High Diversity of Waters, Communities and Stressors—Design of Croatian Fish Index for Rivers (CFIR)

Determination of the ecological state of any water body is crucial for its adequate conservation and is, moreover, required by the Water Framework Directive. Employment of multimetric indices, which integrate various anthropogenic threats to water systems with the responses of different elements of fish communities, improve our understanding and allow us to monitor the ecological state of freshwater bodies. However, design of multimetric indices that describe the ecological state of water bodies based on fish communities has already proved difficult for the Mediterranean region for several reasons, including the frequent presence of species-poor and ecologically tolerant fish communities, high endemicity and a serious lack of localities with undisturbed fish communities. The development of an ecological state index for natural water bodies in Croatia based on fish as a biological element which we present in this paper was also hindered by similar obstacles, particularly due to exceptionally high endemicity present in rivers of the Adriatic watershed, the great number of distinct water types, and significant anthropogenic pressures. Nevertheless, based on comprehensive sampling of fish communities throughout Croatia and following appropriate statistical procedures, we were able to identify stressors acting on certain water bodies, as well as responses of fish communities to them, and, finally, describe the ecological state of natural water bodies throughout Croatia. We also propose measures that will most likely help in achieving improvement and/or maintenance of the ecological states of these water bodies.