Observation of fish migration between the sea and a mediterranean lagoon (Etang de l'Or, France) using multibeam sonar and split beam echo sounder

Abstract

The aim of this paper is to describe the application and results of a new methodology for studying the fish distribution, abundance and amphidromous migration in very shallow waters, taking a Mediterranean lagoon as an example. The study is the `Etang de l'Or' (France). The study was conducted in the main channel connecting the lagoon to the sea (1.5 km long, 12 m wide and 1.5 m deep). Two surveys were achieved, in May and October 1995. The equipment consisted of a split-beam echo sounder (120 kHz) used horizontally and a multibeam sonar (60 beams, 1.5° each, 455 kHz), both used from the shore, aimed perpendicularly to the channel axis. It allowed us to obtain TS distributions, abundance indices (including fish school evaluation) and the direction of fish migration (fish tracking). The sonar was directed on a vertical plan to evaluate the actual number of fish schools crossing the channel and to make behavioural observations. Echo soundings were processed by direct measurement on the echogram for school geometry and through EP500 SIMRAD software for echo integration, TS measurements and fish tracking. The sonar data were recorded on video tape and processed by eye in the laboratory (school counting). During the surveys, individual targets did not show significant migration, while fish schools were clearly migrating towards the lagoon in October. During this survey the mean density was evaluated at 7 g m −3, and the total biomass entering the lagoon at 1.4 tons/day. No relation between current flow and fish movements was observed. The paper demonstrates that multibeam sonar with split-beam echo sounder used horizontally may overcome several serious limitations on the application of acoustics in shallow water.