AUKE BAY LABORATORY (ABL) (cont.)

Physical Characteristics and Ichthyofauna of Eelgrass Meadows in Southeastern Alaska: Establishing Long-term Monitoring Sites

Eelgrass (Zostera marina) meadows were sampled for physical characteristics and fish assemblages and mapped with GPS (global positioning system) at six sites in southeastern Alaska. Our goal was to evaluate physical habitat changes and fish use on both a seasonal basis and from one year to the next. This critical habitat is used extensively by many species (particularly by the egg, larval, and juvenile life stages) including forage and harvested species. Unfortunately, this habitat is vulnerable to human and natural disturbances.

Most sites were sampled in June 2001, 2002, and 2003, and in late January 2003. Surface sediments were also sampled for select contaminants (e.g., PCBs, PAHs, metals) at five of the sites in 2001. A total of 44 seine hauls yielded 58,902 fish of 45 species; 18 of the species are included in a salmon or groundfish fishery management plan in Alaska. The most abundant forage fish or commercially important species captured (mostly as juveniles) were chum salmon, Pacific herring, pink salmon, coho salmon, and Pacific sand lance; mean size of each of these species was less than 100 mm (fork length). The three most abundant noncommercial species captured were shiner perch (Cymatogaster aggregata), threespine stickleback (Gasterosteus aculeatus), and crescent gunnel (Pholis laeta). The median number of species captured was 15 in June and 13 in January. More fish were captured in summer than in winter; mean catch per seine haul was 1,623 fish in June compared to only 60 fish in January. The area of eelgrass meadows mapped with GPS ranged from 434 m² to more than 71,000 m². Eelgrass density ranged from 336 shoots/m² to 1,544 shoots/m², and dry biomass ranged from 36 g/m² to 71 g/m². Mean monthly water temperatures were usually lowest in March (range 3.8°-6.6°C) and highest in August or September (range 11.0°-12.8°C). Contaminants were either not detected or were in low concentrations at all sites.

Eelgrass provides important nearshore habitat for juveniles of many forage fish or commercially important species, especially in summer. Periodic resampling of these sites will allow resource managers to monitor gains or losses in eelgrass habitat and changes in fish communities that may result from human or natural disturbance.

By Scott Johnson and John Thedinga.


Alaska Sablefish Assessment

The final assessment for the combined sablefish stock from the Gulf of Alaska and the Bering Sea/Aleutian Islands region was prepared by ABL scientists during the last quarter of 2003 and presented to the North Pacific Fishery Management Council’s (NPFMC) Groundfish Plan Teams and Scientific and Statistical Committee. The assessment showed that sablefish abundance increased during the mid-1960s due to strong year classes from the late 1950s and 1960s. Abundance subsequently dropped during the 1970s due to heavy fishing; catches peaked at 56,988 metric tons (t) in 1972. The population recovered due to exceptional year classes from the late 1970s; spawning abundance peaked again in 1987. The population then decreased as these exceptional year classes died off.

The longline survey abundance index decreased 7% from 2002 to 2003. This decrease follows recent increases, so that relative abundance in 2003 is 10% higher than in 2000. The fishery abundance index also generally increased and is 6% higher in 2002 than in 2000 (2003 data are not available yet). Spawning biomass is projected to decrease slightly (<1%) from 2003 to 2004. Sablefish abundance is moderate; projected 2004 spawning biomass is 40% of unfished biomass. Abundance has increased from a low during the years from 1998 to 2000. The 1997 year class is an important part of the total biomass and is projected to account for 31% of 2004 spawning biomass. The 1998 year class likely is above average, although not as strong as the 1997 year class.

Because sablefish abundance has been low, we have previously recommended recent acceptable biological catches (ABCs) of less than the maximum permissible. Abundance now has increased to a moderate level. Abundance increased due to conservative quotas in previous years and the strong 1997 year class. The maximum permissible yield from an adjusted F_40% (mortality rate) strategy is 25,400 t for 2004 and 20,700 t for 2005. However, this 2004 maximum yield represents a substantial increase (22%) compared to the 2003 ABC, while abundance is projected to decrease slightly (1%). The probability that maximum permissible yield will reduce spawning biomass below the benchmark B_30% (biomass level) in 5 years is 0.27. We recommended a 2004 ABC less than the maximum permissible, either 23,000 t or 20,700 t for the combined stock. The 23,000 t ABC is a moderate increase (10%) compared with the maximum permissible ABC. This ABC increase represents a balance for a stock that is now at the target abundance but is also projected to decline, and the increase appears to be sufficiently risk-averse given that next year’s assessment will reevaluate the stock status. The 20,700 t ABC is similar to the 2003 ABC of 20,900 t; consistent with the abundance trend, this ABC is the most risk-averse of the two recommended 2004 ABCs. Abundance is projected to decline slightly in 2004 and continue decreasing thereafter. The 2004 ABC of 23,000 t was recommended for the combined Gulf of Alaska and Bering Sea/Aleutian Islands region stock by the committee and was accepted by the NPFMC at its December 2003 meeting.

