Estimating the effects of winter-fishing: mobile echosounding under ice

An echosounder with a tape recorder in a propeller-driven waterproof box (a n echosounder shuttle) was used under the ice to monitor the vertical distr ibution and density of fish in a winter seining area of a Finnish lake in A pril 1997. The surveyed fish were at 3.5-8 m depth, and fish density was si gnificantly lower after seining than before. The seine catch was about 10 k g ha(-1), and the echosounding results showed a decrease of 7 kg ha(-1) in the hauling area. There were some difficulties due to the low temperature, and minor problems with steering and leakage of the shuttle. We conclude th at it is possible to make mobile echosurveys under the ice and estimate den sities of wintering fish. (C) 1999 Elsevier Science B.V. All rights reserve d.