Abstract: Hydroacoustical measurements of fish population in the shallow Malta reservoir (mean depth <3 m) were performed in September and October 2008 using mobile horizontal beam. During the different time of day and night the same transects were covered in two directions, from the dam towards the upper part of the reservoir and back. During the day the number of fish registered on the returned way was lower than on the way forward in 83% of cases, and during the night in 67% of cases, however these differences were not statistically significant. Observation of fish at different distances from the boat have shown small avoidance reaction, especially for the small fish, that was similar during the day and at night, thus suggesting that mainly noise was responsible for the fish avoidance. Nevertheless, in shallow waters boat avoidance by fishes is of much less importance than in marine environment.