The avoidance reactions of fish with respect to a survey vessel were studied during horizontal acoustic applications of a Simrad EY500 split-beam echosounder (120 kHz) in two lakes (Wallersee, Balaton) and two reservoirs (Orlik, Rimov). Three methods were used to assess the avoidance reaction of fish to the survey vessel: (1) comparison of acoustically detected fish biomass at different distances. (2) determination of the fish direction vector (echogram slope) with respect to the transducer and (3) direct acoustic observation of fish behaviour in front of the moving vessel. Comparing acoustic biomass in order to demonstrate avoidance reactions is limited. All fish were divided in two groups according to the slope of their movement: with a positive value of slope (fish swimming away from the transducer) and with a negative slope (fish swimming towards the transducer). Fish avoidance caused higher slope values. Most avoidance behaviour was found with small fish (target strength, TS -40 dB. At distances over 10 in, the avoidance of small boats (5-6 m long, 15-25 HP two-stroke engine) appears not to be a serious problem in shallow waters. (c) 2004 Elsevier B.V. All rights reserved.