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Midwater Assessment & Conservation Engineering Program

Acoustic Measurements of Fish and Plankton in the Ice-Covered Areas of the Eastern Bering Sea

Scientists from the Midwater Assessment and Conservation Enginteering (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 major interannual changes in ice extent and ice persistence in the Bering Sea have been documented, 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 conducted during the ice-free summer months 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 to measure the distribution of fish in ice-covered waters.

MACE scientists worked with the vesselís crew to instrument the U.S. Coast Guard icebreaker Healy with scientific fishery echosounders. The equipment was used during the 13 March to 6 May cruise in the eastern Bering Sea as part of the interdisciplinary BEST (Bering Sea Ecosystem Study) program funded by the National Science Foundation. Although 2008 was a year of exceptionally heavy ice cover in the Bering Sea, Healy's icebreaking capabilities allowed extensive survey coverage of the ice-covered areas. In addition, the NOAA ship Oscar Dyson 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 of the acoustic backscatter is only partially completed, 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 greater than 100 m. From analysis of a subset of data collected during the 2007 and 2008 cruises, the distribution of fish appeared to be more restricted to areas near the shelf break where warmer water was observed during 2008 (Fig. 1).


Figure 1, see caption
Figure 1.  Acoustic backscatter (SA m2 nmi-2) attributed to fish along Healy's trackline in 2007 (left) and 2008 (right).  There was substantially higher ice cover in 2008 than in 2007.  The approximate position of the 200-, 100-, 70- and 50-m isobaths are shown as dotted lines.  Symbol size and color tone is proportional to the intensity of acoustic backscatter.
 

Very few fish were observed in the northern areas where near-bottom waters were cold (< ~1°C), even in areas where walleye pollock are abundant during the summer months. The observations suggest that walleye pollock, a dominant component of the ecosystem, may shift its distribution away from areas where very cold water is present, although complete analyses of the data are required to confirm these trends.

Acoustic backscatter attributed to plankton (primarily euphausiids) was more evenly distributed than backscatter attributed to fish during both years although some patches of abundant euphausiid backscatter were detected 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 walleye pollock and euphausiids are currently under way.

By Alex DeRobertis
 

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