A HYDROACOUSTIC STUDY OF THE PELAGIC SPAWNING BEHAVIOR OF WHITEFISH (COREGONUS-LAVARETUS) IN LAKE CONSTANCE

The horizontal and vertical distribution patterns of pelagic spawning whitefish (Coregonus lavaretus) were studied by echosounding from the end of the growing season (early November) until after spawning (late January). During early November, whitefish inhabited similar depths during day and night, but during spawning displayed vertical migrations of up to 50 m amplitude: whitefish ascended at dusk and descended at dawn. At the end of the spawning season, vertical migrations ceased. juveniles took part in the daily migrations, but only sexually mature specimens invaded the uppermost 10 m of water, while maturing and juvenile individuals and spent females stayed deeper during the night. This vertical distribution pattern is interpreted as a means to increase encounter probability among mature specimens. Diel vertical migration of the entire population is suggested to be an aid to avoid cannibalism on freshly spawned eggs during settling to the lake bottom. Total fish density estimates increased during upward migration and decreased during downward migration; this may be the result of differences in target strength caused by both reduced swimbladder volume and positive tilt while in deep waters. Highly reproducible abundance estimates were obtained during night when whitefish were evenly distributed at the top of their vertical range.