Development of Bulgarian Fish Based Index for Ecological Classification and Monitoring of Natural Riparian Lakes (Type L5/L-EC-1)

The proposed new index BFBI-L5 was developed by using the data from 15 years ichthyological investigations in the Srebarna Lake (Danube floodplain). The method is based on the comparison of composition and structure of the fish community with the reference model ichthyocenosis as defined by historical data and data of specific hydromorphological features of the water body. Fish sampling shall be performed twice a year with standard bottom multi-mesh gillnets (NORDIC type) covering proportionally all main fish biotopes. Eleven indicators calculated through seven ichthyological parameters characterizing both fish community and type specific guilds are used to assess the ecological status of the lake. Each indicator is converted to score values according to its significance and to the specific metrics. The total sum of scores of all 11 indicators is used to calculate the ecological quality ratio (EQR) as the ratio of calculated sum of scores and the reference total score. The normalized EQR values equal the BFBI-L5, with equidistant class boundaries from 0 to 1. Although the index was developed for the lakes of type L5/L-EC1, the methodology for ecological assessment could be adapted also for other lowland natural lakes in Bulgaria, such as the lakes along the Black Sea coast.