RESOURCE ECOLOGY & FISHERIES MANAGEMENT
(REFM) DIVISION (cont.)

2003 Stock Assessment Summary (cont.)

BSAI Walleye Pollock

This year’s walleye pollock assessment features new data from the 2003 bottom trawl survey and age-composition estimates from the 2002 echo-integration trawl (EIT) survey and the 2002 fishery. Estimates of weight-at-age were also revised and added to the assessment based on otolith age-determinations. The 2003 bottom trawl survey biomass estimate was 8.51 million metric tons (t), an increase of 77% from the 2002 estimate of 4.82 million t.

Based on analyses of EBS pollock stock productivity (and the uncertainty about productivity), the risk-averse ABC value for 2004 was estimated to be 2.56 million t (up 10% from 2.33 t for 2003). Although the survey index indicated a large increase in stock size, the increase was moderated considerably considering the large degree of uncertainty in the 2003 survey estimate. The current stock size appears to be stable and is projected to decrease in the near term. However, the authors noted that at current catch levels between 1.3 and 1.5 million t, the stock is expected to remain well above the B_35% level of spawning biomass.

The 2002 bottom trawl survey of the AI region resulted in a biomass estimate of 175,283 t, an increase from the previous estimate conducted in 2000. This results in an ABC and over fishing level (OFL) of 39,438 t and 52,585 t, respectively. A preliminary age structured assessment for pollock in the AI region was presented as an appendix to the SAFE. Preliminary results from this analysis suggest that the current ABC is sufficiently precautionary. It is anticipated that the assessment model results will be used for ABC determinations in future years. For Bogoslof Island pollock, the 2003 EIT survey of the region resulted in a biomass estimate of 198,403 t. The adjusted ABC (considering that this region is treated as an index site for the Bering Sea basin or Donut Hole stock) is 2,570 t.


BSAI Pacific Cod

The 2003 assessment updated last year’s assessment, incorporating new catch and survey information. This year’s EBS bottom trawl survey resulted in a biomass estimate of 605,681 t, a 2% decrease from last year’s estimate. The Aleutian Islands were surveyed in 2002; the biomass estimate was 82,853 t, representing a decrease of 39% from the 2000 estimate. The 2003 estimate of Pacific cod age 3+ abundance was 1,660,000 t. Projected 2003 female spawning biomass for the BSAI stock was 435,000 t, up about 3% from last year’s estimate. The NPFMC’s Scientific and Statistical Committee (SSC) has determined that reliable estimates of B_40%, F_40%, and F_35% exist for this stock, and that this stock therefore qualifies for management under Tier 3 of the BSAI Groundfish Fishery Management Plan (FMP). In 2003, the BSAI groundfish plan team and the NPFMC SSC recommended a conservative harvest policy where the ABC was set equal to the 2002 ABC of 223,000 t. This policy was continued in 2004. The recommended ABC was well below the maximum permissible ABC and OFL for the BSAI stock was 297,000 and 350,000 t, respectively.


BSAI Yellowfin Sole

The 2003 assessment incorporated new catch, age, and survey abundance information. This year’s EBS bottom trawl survey resulted in a biomass estimate of 2,284,800 t, an increase of 14% from last year’s survey. As in previous years, the catchability of yellowfin sole is modeled as a linear function of water temperature. Reliable estimates of B_40%, F_40%, and F_35% exist for this stock and, therefore, qualify for management under Tier 3 of the BSAI Groundfish FMP. The updated point estimates of B_40%, F_40%, and F_35% from the present assessment are 390,400 t, 0.115, and 0.138, respectively. Given that the projected 2004 spawning biomass of 446,400 t exceeds B_40%, the ABC and OFL recommendations for 2004 were calculated under sub-Tier “a” of Tier 3. The author recommended setting F_ABC at the F_40% (=0.115) level, which is the maximum permissible level under Tier 3a. Projected harvesting at the F_40% level gives a 2004 ABC of 113,400 t. The OFL was determined from the Tier 3a formula, where an F_35% value of 0.138 gives a 2004 OFL of 134,700 t. Model projections indicate that this stock is neither overfished nor approaching an overfished condition. Stock projections show the yellowfin sole stock will continue to decline despite low exploitation rates. The decline is due to the low recruitment in the last decade. A Ricker stock-recruitment relationship was fit within the stock assessment model. MSY was calculated from two different spawn-recruitment time-series. This information was used to evaluate the impacts of using a proxy for harvest levels (e.g., F_40%) compared to more direct estimates of stock productivity (i.e., Fmsy based on stock-recruitment pattern estimates).


BSAI Greenland Turbot

The current assessment updated last year’s assessment, incorporating new catch and survey information. The 2003 survey biomass estimate for the EBS shelf was 24,093 t. The 2002 estimates from the EBS slope and the Aleutian Islands were 27,589 t and 9,891 t, respectively. The updated point estimates of B_40%, F_40%, and F_35% from the present assessment are approximately 58,700 t, 0.26, and 0.32, respectively. The assessment authors recommended that the 2004 fishing mortality should be based on the most recent 5-year average of fishing mortality, resulting in a 2004 ABC of 4,740 t. This value was recommended because of uncertainty in the survey biomass estimates and concerns over shifting recruitment patterns since the early 1980s. The recommended OFL for this stock was 19,300 t. Model projections indicate that this stock is neither overfished nor approaching an overfished condition.


BSAI Arrowtooth Flounder

The 2003 assessment updated last year’s assessment with the incorporation of new catch, size composition, and survey abundance information. The 2003 EBS bottom trawl survey resulted in a biomass estimate of 553,900 t, a 55% increase relative to the 2002 trawl survey. In 2002, the RACE Division conducted an assessment of the EBS slope. The biomass estimate for arrowtooth flounder derived from this survey was 61,200 t. The 2002 biomass estimate for the Aleutian Islands stock was 88,700 t. However, this biomass estimate was not included in the model.

The slope survey biomass estimates were incorporated into the model by configuring q for the shelf and slope to be equal to 0.87 and 0.13, respectively. These values reflect the proportion of the EBS stock observed in the shelf and slope regions.

Model runs were evaluated with respect to the estimate of male and female selectivity for the shelf survey and the overall model fit. A prior was placed on the sex ratio estimated from the trawl surveys. The preferred model set the male natural mortality rate at 0.32 and the female natural mortality rate at 0.2. This model configuration was consistent with the author’s hypothesis that differences in sex ratios in observed catches are due to differential sex-specific survival rates.

Reliable estimates of B_40%, F_40%, and F_35% exist for this stock and, therefore, it qualifies for management under Tier 3 of the BSAI Groundfish FMP. The updated point estimates of B_40%, F_40%, and F_35% from the present assessment are 249,100 t, 0.28 and 0.36, respectively. Given that the projected 2004 female spawning biomass of 502,800 t exceeds B_40%, the ABC and OFL recommendations for 2003 were calculated under sub-Tier “a” of Tier 3. The author recommended setting F_ABC at the F_40% (= 0.28) level, which is the maximum permissible level under Tier 3a. Projected harvesting at the F_40% level gives a 2004 ABC of 114,600 t.

The OFL fishing mortality rate is computed under Tier 3a, F_OFL = F_35%= 0.38, and translates into a 2004 OFL of 141,500 t. Model projections indicate that this stock is neither overfished nor approaching an overfished condition.

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AFSC Quarterly
Research Reports
Oct-Nov-Dec 2003