Avoidance Behaviour of Fish with Respect to An Active Pelagic Trawl Observed with High-Frequency Sonar

Direct observation of fish avoidance behaviour with respect to an active pelagic trawl is challenging. A DIDSON acoustic camera was used for direct observation of the fish avoidance behaviour in front of the mouth of an active pelagic surface trawl in the mesotrophic Czech reservoir Zelivka. Based on their reaction and swimming behaviour towards the active trawl the fish (n = 1051) displayed very heterogeneous behavioural patterns, which were classified in 11 different categories of avoidance behaviour. The avoidance behaviour was analysed in detail with Sonar5-Pro post-processing software including variables like total length, distance to the trawl, tortuosity, mean speed of total track, mean speed of first avoidance track, mean speed of total avoidance track, angle between initial and first avoiding track and angle between initial and total avoiding track. 76% of the fish were swimming into the trawl, 24 % out of the trawl. The majority of the fish (81.7%) showed no avoidance reaction, both during day (55.6%) and during night (93.6%). Hence, fish seemed to be more passive during the night than during day. Avoidance behaviour (18.3%) was much more apparent during day (44.4%) than during night (6.4%) indicating that ambient light conditions seem to trigger avoidance behaviour during day. A two-step cluster analysis revealed significant discrimination of three clusters termed as ‘bleak-cluster’, ‘bream-cluster’ and ‘silver carp-cluster’ and were formed by the most abundant fish species in the reservoir. The study might contribute to mitigating selectivity of trawling.