fish

Toxic cyanobacterial blooms constitute a common problem at the global scale. The aim of the project, was to assess fish biomass and their spatial-temporal distribution patterns in the Sulejów Reservoir in relation to cyanobacterial blooms. Additionally the spatial and temporal dynamics of physical variables, inorganic nutrients, zooplankton and phytoplankton chlorophyll a were collected to help understanding which factors are controlling the development of the bloom and determine fish behavior. Fish were sampled by gillnetting during each survey to determine species composition and their size structure. For dominating fish species accumulation of hepatotoxins-microcystins in different parts of fish body were be measured using HPLC methodology. To obtain maps of fish distribution acoustic surveys were performed using Simrad split beam echosounder EY60, 200 kHz.
The main aim of the project has been fully reached and confirmed our hypothesis that fish can escape from cyanobacterial bloom. The echograms of the “bloom border” have shown that fish crowded in a clean water just at the border, while very few fish were found within the bloom. Fish and cyanobacterial biomass were showing opposite trends with less fish encountered as intensity of bloom was increasing. Mean fish density for the investigated area was nearly three times lower in 2013 and 2015 (230 and 188 fish ha-1accordingly), when cyanobacterial blooms were present, than in 2014 (623 fish ha-1) when the bloom was absent. Molecular analyses confirmed that in years 2013-2015 in the Sulejów Reservoir intracellular contained toxins were present, but no toxins were detected in water. Also no microcistins were discovered in the tissue of dominating fish species, roach and bream. This means that either fish were able effectively avoid the intoxication by cyanobacterial bloom, or toxin concentrations were too low to be detected with the methods applied (HPLC with diode array detection).