Night fish avoidance of Microcystis bloom revealed by simultaneous hydroacoustic measurements of both organisms

Simultaneous observations of fish and cyanobacteria were conducted in the shallow Sulejów Reservoir (Poland) during the occurrence of Microcystis bloom. A Simrad EY60 split beam echosounder with a 200 kHz transducer beaming horizontally was applied to assess fish and cyanobacteria spatial distribution. Additionally, fish size distribution and species composition were evaluated with gillnets, and cyanobacterial biomass was determined by using an online phycocyanin fluorescence probe. Physico-chemical parameters and water samples for biological analyses were collected at 14 fixed stations situated along the acoustic transects. We found cyanobacteria represented by the genus Microcystis, with their toxigenic genotypes in all analyzed samples. The hydroacoustic results provided direct evidence for fish night avoidance of the bloom. The biomass of fish and cyanobacteria demonstrated opposing trends and their peak values spatially mismatched. The number of fish caught in gillnets within the bloom area was about half that caught outside the bloom area. In spite of the presence of intracellular microcystins (hepatotoxin) at all stations, no extracellular microcystins were identified in water samples and in fish tissues.