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Midwater Assessment & Conservation Engineering Program

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

figure 5, click to enlarge
Figure 5.  Click image to enlarge.

figure 6, click to enlarge
Figure 6.  Click image to enlarge.

figure 7, click to enlarge
Figure 7.  Click image to enlarge.


Midwater Assessment & Conservation Engineering Program (MACE) MACE scientists completed an echo integration-trawl (EIT) survey of walleye pollock (Theragra chalcogramma) on the eastern Bering Sea shelf between 9 June and 7 August aboard the NOAA ship Oscar Dyson. The main purpose of the survey, which has been conducted since 1979, was to estimate the midwater abundance of walleye pollock.

The 2009 survey was conducted westward from Bristol Bay, Alaska, to the Cape Navarin region of Russia along north-south transects spaced at 20 nautical miles (nmi) apart (Fig. 5). During daylight hours, acoustic backscatter data were collected along transects at five individual echosounder frequencies (18, 38, 70, 120, and 200 kHz).

Opportunistic midwater and bottom trawls were conducted to classify the backscatter attributed to walleye pollock and other organisms. Physical oceanographic data were also collected. Walleye pollock abundance estimates were based on the area scattering detected at 38 kHz.

Nighttime activities included collection of additional physical oceanographic data and trawl hauls, and work with other specialized sampling devices (e.g., a lowered echosounding system to measure target strength, and the Simrad ME-70 multibeam sonar to characterize the seafloor and describe small-scale spatial patterns of the dominant scatterers).

Additional sampling in support of the Bering Sea Integrated Ecosystem Research Program (BSIERP) included conductivity-temeperature-depth (CTD) casts with fluorometer and oxygen samples, XBT casts, and underway water sample collections (salinity, chlorophyll fluorescence, oxygen, nutrients) to calibrate the shipboard seawater monitoring system.

Daytime Methot and Tucker trawl hauls were also conducted to assess the density of Bering Sea euphausiids in midwater and where acoustic estimates were not possible near the seafloor and sea surface. Protocols were developed to measure lengths of euphausiid individuals at sea using a dissecting scope and a modified flatbed scanner.

Temperature profile measurements indicated that 2009 was another cold summer. The range of ocean surface temperatures observed in 2009 (0.9°-8.9°C) was similar to that observed in 2008 (0.7°-8.3°C), and colder than that observed in 2007 (2.0°-10.9°C). Bottom temperatures were coldest on the inner shelf north of St. Matthew Island to about 178°W, and warmest just north of Unimak Island.

Catch composition for midwater and bottom hauls indicated that walleye pollock was the most abundant fish species captured by weight. The next most abundant fish species was Pacific ocean perch (Sebastes alutus), which were primarily captured in two hauls, one near Akutan Island and one on the east edge of Pribilof Canyon.

Walleye pollock were largely concentrated west of 170°W in U.S. waters (Fig. 5). Estimated walleye pollock abundance (to within 3 m of bottom) was relatively low compared to 2008 and prior surveys.

The 2009 biomass in the U.S. EEZ (Exclusive Economic Zone) was 0.924 million metric tons (t); the estimated 2008 biomass was 0.997 million t, and the 2007 biomass was 1.77 million t. The estimated pollock biomass in Russia was 0.005 million t, which represented only 0.6% of the total surveyed biomass. In 2008 about 3% and in 2007 about 6% of the total midwater biomass was in Russia.

Walleye pollock fork length (FL) composition differed by geographic area (Fig. 6). Fish between 35 and 70 cm with few smaller juveniles composed the biomass east of 170° (9.6% of total biomass). Pollock <19 cm FL (age-1 or 2008 year class) and 19-38 cm FL (ages 2-3) were the most numerous west of 170°W (89.8% of total biomass). The walleye pollock length composition for the fish observed in Russia ranged from 13 to 75 cm, but the majority of fish were between 20 and 60 cm.

An index of euphausiid abundance for the Bering Sea shelf for summers 2004-09 was generated from EIT survey backscatter at four frequencies (18, 38, 120, and 200 kHz) and from Methot trawl data (Fig. 7). The time series show that euphausiid backscatter has increased more than three-fold whereas declines were seen in both the midwater (EIT) walleye pollock backscatter and the age 3+ pollock biomass estimated by the stock assessment model. These euphausiid (prey) and pollock (predator) abundance trends may be related, or they may be independent responses to changes in environmental conditions.

Size and shape descriptors for juvenile walleye pollock aggregations (depth, height, width, surface area, volume, and the ratio of surface area to volume) were examined using the multibeam data. These data were analyzed as a function of fish lengths representing ages 1-, 2-, and 3-year olds. Preliminary results show that age-1 pollock formed the shallowest schools, and mixed age groups exhibited the most variation in school structure.

By Taina Honkalehto

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