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

2003 Stock Assessment Summary (cont.)

BSAI Shortraker/Rougheye Rockfish

The assessment of northern rockfish was treated as a separate chapter; therefore, the remaining species of the “other red rockfish” complex, shortraker and rougheye rockfish, were considered jointly in a chapter of the SAFE. A Kalman filter rather than a straight averaging of survey biomass estimates was utilized in the assessment of the shortraker/rougheye complex. The data were updated with 2002 and 2003 catch statistics. For shortraker/rougheye, the F_ABC was set at the maximum value allowable under Tier 5, which is 75% of M. Accepted values for M are rougheye rockfish-0.025, and shortraker rockfish-0.030. Multiplying these rates by the best estimates of species-specific biomass gives the following 2004 ABCs:

BSAI Other Rockfish

Assessment authors conducted a detailed examination of the available information for assessment of the status of the other rockfish complex. Catch distribution maps were created for the light dusky rockfish and shortspine thornyhead complex. In 2002, sharpchin rockfish were added to the other rockfish complex. Eight species have been confirmed or tentatively identified in catches from the BSAI. These species include sharpchin rockfish, dark dusky rockfish, light dusky rockfish, harlequin rockfish, red banded rockfish, redstripe rockfish, yelloweye rockfish, and shortspine thornyhead. No new biomass estimates were available in 2003 for this assessment. The authors recommended breaking shortspine thornyhead out of the other rockfish category. The authors noted that the available information for shortspine thornyheads would place this stock in Tier 5. Authors recommended that management of the other rockfish category (excluding shortspine thornyhead) should be based on a Tier 6 estimate. Based on this recommendation the ABCs and OFLs for 2004 were as shown in the tables below.

BSAI Squid and Other Species Complex

The squid assessment was updated by incorporating new catch information. The 2002 squid catch of 1,748 t was approaching the ABC of 1,970 t. Squid are managed under Tier 6, where the OFL is set equal to the average catch from 1978 through 1995, and ABC is constrained to be no greater than 75% of OFL. The average catch from 1978 through 1995 was 2,624 t. The maximum permissible value of ABC for 2004 is 1,970 t which is the Plan Team’s recommended value.

The “other species” assessment includes sharks, skates, sculpins, and octopi. The assessment was updated by incorporating new catch and survey biomass information from the slope and shelf trawl surveys. For the 2004 fishing year, the author recommended splitting the other species complex into separate ABC and OFLs for sharks, skates, sculpins, and octopus based on survey biomass estimates under Tier 5. If this strategy proved unworkable, the author’s recommended ABC for the other species complex was calculated as 0.75 times the average catch from 1978 to 1995, or 19,320 t. The author’s recommended overfishing level for the other species complex in the year 2004 was calculated as the average catch from 1978 to 1995, or 25,760 t. The rationale for a Tier 6-based ABC and OFL is that there are no reliable estimates of natural mortality for a species complex containing animals with such extremely diverse life histories. The Plan Team recommends setting F_ABC for sculpins and skates at the maximum allowable under Tier 5, which is 75% of M. The NPFMC SSC recommended that this strategy be reviewed.


AI Atka Mackerel

The 2003 AI Atka mackerel stock assessment incorporated new catch data, and the 2002 fishery age composition. The 2002 AI trawl survey biomass estimate was 772,798 t, a 51% increase from the 2000 estimate. Age composition data indicate that the 1999 year-class may be above average.

The 2003 assessment was implemented using AD Model Builder as a modeling platform. Analysis showed the model results were sensitive to the selectivity pattern. To provide a more robust selectivity pattern for the projection, an average pattern for the years 1999-2002 was used.

The SSC has determined that this stock is qualified 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 83,800 t, 0.67, and 0.833, respectively. Projected female spawning biomass for 2004 is 86,000 t, placing Atka mackerel in sub-Tier “a” of Tier 3. The maximum F_ABC value allowable under Tier 3a is F_40% (0.67). Projected harvesting at a fishing mortality rate of 0.67 gives a 2004 catch of 66,700 t, which is the maximum permissible value of ABC under Tier 3a.

The OFL was determined from the Tier 3a formula, where an F_35% value of 0.83 gives a 2004 OFL of 78,500 t. Model projections indicate that this stock is neither overfished nor approaching an overfished condition.


GOA Pollock

The 2003 GOA pollock assessment was updated by incorporating new catch, age composition, and biomass estimates. The 2003 EIT trawl survey of the Shumagin Islands provided a biomass estimate of 135,600 t. The 2003 Shelikof Strait biomass estimate of age-2+ fish was 270,200 t, an increase of 18% from the 2002 biomass estimate. An additional 186,400 t of pollock were encountered during exploratory transects in the four regions: Shumagin Islands, Sanak, the shelf break and Middelton Island. The 2003 NMFS bottom trawl biomass estimate for pollock west of 140^o W was 300,690 t, an 84% increase from 2001. The 2003 preliminary ADF&G trawl survey biomass estimate was 66,989 t, a 30% decrease from the 2002 biomass estimate. Evidence from these surveys indicates that the 1999 year class will be above average.

The stock assessment model was unchanged in 2003. Model exploration focused on evaluating the contribution of each survey time series to the assessment. An evaluation of the appropriateness of fishing mortality and stock biomass reference points used to manage Gulf of Alaska pollock was presented in an Appendix. The model estimate of 2004 spawning biomass was 195,350 t, which is 31% of the estimated unfished spawning biomass. Under the new RPA control rule approved by the Council, pollock fishing in the Gulf of Alaska would have to be stopped if biomass declines to 20% of unfished. Spawning biomass is projected to be below the B_40% value of 248,000 t. This places GOA pollock in Tier 3b. The authors’ 2004 ABC recommendation for pollock west of long. 140^oW was 65,660 t, nearly equal to last year’s projected ABC for 2004.

For walleye pollock in Southeast Alaska (East Yakutat and Southeastern areas), the ABC recommendation is unchanged at 6,520 t. Pollock in the Southeast Outside and East Yakutat areas fall into Tier 5. Under the Tier 5 approach, the 2003 ABC is 6,520 t, based on exploitable biomass of 28,979 t as derived from CPUE data during the 2003 GOA trawl survey and a natural mortality estimate of 0.30. The OFL is 8,694 t.

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AFSC Quarterly
Research Reports
Oct-Nov-Dec 2003