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Groundfish & Shellfish Assessment Programs

2010 Eastern Bering Sea Continental Shelf Bottom Trawl Survey

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Figure 1.  Click image to enlarge.


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Figure 2.  Click image to enlarge.


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Figure 3.  Click image to enlarge.


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Figure 4.  Click image to enlarge.


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Figure 5.  Click image to enlarge.


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Figure 6.  Click image to enlarge.


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Figure 7.  Click image to enlarge.
 
 

The 29th annual bottom trawl survey of the eastern Bering Sea (EBS) continental shelf was conducted between 2 June and 15 August 2010 aboard the chartered fishing vessels Alaska Knight, Aldebaran, and the Vesteraalen. This year's survey expanded northward to include Norton Sound and areas above St. Matthew Island and St. Lawrence Island (Fig. 1) extending west to the U.S.-Russia Convention Line. The purpose of sampling these additional northern stations was to establish baseline information of crab and groundfish species within the northern Bering Sea as a part of a study examining the loss of seasonal sea ice due to climate change. This survey represents the greatest annual coverage of the eastern Bering Sea shelf dating back to the start of the survey time series in the early 1970s.

Scientists from the AFSC, as well as the Alaska Department of Fish and Game, the International Pacific Halibut Commission, and Kawerak Inc. participated in the survey and completed standardized biological sampling of crab and groundfish resources at 142 northern extension stations in addition to the 376 standard annual stations. The northern extension expanded area coverage from the standard 144,493 square nautical miles (nmi2) to 200,088 nmi2, with depths ranging from 11 to 200 m. In addition, the Aldebaran returned to Bristol Bay to resample 23 stations between 24 and 29 July due to the delaying effects of colder than average water temperatures on the red king crab reproductive cycle.

Bottom temperatures measured during the survey ranged from -1.6° to 12.3°C (Fig. 2). Mean bottom temperatures of the standard shelf area in 2010 were slightly warmer (1.33°C) than in 2009 (1.21°C) but continued a cold trend that began in 2006 where the cold pool (<2C) has extended southward into the middle shelf and into Bristol Bay. In the northern shelf area, the cold pool was expansive, covering most of the area between St. Matthew Island and St. Lawrence Island. Nearshore waters above Nunivak Island and all of Norton Sound, however, were relatively warm with bottom temperatures exceeding 6°C (Fig. 2).

Data collections from the EBS shelf trawl survey included 192,000 individual length measurements representing 49 fish taxa; 9,991 age structures representing 17 fish taxa; 7,342 stomach samples representing 56 fish taxa; and 2,230 pathobiology samples from 42 different fish and invertebrate taxa.

Groundfish In the standard area, a majority of trawl catches contained walleye pollock (Theragra chalcogramma), where biomass estimates increased from 2.28 million metric tons (t) in 2009 to 3.75 million t in 2010. 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. Pacific cod abundance also increased, from an estimated 0.43 million t in 2009 to 0.87 million t in 2010, likely due the individual growth of many smaller sized cod observed in 2009. Pacific cod were more broadly distributed across the shelf, however, like pollock, they were less abundant where bottom temperatures were < 0°C (e.g., cold pool). Estimates of 2010 biomass for most of the major flatfish species, including yellowfin sole (Limanda aspera), northern rock sole (Lepidopsetta polyxystra), flathead sole (Hippoglossoides elassodon), arrowtooth flounder (Atheresthes stomias), and Greenland turbot (Reinhardtius hippoglossoides), increased by 18% to 115% compared to 2009 estimates. The most abundant of the flatfishes were yellowfin sole (2.37 million t) and northern rock sole (2.06 million t).

Northern area catches were smaller compared to the standard area, but distributions of some the major species including Alaska plaice (Pleuronectes quadrituberculatus) (Fig. 3), yellowfin sole (Fig. 4), and snow crab (Chionoecetes opilio) extended significantly into the northern shelf. The four most abundant species in terms of estimated biomass in the northern area were yellowfin sole (0.42 million t), snow crab (0.32 million t), purple-orange seastar (Asterias amurensis; 0.28 million t), and Alaska plaice (0.30 million t). Bering flounder (Hippoglossoides robustus), overall a less abundant species, was found to have just over 50% of its estimated biomass in the northern area (Fig. 5).

Commercial Crab In 2010, commercially important crab were caught at all but 21 of the standard stations (Fig. 6). Biomass estimates in metric tons of Bristol Bay legal male red king crab (Paralithodes camtschaticus) have decreased over the last 3 years from 33,541 t in 2007 to 21,347 t in 2010; mature males decreased from 34,262 t in 2009 to 30,248 t in 2010. Mature females in Bristol Bay increased between 2009 and 2010 from 28,758 to 40,797 t. Estimates for legal males in the Pribilof District show little change between 2009 (2,088 t) and 2010 (2,881 t); mature males increased from 2,454 to 3,107 t and mature female estimates have decreased from 590 t in 2009 to 379 t in 2010. The 2010 coefficients of variance (CV) of Bristol Bay mature male and female red king crab biomass estimates ranged from 16% to 27%; the Pribilof District mature male and female red king crab CVs ranged from 36% to 41%.

