Fish abundances and size structure distributions in reservoirs with different trophy level

Hydroacoustical estimation of fish abundance and distribution was performed in three reservoirs with different levels of eutrophication. To analyse TS distribution the echo counting method was applied with TVG set to 40 logR. The Biosonics 101 dual beam echosounder, frequency of 420 KHz, was used for measurements and the ESP software for acoustical data analyses. In the mesotrophic Solina reservoir fish abundance was an order of magnitude lower than in the other two eutrophic reservoirs. Also the fish length distributions had different shapes in al! three reservoirs indicating the changes in fish size structure due to eutrophication. Comparison of the above results shows that hydroacoustically collected data may help to assess the ecological state of inland waters and be used together with other methods in monitoring the water quality.