept at conservative levels due to assessment uncertainties and stock structure issues. Because the model and information have improved and because of positive signs of recruitment, the Council's SSC recommended easing the conservative ABC slightly and adopted a stair-step approach towards the maximum-permissible ABC level, resulting in an ABC increase from 2,540 t in 2008 to 7,380 t for 2009.
BSAI Rockfish: This group comprises 4% of the BSAI groundfish complex. Pacific ocean perch (POP) and northern rockfish dominate, with most of the biomass in the AI.
For POP, the 2004-08 EBS biomasses were all higher than during 1983-2003; the AI biomass appears to be steady, and exploitation rates generally have been light. Northern rockfish biomass appears to have been increasing since 1977 due to favorable recruitment.
Shortraker rockfish biomass has apparently declined slowly since 1980. Biomass of the blackspotted/rougheye rockfish complex in the AI are increasing since 2002, with a huge 1998 year class.
There has been large year-to-year variability in the EBS survey biomass, with 2006 biomass near the high end of 1991-2006 range. Fish previously referred to as rougheye rockfish are now recognized as consisting of two species, the rougheye rockfish (Sebastes aleutianus) and blackspotted rockfish (Sebastes melanostictus).
The blackspotted and rougheye complex has now been separated from shortraker rockfish and is assessed with an age-structured assessment for the first time. The current information on these two species is insufficient to support species-specific assessments.
The previous biomass estimate (based on surveys and a multispecies surplus production model) for this group was 10,800 t in the 2006-07 assessments (projected to 2008). However, the age-structured model resulted in a 2008 biomass estimate that is 76% higher.
The substantial increase in biomass between the last full assessment and this year's assessment is almost entirely due to the adoption of an age-structured model with a selectivity curve that implies that fish are not caught by the survey until they are relatively old. Because the complex is now assessed with an age-structured model, there is an estimate of spawning biomass for 2009 of 6,540 t which is expected to remain stable.
BSAI Atka Mackerel: The Atka mackerel model estimate of biomass increased by 13% due principally to new data indicating that the magnitude of the 2004 year class was more abundant. A number of model changes were also implemented following the extensive peer review conducted during summer 2008.
The projected female spawning biomass for 2009 is estimated at 132,300 t, roughly 54% of unfished spawning biomass and above B40% level (97,800 t) (Fig. 3). The 2009 ABC recommendation is 83,800 t, a 38% increase from the 2008 ABC level.
