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Midwater Assessment & Conservation Engineeering (MACE) Program

Winter Surveys in the Gulf of Alaska

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Figure 1. Acoustic backscattering (SA) attributed primarily to walleye pollock (vertical lines) along transects during the February 2010 acoustic-trawl surveys of the Shumagin Islands and Sanak Trough in the Gulf of Alaska.  Click to enlarge map.
 
   
see caption  
Figure 2 . Acoustic backscattering (SA) attributed primarily to walleye pollock (vertical lines) along transects during the March 2010 acoustic-trawl surveys of Marmot Bay, Shelikof Strait and the Gulf of Alaska shelf break from Barnabus Trough to Chirikof Island.  Click to enlarge.
 
   
see caption  
Figure 3. Acoustic backscattering (SA) attributed primarily to walleye pollock (vertical lines) along transects during the March 2010 acoustic-trawl surveys of the Gulf of Alaska along the southern edge of the Kenai Peninsula and throughout Prince William Sound. Click to enlarge.
 

Scientists from the Midwater Assessment and Conservation Engineering (MACE) Program conducted another in a series of winter echo integration-trawl surveys aboard the NOAA ship Oscar Dyson in the Gulf of Alaska (GOA). These surveys provide updates on the abundance, distribution, and biological composition of prespawning walleye pollock (Theragra chalcogramma). Areas surveyed between 22 February and 9 March 2010 included the Shumagin Islands (Shumagin Trough, Stepovak Bay, Renshaw Point, Unga Strait, and West Nagai Strait), Sanak Trough, Morzhovoi Bay, Pavlof Bay, Kenai Peninsula bays, Prince William Sound, and Marmot Bay. Areas surveyed between 18 and 30 March were Shelikof Strait and the shelfbreak east of Chirikof Island. All surveys were conducted 24 hours per day.

Preliminary survey results indicated that the 2010 walleye pollock abundance in the Shumagin Islands area was roughly one-third of the 2009 estimate (a 5-year high) but was more comparable to what was observed in 2007. Dense juvenile schools were detected in easternmost Shumagin Trough (Fig. 1). Adult walleye pollock densities were again low off Renshaw Point, where the highest quantities of adults had been detected prior to 2007. The length distribution of walleye pollock in the Shumagin Islands area consisted primarily of fish between 27 and 43 cm fork length (FL; age 3, 4, and 5), and a mode at 23-cm FL (age 2). In Sanak Trough the 2010 biomass estimate fell between the 2008 and 2009 estimates. The majority of the biomass in Sanak Trough was located over the center of the trough (Fig. 1) and the size composition was unimodal, with most fish between 50 and 70 cm FL. Morzhovoi Bay biomass estimates were less than two previous surveys (2006 and 2007) and few pollock were encountered in Pavlof Bay.

The preliminary walleye pollock biomass estimate in Shelikof Strait was almost twice that reported in the previous 3 years, and for the first time since 2000, the highest fish densities were observed along the west side of the strait proper between Cape Unalishagvak and Cape Kuliak (Fig. 2). Dense aggregations were also detected along the eastern sides of the southernmost transects. In the Shelikof Strait area, the size composition consisted of a mixture of 20-40 cm FL (age 2 and 3), and larger adult fish. The latter dominated the deepest part of the strait between Cape Kuliak and Wide Bay, and the younger fish dominated elsewhere. In Marmot Bay at Kodiak Island, dense midwater pollock schools were detected to the northwest of Spruce Island and in Spruce Gully. The size composition for Marmot Bay was dominated by fish measuring 40-50 cm FL.

The 2010 pollock biomass estimate along the GOA shelfbreak in the vicinity of Chirikof Island was substantially higher than the 2009 estimate but still approximately half that seen in 2008.
All of the bays along the southern edge of Kenai Peninsula that were surveyed contained aggregations of mostly adult walleye pollock, as did the eastern side of Prince William Sound (Fig. 3). Most of the fish in Prince William Sound were below 400 m in the water column. This was the first winter MACE survey of the Kenai Peninsula bays and Prince William Sound using an echosounder that effectively sampled deeper than 400 m.

Preliminary analysis of maturity stages indicated that survey timing was appropriate for all areas except Sanak Trough and Morzhovoi Bay, where relatively large percentages of the post-spawning adult females were caught.

By Michael Guttormsen


 

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