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Resource Assessment & Conservation Engineering (RACE) Division

Midwater Assessment & Conservation Engineering (MACE) Program

Oscar Dyson Gulf of Alaska Survey

An echo integration-trawl survey of the GOA was conducted 1-30 July 2005 using the NOAA research vessel Oscar Dyson. The principal objective of the survey was to collect echo integration and trawl data to estimate walleye pollock abundance and distribution. The survey originally began on 1 June but the vessel returned to Kodiak on 3 June because of mechanical problems. Repairs were completed and the cruise resumed on 1 July. Pre-existing equipment problems mandated a 5-nautical mile (nmi) minimum distance from shore for safety concerns. This limitation prevented the vessel from surveying along the proposed trackline where significant pollock quantities were found during the 2003 GOA survey, including the Shumagin Islands area, Nakchamik Island, Alitak Bay, and Marmot Bay.

Acoustic data were collected with a Simrad EK60 quantitative echosounding system operating at four frequencies (18, 38, 120, and 200 kHz). Results presented here are based on 38 kHz data. Midwater and near-bottom echosign was sampled using an Aleutian Wing (AWT) 30/26 midwater trawl. On- or near-bottom echosign was sampled with a Poly Nor’eastern (PNE) trawl with roller gear. A midwater Methot trawl was used to target age-0 pollock and macrozooplankton.

The survey design consisted of parallel transects at 20-nmi spacing along the GOA shelf and shelfbreak from the Islands of Four Mountains eastward to near the Shumagin Islands and then along the shelfbreak and seaward from the Shumagin Islands to Amatuli Trough (Fig. 2 below). Alternate transect pairs extended about 30 nmi beyond the shelf break. Barnabas and Chiniak Troughs were also surveyed using 6-nmi spacing and Shelikof Strait was surveyed using 10- or 20-nmi spacing. Echo integration and trawl data were collected along the transects only during daylight hours. Nighttime operations included additional trawling effort, physical oceanographic sampling, and rerunning portions of the tracklines for day-night comparisons.

backscattering maps
Figure 2.  Relative backscattering attributed to pollock along tracklines during the 2005 echo integration-trawl survey in the Gulf of Alaska, OD2005-01.

Pollock was the most abundant species caught in the 28 AWT hauls, comprising 59% of the total catch by weight, followed by Pacific ocean perch (35%). In the 10 PNE trawl hauls, Pacific ocean perch (32%) was the most abundant species caught, followed by pollock (20%) and Atka mackerel (20%). Euphausids (41%) were the dominant species in Methot trawls. Most of the adult pollock echosign was detected in Chiniak and Barnabas Troughs and along the shelfbreak south of Kodiak Island (Fig. 2). Dense age-0 and age-1 echosign was detected in the southern part of the Shelikof Strait sea valley. No pollock were detected along the offshore transects.

By Mike Guttormsen


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