Abundance, distribution, and habitat use of fishes in a large river (Danube, Austria): mobile, horizontal hydroacoustic surveys vs. a standard fishing method

We tested a novel approach for fish abundance and size estimates along a 3.6-km reach in the main stem of a free-flowing section of the Danube River east of Vienna. We deployed an adaptive resolution imaging sonar (ARIS) with horizontal beam orientation from a moving boat in order to assess spatial and temporal fish abundance distributions in different mesohabitats. To validate the results obtained from imaging sonar, hydroacoustic abundance and fish length estimates were compared with those received from boat electrofishing surveys conducted at the same time and place. Furthermore, fish occurrence was related to hydraulic conditions based on a three-dimensional hydrodynamic model in order to reveal patterns of habitat use. Hydroacoustic abundance expressed as catch per unit effort estimates deviated significantly from fish abundances assessed by boat electrofishing, but when comparing relative abundances both methods yielded a similar spatial and temporal pattern. Total fish length estimates obtained from the two methods did not differ significantly. The mobile application of the non-invasive ARIS technology offers the opportunity to quickly gain detailed information on fish abundance and position of individuals. This yields more specific information on temporal and spatial distribution patterns, habitat use and fish size distributions in large rivers.