Under ice density and mobility of fish in winter-seining area of two Finnish lakes as revealed by echo-survey

The density and movements of fish under ice were studied with single-beam m obile surveys and fixed location split-beam surveys, as well as exploratory fishing in winter-seining areas of two shallow Finnish lakes during winter 1999. Fish schooled near the bottom during the day but the schools dispers ed and fish ascended at night. Single and split-beam target strength distri butions corresponded fairly closely with the length distribution of seine c atch samples. Estimated fish densities were greater at night than in daylig ht. The swimming speed of smelt (Osmerus eperlanus) was 0.18 m .s(-1) in da ylight and 0.36 m .s(-1) at night. The corresponding figures for vendace (C oregonus albula) were 0.11-0.17 and 0.05-0.08 m .s(-1). Nights immediately before and after fishing were the best periods to estimate the effect of se ining on fish density. The hydroacoustic fish density estimates at night co rresponded closely with seine catches. We conclude that it is possible to e stimate the effects of winter-seining by mobile under-ice echo-surveys and catch samples. (C) 2000 Ifremer/CNRS/INRA/IRD/Cemagref/Editions scientifiqu es et medicales Elsevier SAS.