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Groundfish Assessment Program

Annual Eastern Bering Sea Continental Shelf Bottom Trawl Survey: Groundfish

Figure 1, click to enlarge
Figure 1.  Click image to enlarge.

Figure 2, click to enlarge
Figure 2.  Click image to enlarge.

Figure 3, click to enlarge
Figure 3.  Click image to enlarge.

The twenty-seventh in the series of annual bottom trawl surveys of the eastern Bering Sea (EBS) continental shelf was completed on 28 July 2008 aboard the AFSC chartered fishing vessels Arcturus and Aldebaran. Scientists 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 (snow) crabs and commercial fish species in response to the changing climate (Fig. 1). Abundance and biomass estimates and analyses of size and age composition will be generated for selected commercial groundfish species and used in the annual stock assessments of EBS groundfish.

Data collections from the EBS shelf trawl survey included: 163,523 individual length measurements representing 22 fish taxa; 8,427 age structures representing 12 fish taxa; 6,730 stomach samples representing 12 fish taxa, and 3,442 pathobiology samples from 6 different fish and invertebrate taxa. In addition to standard survey operations, there were 25 special research projects conducted during the 2008 trawl survey including the third year in a series for two projects: collecting acoustic data on midwater walleye pollock (Theragra chalcogramma) to augment the biennial Midwater Assessment and Conservation (MACE) Program echo integration-trawl time series, and collecting summer bongo samples to monitor distribution and abundance of zooplankton on the EBS shelf.

It was also the first year for collecting a synoptic environmental dataset for the North Pacific Research Board (NPRB) Bering Sea Integrated Ecosystem Research Program (BSIERP). Data collections included profiles of light intensity, water temperature, and salinity for each trawl station. Using a newly installed sea chest, the Aldebaran made underway surface temperature, salinity, nitrate, chlorophyll and dissolved oxygen measurements during the survey. In addition, temperature and salinity profiles were collected from most of the bottom trawl operations by attaching a conductivity-temperature-depth (CTD) profiler to the bottom trawl headrope.

Bottom temperatures on the EBS shelf were on average cooler in 2008 as compared to 2007 (Fig. 2), and the cold pool (<2°C) extended farther south and east toward the Alaska Peninsula and into Bristol Bay. The cooler than average bottom temperatures also delayed molting and spawning of female red king crab in Bristol Bay, so 32 stations had to be resampled at the end of the survey after female red king crab had completed molting and spawning (Fig. 1).

Ninety-five percent of the trawl catches contained walleye pollock; the estimated total biomass decreased to 3.03 million metric tons (t) in 2008 from 4.34 million t in 2007 (Fig. 3). 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. The largest catches of walleye pollock were concentrated along the northwest outer shelf and near the Pribilof Islands where bottom temperatures were above 0°C; large catches of walleye pollock were also observed north of the Alaska Peninsula near Unimak Island.

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 third straight year (0.40 million t; Fig. 3); however, the high abundance of 25 to 40 cm Pacific cod indicated a strong incoming 2006 year class. For all flatfishes except for Alaska plaice (Pleuronectes quadrituberculatus), the estimates of total biomass declined from 2007 to 2008, but trends were less clear because of the variance associated with these estimates (Fig. 3).

By Robert Lauth

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