Influence of scuba divers on the avoidance reaction of a dense vendace (Coregonus albula L.) population monitored by hydroacoustics

The avoidance reactions of a dense population of vendace (Coregonus albula (Linnaeus)) to scuba divers were studied in November 2005, at two different sites in the mesotrophic Bigge reservoir, Germany, using hydroacoustics to observe the fish behaviour. Two mobile scientific echosounders (Simrad EY 500 and EK 60, both operating at 120 kHz frequency) were used concurrently. Hydroacoustic data acquisition was conducted by vertical echosounding from two boats moving along four transects perpendicular to the diver transect (60 m long); the estimated fish abundances obtained from the two echosounders were similar in magnitude. The fish distribution, as shown by the echograms, was strongly influenced by the scuba divers, and due to the avoidance reactions the divers did not observe any fish. During the dive, the overall mean fish densities over both dive spots were substantially lower (4911 fish ha−1, S.D. = 3164, n = 16) than during the two reference surveys carried out before the dives (15,306 fish ha−1, S.D. = 5361, n = 32). The observed influence of the scuba divers on the fish distribution was temporary, and 1.5 h after the end of diving, the fish aggregated again on the dive spot, showing a similar spatial distribution to that recorded prior to the dives.