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Midwater Assessment & Conservation Engineering Program

Echo Integration-Trawl Survey of Walleye Pollock in the Eastern Bering Sea

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

MACE Program scientists completed an echo integration-trawl (EIT) survey of walleye pollock on the eastern Bering Sea shelf between 2 June and 31 July aboard the Oscar Dyson. The main purpose of the survey, which has been conducted since 1979, was to estimate the midwater abundance of walleye pollock.

The 2008 survey was conducted westward from Bristol Bay, Alaska, to the Cape Navarin region of Russia along north-south transects spaced at 20 nmi apart (Fig. 2). During daylight hours, acoustic backscatter data were collected along transects at five frequencies (18, 38, 70, 120, and 200 kHz), and opportunistic midwater and bottom trawls were conducted to classify the backscatter. Walleye pollock abundance estimates were based on the area scattering detected at 38 kHz. Nighttime activities included additional physical oceanographic data collections, trawl hauls for species classification, and work with other specialized sampling devices (e.g., a lowered transducer system for target-strength measurement, a free-drifting acoustic buoy to measure potential fish avoidance behavior to the vessel, and tests of a new generation multibeam sonar). Macrozooplankton and micronekton layers (principally euphausiids) were also sampled with a Methot trawl as part of the large, multidisciplinary BSIERP study.

figure 3, see caption
Figure 3.  Estimated numbers of walleye pollock (millions) by size class for the areas (a) east of long. 170°W, (b) west of long. 170°W, and (c) in the vicinity of Cape Navarin, Russia, during the 2008 echo integration-trawl survey on the eastern Bering Sea shelf.  All acoustic estimates are for fish in the water column to within 3 m of the seafloor.

Physical oceanographic data included trawl temperature-depth profiles, numerous CTD casts (e.g., temperature, salinity, and chlorophyll measurements) and XBT (expendable bathythermograph) profiles along selected transects. Temperature profile measurements indicated that 2008 was another cold summer, similar to 2007 and 2006.

Walleye pollock were largely concentrated west of 170°W in U.S. waters (Fig. 2). Estimated walleye pollock abundance (to within 3 m of bottom) was relatively low compared with recent surveys. Biomass in the U.S. EEZ (Exclusive Economic Zone)was 0.94 million t, which was a little over half that observed in 2007 (1.77 million t). The estimated pollock biomass in Russia was 0.03 million t, which represented only 3% of the total surveyed biomass. In 2007, about 6% of the total midwater biomass was in Russia.

Walleye pollock length composition differed by geographic area (Fig. 3). Large, older fish between 50 and 60 cm with a few juveniles comprised the biomass east of 170° (11% of total biomass was in this area). Age-2 fish (2006 year class) at 20 to 25 cm, some age-3 fish, and older fish between 40 and 55 cm numerically dominated the biomass west of 170°W (86% of total U.S. EEZ biomass was in this area).

Other notable fishes encountered during the survey included a few midwater schools of Arctic cod (Boreogadus saida) in the northwestern part of the survey area, along with the usual aggregations of Pacific ocean perch (Sebastes alutus) observed near Zhemchug Canyon.

After completion of the stock assessment survey portion of the 2008 survey, a paired codend experiment was conducted to examine effects of codend liner mesh size on catch composition. A vessel comparison experiment was also continued to assess whether differences occurred between acoustic (and trawl) data collected by the Oscar Dyson versus the Miller Freeman. The EBS EIT survey has traditionally been conducted with the Miller Freeman but because the Oscar Dyson has become the primary vessel for this survey since 2007, results of the vessel comparison experiment will provide valuable information to evaluate the consequences of this vessel change.

By Taina Honkalehto

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