Comparison of sonar-, camera- and net-based methods in detecting riverine fish-movement patterns

Monitoring of fish movement is important in understanding and optimising the functionality of fishways and in restoring riverine connectivity. This study compared fish monitoring data (ARIS sonar-based and GoPro camera-based), with catches in a multi-mesh stow net following downstream passage in a small river in Bavaria, Germany. In terms of the number of individuals, the sonar-based system (detection rate = 62.6% of net-based catches) outnumbered the counts of the camera-based system (45.4%). Smaller specimens of <100 and <150 mm were under-represented with the sonar and the camera-based systems respectively. Species identification based on the camera system was similar to that for net-based catch, whereas no proper species identification could be performed with sonar data. In conclusion, the sonar-based system can be recommended for the counting of fish. 100 mm during night and turbid conditions, unless species identification is necessary. During daylight and with clear water, cameras can be a cheaper and promising option to monitor species compositions of fish >150 mm.