Pilot underwater visual census study in a Bulgarian sub-Mediterranean reservoir

In order to test the suitability of underwater visual census method in freshwater basins, it was applied and compared with standard sampling methods - gill nets, as well as questionnaires concerning the fish fauna's composition in Stoikovtsi dam (SW Bulgaria). The number of registered species per method was established as: questionnaires > underwater counting > gill nets. The reliability of questionnaires is uncontrolled, whereas underwater counting is nature friendly and permits the calculation of standard values as specimens per unit area, in addition to gill nets, where only CPUE can be estimated. Habitat preference and other specific features can be established, in addition to classical methods (gill nets). The major limitations of underwater counts in fresh waters are turbidity and the diver's experience.