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Midwater Assessment & Conservation Engineering Program

Acoustic-Trawl Survey of Walleye Pollock in the Eastern Bering Sea

Scientists with the Midwater Assessment & Conservation Engineering (MACE) program completed an acoustic-trawl (AT) survey of walleye pollock on the eastern Bering Sea shelf between 5 June and 7 August aboard the NOAA ship Oscar Dyson. The main purpose of the survey, conducted since 1979, was to estimate the midwater abundance of walleye pollock.

The 2010 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. 14). Acoustic backscatter data were collected along transects at five individual echosounder frequencies (18, 38, 70, 120, and 200 kHz) during daylight hours. Opportunistic midwater 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 backscatter detected at 38 kHz. Nighttime activities included collection of additional physical oceanographic data and trawl hauls, and work with other specialized sampling devices. Additional sampling in support of the Bering Sea Integrated Ecosystem Research Program (BSIERP) included conductivity-temperature-depth (CTD) casts with fluorometer and oxygen samples, expendable bathythermograph (XBT) casts, and underway water sample collections (salinity, chlorophyll fluorescence, oxygen, nutrients) to calibrate the shipboard seawater monitoring system. Daytime Methot and Tucker trawls were also conducted in support of an acoustics-based abundance estimate for euphausiids in midwater and where acoustic estimates were not possible near the seafloor and sea surface.

see caption
Figure 14. Transect lines with haul locations for Aleutian wing trawl, Methot trawl, and Tucker trawl during the summer 2010 acoustic-trawl survey of walleye pollock on the Bering Sea shelf. Transect numbers are underlined.

Temperature profile measurements indicated that summer 2010 was a warmer summer than 2008-09. Ocean surface temperatures observed in 2010 (1.8° – 12.3°C) were higher than those in 2009 (0.9° – 8.9°C) and 2008 (0.7° – 8.3°C), and the values ranged more widely as well. Bottom temperatures were coldest near St. Matthew Island (-1.6°C), cool along the shelf as far east as False Pass, and warmest late in the survey off Cape Navarin.

Catch composition for midwater hauls indicated that walleye pollock was the most abundant species captured by weight (89%). The next most abundant species was jellyfish (Chrysaora melanaster) at 10%. Jellyfish catch increased compared to 2009, when they comprised only 3% of the total catch by weight.

Pollock were found throughout the surveyed area, but most were found to the south and west of St. Matthew Island (Fig. 14). Estimated walleye pollock abundance (to within 3 m of bottom) was relatively high compared to recent surveys (2007-09). The 2010 preliminary pollock biomass estimate in the U.S. Exclusive Economic Zone was 2.323 million t. The 2009 biomass was 0.924 million t, and the estimated 2008 biomass was 0.997 million t. The estimated pollock biomass in Russia was 0.131 million t and represented 5.3% of the total midwater biomass; the 2009 estimate for Russia (0.005 million t) represented only 0.6% of the total surveyed biomass in that year.

Walleye pollock fork length (FL) composition differed by geographic area (Fig. 15). Fish between 35 and 70 cm with very few smaller juveniles made up the biomass east of 170°W (7.3% of total biomass). Length distributions were very similar for all fish west of 170°W (87.4% of total biomass) and those in Russia. The highest number of fish were in the 20-30 cm FL range (likely the 2008 year class), followed by the 2009 year class (<20 cm FL) and then by fish between 31 and 50 cm.

An experimental sideways-looking 70 kHz transducer was installed on the Oscar Dyson centerboard to investigate vessel-avoidance behavior by pollock during the survey. The degree to which pollock react to the vessel will be assessed by comparing pollock estimates of abundance and depth distribution to the side of the vessel with those observed directly below the vessel with a traditional downward-looking transducer. This method will enable pollock vessel-avoidance to be monitored continuously during a survey. First field tests of a non-extractive stereo camera-trawl system (Cam-Trawl) were also successfully completed during the survey. The Cam-Trawl consists of a stereo camera system attached to the back of a large trawl with no codend. The device is designed to determine the species identification, density, and size of animals as they pass by the camera and out through the rear of the open trawl.

see caption
Figure 15. Population numbers and biomass at length estimated for walleye pollock between 16 m from the surface and 3 m off the bottom from the summer Bering Sea shelf acoustic-trawl survey in three geographic regions.


By Taina Honkalehto and Abigail McCarthy

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