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Midwater Assessment & Conservation Engineering Program

Summer 2007 Pollock Acoustic-Trawl Survey

Figure 7 map, see caption
Figure 7.  Click image to enlarge.

Figure 8 map, see caption
Figure 8.  Click image to enlarge.

Midwater Assessment and Conservation Engineering (MACE) Program scientists conducted an acoustic-trawl survey from 2 June to 30 July 2007 on the Bering Sea shelf between 161°W and 175°E aboard the NOAA ship Oscar Dyson. The principal objective of the survey was to collect echo integration and trawl data to estimate the abundance and distribution of walleye pollock. The survey design consisted of generally north-south oriented, parallel transects spaced 20 nautical miles (nmi) apart (Fig. 7).

Acoustic and trawl data were collected during daylight hours. Nighttime operations included additional trawling, conductivity-temperature-depth-fluorometer (CTD) measurements and gear tests of a multiple opening/closing codend device for large trawls. Additional research projects included deployment of an underwater video recording system to examine trawl effects on the seafloor, trawl selectivity experiments, collection of acoustic-doppler-current-profiler (ADCP) data to investigate pollock movement, and measurement of underwater light level data to assess the effect of light intensity and light penetration on the distribution and behavior of walleye pollock.

Seabird observers recorded underway line-transect data on seabird abundance throughout the cruise. Permission was received to survey Russian waters in the Cape Navarin region, and a guest Russian scientist from TINRO-Centre in Vladivostok participated aboard the Oscar Dyson. Very few walleye pollock were observed on the eight easternmost transects between Bristol Bay and north of Unimak Island (Fig. 7). Walleye pollock numbers increased slightly north of Unimak Pass and along the 100-m isobath surrounding the Pribilof Islands.

West of the Pribilof Islands, pollock concentrations increased gradually towards the north and west. A large aggregation of age-1 juveniles was observed along the southern section of transect 19. The highest concentrations of pollock were observed between 175 and 179W. Adult walleye pollock (larger than 30-cm fork length (FL)) were most abundant offshore of the 100-m isobath along transects 24 and 25, and juveniles (smaller than 30-cm FL) were most numerous on transect 27. Walleye pollock size composition ranged from 10- to 74-cm FL across the shelf (Fig. 8).

East of the Pribilof Islands (east of 170W) the dominant length mode was 49-cm FL. West of 170W the dominant length modes were 45-cm FL for adult walleye pollock and 14-cm for juveniles.

In the Cape Navarin area of Russia, juveniles dominated the population numerically, with modes at 15 and 21 cm, followed by adults at 44 cm. Of the total estimated 2007 walleye pollock biomass in the Bering Sea, 13% was east of 170W, 82% was west of 170W in U.S. waters, and 5% was in Russian waters. More juvenile walleye pollock, especially age-1s, were observed in 2007 than in 2006.

In addition, several days were dedicated to trawl-oriented research projects. Trawl selectivity experiments were carried out to estimate pollock escapement from the Aleutian wing trawl (AWT), using 1-inch monofilament pocket nets attached to different parts of the trawl meshes, a DIDSON acoustic imaging device, and still stereo camera to observe fish size and position.

Two trials were completed; one with eight and the second with six trawl hauls. The results of these trials will be used to estimate differential escapement of small pollock relative to that of large pollock. Additionally, field tests were conducted with a multiple opening/closing codend device (MOCC) attached to the AWT. The MOCC is controlled by an acoustic link to the ship and contains three codends allowing up to three discrete samples to be collected during a single trawl haul.

Four replicate trawl hauls of three codends each were made to estimate the transit time for fish to pass through the length of the AWT and into the MOCC in order to determine the appropriate time interval for tripping the MOCC codends. Initial results suggested that about 10 minutes were required for fish to transit the AWT.

By Taina Honkalehto

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