Going with the flow: Spatio-temporal drift patterns of larval fish in a large alpine river

Fish larval drift is an essential step in the life cycle of riverine fish species as it determines dispersal and colonisation. Anthropogenic flow alterations and interruption of longitudinal and lateral connectivity by river damming and straightening can severely affect larval drift patterns. In this study, we characterised spatio-temporal drift patterns of fish larvae in the heavily regulated large alpine River Inn and within a constructed nature-like fish bypass. Drift was investigated in the main reproduction period of the fish fauna in this river, ranging from mid-April to end of June. Diel patterns were assessed by samples taken during day, dusk, night, and dawn each day of sampling. To obtain robust species identification, we used DNA barcoding for genetic verification of phenotypically pre-sorted groups. From a total of 1,069 fish larvae and 283 fish eggs caught, DNA barcoding revealed 16 species from five families, including several target species of conservation. We found strong evidence that several endangered species, such as Chondrostoma nasus, Thymallus thymallus, Cottus gobio, and Aspius aspius successfully reproduced in the bypass system. Genetic species verification showed a high level of homogeneity in most phenotypically pre-sorted groups. Distinct seasonal patterns were observed, with the majority of fish species in the drift observed in mid-June. Thymallus thymallus and Cottus gobio dominated larval drift at the beginning of the observation followed by mainly unimodal abundance peaks of several Cyprinid species. Nocturnal drift prevailed in all species. Our results on the spatial occurrence and abundances of fish larvae highlight the importance of bypass systems in heavily modified waterbodies to provide valuable spawning habitats and drift corridors around dams. Moreover, the distinct species-specific time patterns of larval drift represent a first basis to direct future discharge and river management plans in large alpine rivers towards a protection of the sensitive larval stages of specific target species of conservation. This includes bypass and turbine operation modes in the period of highest drift activity, as well as the construction of nature-like bypass systems and creation of spawning grounds therein.

Automated summary

The study investigated fish larval drift patterns in the heavily regulated River Inn in the alpine region, revealing multiple species drifting during the main reproduction period, including endangered species such as Chondrostoma nasus, Thymallus thymallus, Cottus gobio, and Aspius aspius. Genetic verification showed high homogeneity among phenotypically grouped larvae, with distinct seasonal patterns observed. This highlights the importance of bypass systems in providing spawning habitats and drift corridors for conservation target species in heavily modified waterbodies.