Abundance and horizontal distribution of Lake Constance pelagic whitefish (Coregonus lavaretus L.) during winter

Abstract

On seven dates in winter/early spring 1989/90, hydroacoustic data were sampled in the main basin of Lake Constance with a single-beam echosounder. In the pelagic zone, acoustic backscattering is almost entirely due to pelagic whitefish during that time of the year. For each sampling date, whitefish abundance was estimated for three size-classes (from -32 dB to -52 dB) and the horizontal distribution of the stock was plotted. Total population estimates for all size-classes combined agreed extremely well among four surveys before and after spawning time, ranging from 5.3 to 5.9 million individuals. During spawning and in early spring only part of the population was accessible to acoustic sampling. Horizontal distribution patterns were very consistent among five surveys in winter, showing a pronounced density gradient increasing from the southwestern to the northeastern shore. Pelagic whitefish thus maintain their characteristic overwintering distribution pattern for at least three months.