Fish exploration of reservoirs and lakes: A novel way of mobile echosounding
The use of echosounders can be perceived as an extremely powerful tool for exploring aquatic ecosystems. Their undeniable advantage is their friendliness to the observed organisms, which are not distracted or damaged. For example, in marine systems, the use of echolsouders is a fundamental approach to studying fish communities and other pelagic organisms. When studying the fish communities of European lakes and reservoirs, we found that obtaining quantitative information with scientific echosounders is much harder. This is related to the fact that most fish live in shallow layers (0-4 m below sea level) in many waterbodies. Paradoxically, exploration of these shallow layers appears to be much more difficult than exploration of deep layers.
One hope was to use so-called horizontal surveys with the ultrasonic cone pointing horizontally below the surface The complex evaluation of horizontal echosounding tests showed a high level of interference of the surface-reflected ultrasound waves affecting the signal quality (Balk et al., 2017).
Fig. 1: New mobile up-looking system with the transducer lowered under the fish layer (Kubečka et al., 2015, Baran, et al., 2017). Upper figure - transport position, bottom position during the survey.
The possibilities of refinement of horizontal surveys will be further studied, however, an alternative system has been developed to obtain quality information about the open water fish of reservoirs and lakes, using an acoustic transmitter forward of a research vessel and immersed under a layer of surface fish to a depth of about 8 m (Figure 1). Unlike the in horizontal applications the acoustic signals obtained by uplooking system are acoustically very clean, noise free and allow to monitor not only fish but also some forms of aquatic invertebrates. The latest results of the comparison with the samples of direct catches by trawling show very good agreement (Baran et al., 2017) and therefore this method is a very promising approach to studying the quantity, spatial occurrence and behavior of fish in large waters.
Fig. 2 Mounting of echolocation transmitters on the submersible platform in front of the research vessel
Fig. 3: Up-looking hydroacoustic survey with plunged transducers submerged in front of the research vessel.
Balk H., Søvegjarto, B.S. Tušer M., Frouzová, J., Muška M., Draštík, V., Baran, R., Kubečka J., 2017 Surface-induced errors in target strength and position estimates during horizontal acoustic surveys. Fisheries Research, 188: 149-156. http://dx.doi.org/10.1016/j.fishres.2016.12.017
Baran R., Jůza, T., Tušer, M., Balk, H., Blabolil P., Čech, M., Draštík, V., Frouzová, J., Jayasinghe A.D., Koliada, I., Mrkvička T., Muška, M., Ricard, D., Sajdlová, Z., Vejřík, L., Kubečka, J. 2017: A novel upward-looking hydroacoustic method for improving pelagic fish surveys. Scientific Reports | 7: 4823 | DOI:10.1038/s41598-017-04953-6 1
Kubečka, J., Tušer, M., Komárek V., 2015 Czech patent. Device for remote investigation of aquatic animals. CZ. patent No. 305610 PV 2014-608