From 2009 to 2010, the estimated biomass of legal male blue king crab (P. platypus) in the Pribilof District increased from 170 to 202 t, while mature males decreased from 452 to 322 t, with a CV for legal and mature males of 46%48%. In the St. Matthew Island Section of the Northern District, both mature and legal-sized males increased from 2009 estimates of 4,622 and 2,390 t to 8,141 and 4,317 t, respectively, above the average estimates from the previous 20 years. The CV for the St. Matthew Island Section mature male biomass was 26%.

Mature male Tanner crab (Chionoecetes bairdi) abundance decreased between 2009 and 2010 from 30,281 to 27,949 t; mature females also decreased from 14,832 to 5,922 t. Legal males showed an increase from 7,027 to 7,955 t. The CVs for mature male and female biomass ranged from 16% to 20%. Based on the mature male biomass, the Tanner crab stock was declared overfished in September 2010, and based on the female biomass the fishery was closed for the 2010-11 fishing season.

Estimated biomass of legal male snow crab (C. opilio) decreased from 149,714 to 136,140 t between 2009 and 2010, while mature male biomass increased from 103,550 to 107,131 t. Mature female biomass also increased from 68,026 t in 2009 to 132,166 t in 2010, with CVs for mature males and mature female snow crab ranging from12% to 18%.

The 2010 biomass estimates for legal-sized males of commercial crab stocks in the standard survey area of the eastern Bering Sea are shown in Table 1.

Table 1.  The 2010 biomass estimates in metric tons (t) and pounds (lb) with 95% confidence intervals (+ 1.96*SE) for legal-sized males of commercial crab stocks in the eastern Bering Sea.
  2010 Legal-sized male biomass
Bristol Bay District red king crab (Paralithodes camtschaticus) 21,347 (7,504) (t) 47,062 (16,544) (lb)
Pribilof District red king crab (P. camtschaticus) 2,881 (2,049) (t) 6,351 (4,517) (lb)
Pribilof District blue king crab (P. platypus) 202 (191) (t) 445(421) (lb)
St. Matthew Island Section blue king crab (P. platypus) 4,317 (2,165) (t) 9,517 (4,773) (lb)
Southern Tanner crab, all Districts (Chionoecetes bairdi) 7,955 (3,172) (t) 17,536 (6,993) (lb)
Snow crab, all Districts (C. opilio) 136,140 (31,567) (t) 300,134 (69,593) (lb)
Snow crab, all Districts > 4.0 inches 88,788 (24,996) (t) 195,472 (55,106) (lb)

Because stock assessment models rely on growth increments and mating success, the delayed molt and mating cycle of female red king crab due to cold bottom water temperatures necessitated a return to Bristol Bay. By the time the resample was completed on 29 July, 96% of mature females had completed the molt and extruded new clutches.

Complete crab survey results from the EBS bottom trawl survey can be found on the AFSC website at http://www.afsc.noaa.gov/Kodiak/default.htm.

Northern Bering Sea crab catch distribution crab caught at each station by species is shown in Figure 7. Red king crab were caught at 22 of the 44 total stations within the Norton Sound Section of the Northern District. The 2010 biomass estimate of legal-sized males was 1,030 t, and was 1,656 and 263 t for mature and immature males, respectively. The biomass estimate of mature female red king crab was 347 t. The CV for all males and mature female biomass ranged from 31% to 49%.

The 2010 biomass estimate of legal-sized blue king crab males was 45 t, the estimate of mature and immature males was 566 and 590 t, respectively. The biomass estimate of mature female red king crab was 730 t. The CV range for blue king crab males and mature female biomass was 21%-100%. The majority of both mature and immature males and females were distributed off the northwest coast of St. Lawrence Island, while a smaller abundance of mature male and female blue king crab were distributed in shallow water south of Bering Strait.

Two immature Tanner crab, one male and one female, were caught during the survey at one station (AA-23; Fig. 7).

Snow crab were caught at 118 of the 145 total northern extension stations, although legal-sized males were only caught at one station (BB-07). The 2010 biomass estimates of legal-sized males was 8 t, of mature and immature males was 8 and 182,348 t, respectively; the biomass estimate of mature female snow crab was 30,277 t. The CVs for snow crab males and mature female biomass ranged from 13% to 100%. Both immature male and female snow crab were widely distributed, while the majority of mature females were caught in the southwestern portion of the NBS.

Crab Special Projects In addition to the assessment survey, a number of special projects were carried out to collect biological data on several crab species. Stomach samples from male and female Tanner crab as well as possible prey items were collected for diet studies. Hemolymph samples from red and blue king crab were taken for population genetics; samples were also collected from snow and Tanner crab to monitor bitter crab syndrome. Egg clutches from red king crabs were collected to investigate reproductive potential including egg quality and fecundity variability.

By Dan Nichol, Robert Foy, and Robert Lauth
 

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