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Groundfish Assessment Program

Annual Eastern Bering Sea Bottom Trawl Survey: Groundfish

Figure 1 map, see caption
Figure 1.  Click image to enlarge.

Figure 2, see caption
Figure 2.  Click image to enlarge.

Figure 3 map, see caption
Figure 3.  Click image to enlarge.

Figure 4, see caption
Figure 4.  Click image to enlarge.

The twenty-sixth in the series of annual bottom trawl surveys of the eastern Bering Sea (EBS) continental shelf was completed on 2 August 2007 aboard the AFSC chartered fishing vessels Arcturus and Aldebaran. Scientific staff from the AFSC, Alaska Department of Fish and Game, and the International Pacific Halibut Commission participated in the survey and completed standardized biological sampling of crab and groundfish resources at 376 stations (Fig. 1). Three-hundred fifty-six of these stations have been sampled annually since 1982, and the additional 20 stations in the northwest have been sampled every year since 1987 to investigate the northern distribution and abundance of opilio crabs and commercial fish species in response to the changing climate (Fig. 1).

Bottom temperatures on the EBS shelf were on average cooler in 2007 as compared to 2006, and the cold pool
(<2°C) extended farther south and east toward the Alaska Peninsula and into Bristol Bay (Fig. 2).

The largest catches of walleye pollock (Theragra chalcogramma) were concentrated along the northwest outer shelf and near the Pribiloff Islands where bottom temperatures were above 0°C (Fig. 3); large catches of walleye pollock were also observed north of the Alaska Peninsula near Unimak Island (Fig. 3).

Ninety-five percent of the trawl catches contained walleye pollock and the estimated total biomass increased to 4.16 million metric tons (t) in 2007 from 2.85 million t in 2006 (Fig. 4). Catches of walleye pollock from the inner and middle shelves were composed mainly of 1-year olds that ranged in size from 10 to 20 cm. Similar to walleye pollock, Pacific cod (Gadus macrocephalus) were broadly distributed across the EBS shelf and caught at nearly all stations. Total biomass declined for the second straight year (0.42 million t; Fig. 4); however, numbers of Pacific cod ranging in size from 10 to 20 cm were almost four times greater compared to 2006, which suggests there may be a large incoming year class. For all flatfishes except for yellowfin sole (Limanda aspera), the annual trend of total biomass declined from 2006 to 2007, but trends were less clear because of the variance associated with these estimates (Fig. 4).

In addition to abundance and biomass estimates, analyses of size and age composition will be completed for selected commercial groundfish species using collections of 64,903 length measurements and over 8,000 age structures. There were 22 special research projects conducted during the 2007 trawl survey (see April-May-June 2007 Quarterly Report for a list of the projects). For one of these projects, the Arcturus collected midwater acoustic data from transects across sampling grid cells where there were high densities of walleye pollock.

The AFSC Midwater Assessment and Conservation Engineering (MACE) Program is evaluating the feasibility of using acoustic data from annual EBS trawl survey trawl charter vessels for estimating the midwater component of walleye pollock biomass during years without acoustic surveys. The Groundfish Assessment and Recruitment Processes Programs are also collaborating on the routine collection of bongo plankton samples from the EBS survey charter vessels. Plankton tows were conducted once daily from the Arcturus during the first and second legs. The goal is to expand coverage of plankton tows to include both vessels for the entire survey area.

By Robert Lauth

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