Ichthyofaunal and hydromorphological changes downstream of a major hydroelectric dam in the Southern Balkans

Large hydropower dams usually have serious impacts on river fish communities, yet there are few studies documenting their specific effects in the Balkans and Eastern Mediterranean countries. This study describes before-and-after changes in ichthyofaunal composition under the new hydroelectric flow regimes downstream of the Ilarion Hydroelectric Dam, located on Greece’s longest river, the Aliakmon. The dam became operational in 2012 and is located approximately 7 km upstream of the Polyphytos hydroelectric reservoir, which was created in 1972. The investigated reach below the Ilarion Dam is thus confined between two large artificial hydroelectric reservoir systems, forming a highly regulated and spatially restricted lotic–lentic transition zone. Contemporary electrofishing surveys were compared with historical surveys predating the construction of the dam and its upstream reservoir. Results indicate marked compositional shifts in the fish assemblage, with four native rheophilic species not detected in the contemporary downstream surveys, while at least three other species showed marked declines. In contrast, generalist and lentic species increased in abundance, including the non-native lentic species Lepomis gibbosus and Gambusia holbrooki . Our analyses of available hydrological data from recently installed monitoring stations show altered flow conditions, including hydropeaking and modified water-temperature regimes downstream of the dam, together with the destabilizing effects of water-level fluctuations in the downstream reservoir. Furthermore, topographic maps of the river reach indicate substantial alteration of the riverbed associated with gravel quarrying during dam construction, conditions that may also promote lotic habitat degradation. Although this study is not based on long-term monitoring, it provides insights that may help guide further research and adaptive management in highly regulated hydropower river systems.