Results of the echo integration-trawl survey of walleye pollock (Theragra chalcogramma) on the U.S. and Russian Bering Sea shelf in June and July 2007

Abstract

Eastern Bering Sea shelf walleye pollock (Theragra chalcogramma) abundance and distribution in midwater were assessed between 2 June and 30 July 2007 using echo integration-trawl techniques aboard the NOAA ship Oscar Dyson. The survey also assessed walleye pollock in the Russian Cape Navarin area.  In general, ocean conditions were cold in 2007, as in 2006.  Surface waters averaged 5.0°C in June and early July, but warmed to an average of 8.9°C in mid- to late July.  The water temperature at 60 m did not exceed 3°C, except for parts of the outer shelf and slope between Unimak Island and Zhemchug Canyon. Few walleye pollock were found in the survey region between Port Moller and central Unimak Island.  A few very dense aggregations were observed along the 100-m isobath east of the Pribilof Islands. More pollock were observed west of the Pribilof Islands on the outer shelf, where a large aggregation of 1-year-olds was observed.  Walleye pollock adults and juveniles (primarily age-1 and a few age-2s and -3s) were also abundant in a large area west of St. Matthew Island.  Estimated walleye pollock abundance in midwater (between 14 m from the surface and 3 m off bottom) in the U.S. EEZ portion of the Bering Sea shelf was 9.21 billion fish weighing 1.77 million metric tons (t); in the Russian EEZ, it was 1.08 billion fish weighing 0.10 million t (6% of the total midwater biomass). East of 170°W (13% of total biomass) the predominant length mode was 50 cm.  West of 170°W within the U.S. EEZ (81% of total biomass) modal lengths were 14 and 45 cm.  In Russia modal lengths were similar to those in the western U.S. EEZ.  Fewer walleye pollock were observed east of 170°W than in recent years.  Inside the U.S. EEZ, juveniles were dominant numerically (61% age-1s, and 11% age-2s) but these two age groups represented only 11% of the total biomass.  By contrast, ages 3+ totaled 28% of the population numerically, and made up 89% of the total biomass.  Vertical distribution of walleye pollock is discussed, as well as horizontal and vertical distribution of non-pollock backscatter.

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