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Midwater Assessment & Conservation Engineering Program

Eastern Bering Sea Acoustic-trawl Survey

figure 3, click to enlarge
Figure 3,  Acoustic survey trackline for the 2007 acoustic-trawl survey of the eastern Bering Sea shelf.

Midwater Assessment and Conservation Engineering (MACE) Program scientists began an acoustic-trawl survey of the eastern Bering Sea aboard the NOAA ship Oscar Dyson on 1 June. This survey will continue through 30 July 2007, spanning the area along the outer Bering Sea continental shelf from outside of Bristol Bay to the U.S.-Russia Convention Line. Permission to survey in the Russian exclusive economic zone (EEZ) has been granted – allowing the survey area to be extended into the Navarin shelf region.

The principal objective of the survey is to collect echo integration and trawl data to estimate the midwater abundance and distribution of walleye pollock (Theragra chalcogramma). The survey design consists of north-south oriented, parallel transects spaced 20 nmi apart (Fig. 5). In support of the survey objectives, acoustic and trawl data are collected during daylight hours. Nighttime operations – in addition to some limited daylight research time - include additional trawling, target strength data collection, gear tests of a multiple opening/closing codend device, net selectivity experiments, and CTD deployments.

By Neal Williamson

Acoustic Surveys in Ice-Covered Areas of the Eastern Bering Sea

figure 4, click to enlarge
Figure 4, click image to enlarge

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

Scientists from the MACE program conducted a study of the abundance and distribution of fish and plankton in the eastern Bering Sea in relation to seasonal sea ice cover. The goal of this work was to improve our understanding of the relationship between the abundance of fish and other animals and the position of the Bering Sea ice edge. Although there are major interannual changes in ice extent and ice persistence in the Bering Sea, little is known about the impacts of ice cover on the distribution of fish and plankton during winter months when much of the shelf is covered by ice.

Previous work has shown that many species in the Bering Sea avoid the cold temperatures, and it is likely that areas of melting sea ice act as a barrier for fish due to the formation of very cold water at all depths. Because ice cover makes it difficult to sample fish with conventional sampling gear such as nets, acoustics was used measure the distribution of fish in ice-covered waters.

MACE scientists in collaboration with the vessel’s crew surmounted several technical challenges and instrumented the U.S. Coast Guard (USCG) icebreaker Healy with scientific fishery echosounders. The equipment was used during an 10 April - 12 May cruise in the eastern Bering Sea as part of the interdisciplinary BEST (Bering Sea Ecosystem Study) program (see next article).

Healy’s icebreaking capabilities allowed the ice-covered areas to be surveyed for the first time, while the NOAA ship Miller Freeman used the same instrumentation to survey in open water, and in the marginal ice zone at the same time. A multifrequency echosounding technique was used to distinguish acoustic backscatter from plankton and fish. Although analysis is ongoing, preliminary results indicate that the abundance of fish in the water column in the cold and shallow ice covered areas of the inner and middle shelf (depths <100 m) was much lower than in the ice free regions of the outer shelf at depths > 100 m (Figs. 4 and 5).

Backscatter attributed to plankton (primarily euphausiids) was more evenly distributed, and euphausiids were at times abundant in the ice covered areas. More comprehensive analyses of the acoustic backscatter from both vessels in relation to water column characteristics, sea ice cover, and the summer distribution of pollock and euphausiids are currently under way.

By Alex De Robertis

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