Evaluation of a variable angle scanning method to estimate relative abundance and distribution of fish using a single-beam echosounder in shallow lakes

Abstract

The validity of a hydroacoustic procedure was assessed using a combination of horizontal and vertical scanning to map the distribution of targets and to estimate target density in a shallow lake. Three distribution patterns were created using 37–50 artificial targets (metal hex nuts) anchored at known positions. Real and acoustic maps were qualitatively similar. Aggregation indices estimated by hydroacoustics were within 15% of the real values. Target density ranged from 1 to 8 targets per 100 m−3. Estimated target densities were within one target of the real values for 88% of our observations. The variable angle approach was used also to monitor daily and seasonal variations in fish distribution and relative abundance outside the littoral zone. Dace Phoxinus eos × P. neogaeus appeared to use the littoral as a refuge during the day and to migrate to the pelagic zone at dusk. The movements of dace outside the littoral zone were limited to the months of June-August. The variable angle acoustic approach can be useful to estimate fish distribution and relative abundance in shallow lakes.