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Status of Stocks & Multispecies Assessment Program

Groundfish Stock Assessment Summaries for 2007:  Bering Sea / Aleutian Islands (BSAI)

The sum of the ABCs for 2007, as recommended by the Scientific and Statistical Committee, is just over about 2.7 million t, 10% lower than the sum of the 2006 ABCs. This drop was driven by the large reduction in ABC from EBS pollock (1.93 million t in 2006 compared to 1.394 million t in 2007). In 2007, an additional summer EIT survey of the EBS shelf region is tentatively planned along with the winter Bogoslof region EIT survey.

The purpose of this extra survey is two-fold—to evaluate the condition of pollock stocks during a period of recent relatively rapid declines and to allow for inter-ship calibrations between the NOAA ships Miller Freeman and Oscar Dyson. The standard summer bottom trawl survey is also planned.

These surveys (budget outlook permitting) will provide critical information for developing 2007 assessments and further management recommendations. Summaries of each stock/assemblage group are presented as follows.

  Figure 4, see caption
Figure 4.  Estimated age 3+ EBS mid-year pollock biomass under, 1978-2007. Approximate upper and lower 95% confidence limits are shown by dashed lines. Superimposed is the estimate of mid-year age 3+ biomass from last year’s assessment.

EBS Pollock:  The 2006 bottom-trawl survey estimate of the pollock stock in this region was close to expected based on projected declines from 2005. However, the 2006 EIT survey was about 30% lower than expected which exacerbated estimates of the level and rate of decline estimated since 2003 (Fig. 4).

The Council recommended a 2007 ABC of 1.394 million t which was used for setting the TAC (also set at 1,394,000 t). This ABC level was reduced from 1.512 million t due to concerns over the continued decline in biomass and increasing trends in harvest rates, uncertainty in incoming year-class strength and unexpected shifts in spatial distribution. Projections for 2007 indicate that the decline will ease by 2008 as the apparently above-average 2005 year class ages.

AI Pollock:  The Council accepted the age-structured analysis and based on new survey and age composition data, recommended the maximum permissible Tier 3 ABC levels of 44,500 t. However, due to current regulations, the TAC is set to 19,000 t. The success of the AFSC cooperative research program to survey the Aleutian Islands during the winter (when pollock fishing traditionally occurred in this region) has been heralded as a promising approach to undertake fine-scale area and temporal management to address Steller sea lion recovery concerns. This approach allows for directed pollock fishing while also minimizing the potential interaction effects with Steller sea lions. In 2006, the project extended over 6 weeks and provided eight synoptic surveys during this period. This allowed for unique quantification of diurnal and within season pollock movement patterns.

Bogoslof Island Pollock:  An updated age-structured assessment was presented based on the one developed in 2005. This year the authors explored the effect of Donut Hole catches in the 1980s on the stock assessment results. They assumed that 75% of the Donut Hole catches came from the Bogoslof stock, which is in accord with past practices of international pollock workshops (which used a range from 60% to 80%). Little is known about the actual degree of interchange between Bogoslof fish and central BS and EBS pollock. The 2007 ABC was specified as in previous years which (based on the most recent survey estimate) results in a value of 5,220 t. The Council sets the TAC for this area to 10 t to account for bycatch in other directed fisheries.

BSAI Pacific Cod:  The 2006 EBS shelf bottom-trawl survey estimate (518,000 t) for Pacific cod, down about 14% compared to the 2005 estimate. The assessment evaluated an array of model configurations including a number of enhancements over previous analyses. Regardless of configuration, the models indicated that a series of poor year classes (2000-2004) are imminent and contribute to the projected declines. The Council-selected model resulted in an ABC (and TAC, not counting state-waters allocation) of 176,000 t compared to the 2005 level of 194,000 t.

BSAI Flatfish:  Combined, the five main species groups of flatfish continue to trend upwards with a 14% increase from the 2005 level and now represent about 45% of all groundfish biomass. This year the Council accepted that the estimates of Fmsy and the associated uncertainty were adequately estimated (based on fitting a stock-recruitment relationship within the integrated assessment model) for yellowfin sole and northern rock sole. This results in management under Tier 1 and implies somewhat higher ABCs than estimated under Tier 3 methods. For example, the 2007 yellowfin sole ABC under Tier 1 is estimated at 225,000 t while under Tier 3, the value is 136,000 t (the 2006 Tier 3 ABC was 121,000 t). Nonetheless, the 2007 yellowfin sole TAC was set to 136,000 t.

The arrowtooth flounder 2007 ABC increased 16% to 158,000 t which led to a TAC of only 20,000 t. Northern rock sole 2007 ABC increased by 57% (since it shifted to Tier 1) to 198,000 t but the TAC was set at 55,000 t. The flathead sole ABC increased by 32% to 79,200 t while the Council set the TAC to 30,000 t. Alaska plaice 2007 ABC was stable (up 1%) at a value of 190,000 t and a 2007 TAC of 25,000 t. The other flatfish assemblage 2007 ABC increased 18% to 21,400 t but the TAC was set to 10,000 t.

  Figure 5, see caption
Figure 5.  Abundance-at-length (cm) for Greenland turbot observed from the summer NMFS shelf trawl surveys, 1985-2006 (sexes combined, all strata except for 1986 where only strata 1-6 were sampled).

BSAI Greenland Turbot:  The data and model for 2006 was revamped. A new version of the software was used (stock synthesis 2) and a number of components of the model were re-evaluated. Additionally, the recent length frequency data from the shelf survey indicates that a recruitment event has occurred recently (Fig. 5). A number of issues remain with this assessment, in particular the fact that this species tends to be distributed in more northerly regions and the degree of mixing between other regions (e.g., north of the U.S. convention line) is unknown. Assessment uncertainties and stock structure issues have led the Council to recommend conservative ABC levels, which for 2007 is 2,440 t, well below the maximum permissible value of 12,680 t.

BSAI Rockfish:  In the past year, rockfish stock assessments received a review by the NMFS Center of Independent Experts (CIE), and also a workshop on modeling approaches for rockfish was conducted. These led to improvements to this year’s assessments and some important changes. For example, the BSAI Pacific ocean perch assessment includes a model estimate for natural mortality, resulting in an upward revision from 0.05 to 0.06. Based on recommendations from the CIE, the initial age composition was assumed to be in equilibrium with an unfished population, instead of reflecting variation in recruitment strength for each cohort, as previously assumed. This results in a 2007 Pacific ocean perch ABC of 21,900 t, an increase of 48% higher than the 2006 level.

For northern rockfish the 2007 ABC decreased by 4% relative to the 2006 value to 8,190 t. The shortraker rockfish 2007 ABC recommendations decreased by 27% from last year’s levels giving 424 t. The ABC for rougheye rockfish dropped from 224 t to 202 t while other rockfish dropped from 1,400 t to 999 t.

BSAI Atka Mackerel:  New information from the fishery and surveys resulted in slightly different model results compared to the 2005 assessment. In particular, the revised estimates of age structure resulted in projected 2007 age 3+ biomass estimates of 364,200 t, down about 18% from last year’s estimate for 2006. The projected female spawning biomass for 2007 is estimated at 129,900 t, roughly 55% of unfished spawning biomass and above B40% level (95,000 t). The 2007 ABC recommendation is 74,000 t, down from the 2006 level of 110,000 t.

By Jim Ianelli and Anne Hollowed (REFM), and Phil Rigby (ABL)

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