An evaluation of artificial floating littoral zones to support fish communities in reservoirs

Artificial water level fluctuations (WLF) in reservoirs impact fish communities by degrading littoral habitats. To mitigate these negative effects, artificial floating islands that mimic natural littoral zones appear as a promising mitigation tool. However, their effectiveness in supporting fish communities remains poorly documented. In this study, three artificial floating littoral zones (FLOLIZ, 70 m2 surface area) were installed in a French hydropower reservoir subject to extreme WLF. Fish communities were assessed in spring and summer over four years in FLOLIZ and in control littoral stations during daytime and nighttime. Fish, especially juveniles, did not appear more frequently in FLOLIZ than in control littoral stations during daytime. At night, both adult and juvenile fish were more abundant in the littoral zone of the reservoir. During the day, the fish community in FLOLIZ was mainly composed of juvenile Chondrostoma toxostoma and adult Perca fluviatilis. Differences occurred only for a few species and life stages (juvenile vs adult); however, in general, results indicated greater abundance or richness in control littoral stations. These results do not support the effectiveness of FLOLIZ in mitigating deleterious effects of artificial WFL. The distance of FLOLIZ to the littoral zone could explain these results. Further studies in different environmental conditions, in different ecosystems and with different FLOLIZ designs are needed to provide additional information on the effectiveness of such structures as a mitigation tool.