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Shellfish Assessment & Groundfish Assessment Programs

Annual Eastern Bering Sea Continental Shelf Bottom Trawl Survey

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-eighth annual bottom trawl survey of the eastern Bering Sea (EBS) continental shelf was conducted between 28 May and 2 August 2009 aboard the chartered fishing vessels Arcturus and Aldebaran.

Scientists from the AFSC, Office of Sustainable Fisheries, 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 covering an area of 144,493 square nautical miles and bottom depths ranging from 20 to 200 m. In addition, both vessels returned to Bristol Bay to resample 32 stations between 27 and 30 July due to the delaying effects of colder than average water temperatures on the red king crab reproductive cycle (see shaded area in Fig. 1).

Trawl surveys of the EBS shelf started in the early 1970s when the Bureau of Commercial Fisheries and the commercial fishing industry worked together to develop a red king crab (Paralithodes camtschaticus) fishery by surveying different parts of the shelf. An environmental assessment related to oil exploration was the motivation for the first large-scale survey of the EBS shelf in 1975. Trawl surveys continued through 1981 with a nonstandard station distribution and using a slightly different survey trawl than is presently used.

Since 1982, 356 stations have been systematically sampled during the May-August time period using the same standardized set of survey trawl gear (Fig. 1). An additional 20 stations in the northwest were added to the standard area in 1987 to investigate the northern distribution and abundance of snow crab (Chionoecetes opilio) and commercial groundfish in response to the changing climate (Fig. 1).

Abundance estimates and analyses of size or age composition generated from this survey time series are used in stock assessments for management of commercial crab and groundfish species by the North Pacific Fishery Management Council.

Eastern Bering Sea Shelf Water Temperatures

Bottom temperatures measured during the survey ranged from -1.7° to 6.3°C (Fig. 2). The weighted average bottom temperature of the EBS shelf survey area was slightly higher in 2009 (1.23°C) compared to 2008 (1.13°C) with the cold pool (<2°C) extended southward down the middle shelf and into Bristol Bay.

Commercial Crab

  biologists measuring red king crab
Figure 4.  Biologists Jan Haaga (AFSC, Kodiak) and Molly Fox (University of Alaska Fairbanks) measure red king crab during the 2009 Bristol Bay retow after mature females had completed the mating and molting cycle and extruded new clutches.  Photo by Ken Weinberg.

Over 46,800 individual crab, including red king crab, blue king crab (P. platypus), snow crab, Tanner crab (C. bairdi), Tanner/snow hybrid (C. bairdi x opilio), and hair crab (Erimacrus isenbeckii) were sampled (Fig. 3); other biometric data such as shell and egg condition, weight, chela height, and presence of parasitism were also recorded.

From 2008 to 2009, estimates of legal red king crab abundance in the Bristol Bay and Pribilof Districts decreased, with a substantial decrease in the number of small males in the Bristol Bay District. Female red king crab in both districts also showed an overall decrease in abundance.

Legal blue king crab in the Pribilof District showed a slight increase; legal blue king crab in the St. Matthew District decreased while pre-recruit males increased in abundance. It is noted that estimates for blue king crab have low precision due to the few numbers of crabs caught during the survey.

Legal pre-recruit and large female Tanner crabs in all districts decreased since 2008, and there was a slight increase in the number of small females. Legal male snow crab abundance increased slightly while small female snow crab abundance increased significantly in all districts.

The 2009 point estimates for all legal-sized males of commercial crab stocks in the eastern Bering Sea are shown in Table 1.

Table 1.  The 2009 point estimates (± 1.96 SE) for legal-sized
males (millions of crab) of commercial crab stocks in the EBS.
  2009 legal-sized males
(millions of crab)
Bristol Bay District red king crab 8.5 ± 6.6
Pribilof District red king crab 0.7 ± 0.9
Pribilof District blue king crab 0.07 ± 0.08
St. Matthew Island Section blue king crab 1.4 ± 0.7
Tanner crab, all districts 6.9 ± 2.8
Snow crab, all districts 371.8 ± 86.4
Snow crab, all districts > 4.0 inches 130.1 ± 42.4

For the fourth year in a row, a return to Bristol Bay to resample predetermined stations for red king crab was required. Many of the mature female red king crab caught at the beginning of the survey had not extruded new clutches of eggs due to cold water temperatures.

Because spawning stock assessment models rely upon future recruitment and mating success, the delay of the new clutches necessitated the return to Bristol Bay. At the time of the resampling, virtually all mature females had completed the mating and molting cycle and extruded new clutches (Fig. 4).

These data with expanded description of the survey and additional station information are available in the NOAA Technical Memorandum "The 2009 Eastern Bering Sea Continental Shelf Bottom Trawl Survey: Results for Commercial Crab Species" posted on the AFSC Web site at


Data collections from the EBS shelf trawl survey included 149,098 individual length measurements representing 22 fish taxa; 9,264 age structures representing 13 fish taxa; 5,622 stomach samples representing 19 fish taxa; and 1,212 pathobiology samples from 40 different fish and invertebrate taxa.

A majority of trawl catches contained walleye pollock (Theragra chalcogramma), although the estimated total biomass decreased from 3.03 million metric tons (t) in 2008 to 2.28 million t in 2009.

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. There was a marginal increase in Pacific cod total biomass from 0.40 to 0.42 million t and a much higher proportional increase in population due to higher numbers of 15-20 cm and 40-50 cm Pacific cod.

For yellowfin sole (Limanda aspera), rock sole (Lepidopsetta spp.), flathead sole (Hippoglossoides spp.), arrowtooth flounder (Atheresthes stomias), and Greenland turbot (Reinhardtius hippoglossoides), the estimates of total biomass declined 13%-25% from levels estimated for 2008. A 4% increase in biomass was observed for Alaska plaice (Pleuronectes quadrituberculatus) and a 21% increase in Pacific halibut (Hippoglossus stenolepis).

Special Projects

In addition to standard survey operations, numerous special research projects were conducted during the 2009 trawl survey. It was the fourth year in a series for collecting acoustic data on midwater walleye pollock to augment the biennial Midwater Assessment and Conservation (MACE) Program echo integration-trawl time series, and also the fourth consecutive year for collecting summer bongo samples to monitor distribution and abundance of zooplankton on the EBS shelf. It was also the second year for collecting a synoptic environmental dataset for the National Science Foundation (NSF)/North Pacific Research Board (NPRB) Bering Sea Integrated Ecosystem Research Program (BSIERP).

Profiles of light intensity, water temperature and salinity were collected by attaching a light meter and conductivity-temperature-depth (CTD) profiler to the bottom trawl headrope. A sea chest onboard the Aldebaran also made continuous measurements of surface temperature, salinity, nitrate, chlorophyll and dissolved oxygen during the course of the survey.

Several special crab studies were conducted including reproductive potential of snow and red king crab, hemolymph collections to monitor bitter crab syndrome, and population genetics studies of Paralithodes spp.

From 19 to 25 July, the AFSC also participated in a cooperative study with the Bering Sea Fisheries Research Foundation to estimate the efficiency of the EBS survey trawl for snow crab (see following report).

By Bob Foy and Bob Lauth

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