By Michael Sigler.


Stock Assessment of Gulf of Alaska Slope Rockfish and Pelagic Shelf Rockfish

Updated stock assessments of slope rockfish and pelagic shelf rockfish in the Gulf of Alaska were completed in November 2003 by ABL scientists. In contrast to previous years, the stock assessment of slope rockfish was divided into three separate assessments: Pacific ocean perch, northern rockfish, and shortraker/rougheye and other slope rockfish.

The Pacific ocean perch assessment features a new index of biomass from the 2003 biennial trawl survey, age compositions from the 1998, 1999, and 2002 fisheries, and catch from the 2003 fishery. The 2003 survey biomass index was much lower and more precise than the previous three surveys. A preliminary evaluation of uncertainty presented in 2002 indicated some potential problems with model specification; to resolve these issues, the authors constructed a new length-age transition matrix and relaxed some model constraints. The base model from last year was contrasted with four alternative versions. A model that includes two length-age transition matrices, one for earlier length data and one for recent length data, and that estimates catchability (q) and natural mortality (M) simultaneously was chosen to compute ABC. The length-age matrix for the earlier data was constructed to reflect slower growth of Pacific ocean perch when biomass was high. This model fit the data much better than last year’s model, especially fishery size and survey age data. Compared to the other models this updated model fit the data better and provided reasonable estimates of q, M, and current and historical stock status. The Pacific ocean perch ABC for 2004 from the recommended model with harvesting at F_40% is 13,340 t. The estimated spawning biomass of 95,760 t is greater than B_40% (89,800 t), where B_40% is determined from the average recruitment of the 1977-97 year classes. Compared to the previous year’s assessment, the 2004 estimated biomass and ABC are similar but slightly lower

The assessment for northern rockfish used an age-structured model identical to that used in last year’s assessment. New input data included biomass estimates from the 2003 biennial trawl survey, fishery catch from 2002 and preliminary catch for 2003, age compositions from the 2001 biennial survey and 2002 fishery, and length compositions from the 2003 fishery. Based on the model, the estimated exploitable biomass and recommended ABC for Gulf of Alaska northern rockfish in 2004 are 95,149 t and 4,870 t, respectively. The northern rockfish stock is thought to be decreasing because of recent weak recruitment. Compared to 2003, the 2004 ABC decreased approximately 12%.

As in previous years, the assessments for shortraker/rougheye rockfish and other slope rockfish in the Gulf of Alaska were not based on modeling, but instead relied on biomass estimates provided by trawl surveys. Exploitable biomass for each of these two management groups was estimated by the average biomass in the three most recent biennial trawl surveys, excluding the estimated biomass in the 1-100 m stratum. The 1-100 m depth stratum was removed from the estimate because most rockfish in this stratum are small juvenile fish and, thus, are not considered exploitable. This results in an exploitable biomass of 73,000 t for shortraker/rougheye rockfish and 89,460 t for other slope rockfish. Applying a combination of F = M and F = 0.75M rates (depending on the species) to these values of exploitable biomass results in recommended ABCs for 2004 of 1,760 t for shortraker/rougheye rockfish and 3,900 t for other slope rockfish. The ABC for shortraker/rougheye was subsequently lowered to 1,318 t at the December 2003 NPFMC meeting to ensure that shortraker rockfish would not be proportionately over-harvested within the group.

A major change occurred in this year’s assessment for pelagic shelf rockfish in the Gulf of Alaska, as an age-structured model was used for the first time to determine exploitable biomass and ABC for light dusky rockfish, the predominant species in the assemblage. This model is a modified version of the northern rockfish model and was first developed in preliminary form in 2002. In 2003, substantial refinements were made to the 2002 base model, and all available data through 2003 were incorporated. The model estimate of current exploitable biomass for light dusky rockfish is 50,380 t, and recommended ABC for 2004 based on an F_40% harvest rate (0.123) is 4,000 t. Exploitable biomass for the three other species in the assemblage (dark dusky, yellowtail, and widow rockfish) is computed using their average biomass estimates for the last three biennial trawl surveys in 1999, 2001, and 2003, which total 7,020 t. Applying an F = 0.75M rate to this value of exploitable biomass yields a recommended ABC of 470 t. Therefore, for the pelagic shelf rockfish group as a whole, total exploitable biomass is 57,400 t, and recommended ABC is 4,470 t. This ABC is a decrease of nearly 19% compared to the 2003 value.

By Dana Hanselman, Dean Courtney, and Dave Clausen.

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AFSC Quarterly
Research Reports
Oct-Nov-Dec 2003