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

Abstract

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.

Effects of a Hydropower-Related Temporary Stream Dewatering on Fish Community Composition and Development: From Ecology to Policy

“…To avoid negative population effects, the dewatering should be applied outside the seasons of spawning, larval drift, and juvenile development. In this study, the main season for larval drift was spring to summer and the most dense fish population in bank habitats was detected in summer to autumn, matching previous findings of the River Inn system (Nagel et al, 2021). Consequently, the least harmful time span for dewatering in this project area would be late autumn to early spring when least fish are present in the bank habitats.…”

“…In contrast to fish stranding, community effects caused by dewatering were rather strong, most likely due to the restricted habitat availability in 2021 when the dewatering phase lasted several weeks from the middle of May to the beginning of August. This time of the year falls within the main larval drift phase of riverine fish at the River Inn (Nagel et al, 2021), and these types of bank habitats usually serve as a crucial refuge area for the development and growth of fish larvae and young-of-the-year fish (Pander et al, 2017;Stoffers et al, 2022).…”

Water level induced changes of habitat quality determine fish community composition in restored and modified riverbanks of a large alpine river

“…Considering the abiotic differences between habitats and particularly seasons and results derived from lowland rivers (Erös et al, 2008;Meulenbroek et al, 2018;Ramler & Keckeis, 2020), a large difference in overall fish community composition between CP, BR, and RR as well as a strong seasonal change of community composition was expected. Surprisingly only the latter was observed in this alpine river and the strong seasonal differences detected were mainly attributed to higher spe- matching the observed times of spawning and subsequent larval drift (Nagel et al, 2021). Since some of those species are ubiquitous schooling fish (e.g., A. alburnus), their occurrence can be dependent on a distinct pattern of habitat use according to reproduction, food availability and predator density (Brown et al, 2011;Pitcher, 1986) being strongly governed by seasonality (Pander & Geist, 2010).…”

“…The set of species detected by SIMPER indicated strong seasonal habitat preferences of A. alburnus (much more abundant during summer) and S. cephalus , A. bipunctatus , C. gobio , G. gobio , and C. nasus being mostly present in the bank habitats during autumn. Depending on the individual species, the observed abundance shifts can result from habitat use change, recruitment (particular in autumn season as most caught species spawn in spring or early summer) as well as a combination of both, matching the observed times of spawning and subsequent larval drift (Nagel et al, 2021). Since some of those species are ubiquitous schooling fish (e.g., A. alburnus ), their occurrence can be dependent on a distinct pattern of habitat use according to reproduction, food availability and predator density (Brown et al, 2011; Pitcher, 1986) being strongly governed by seasonality (Pander & Geist, 2010).…”

The overlooked contribution of a fish bypass channel to the density and diversity of macroinvertebrate drift in a heavily modified river system

“…No additional sites in the main stem were sampled, as this study explicitly aimed to assess spatial variation of macroinvertebrate drift from the inflow to the outflow of the bypass. The drift nets used consisted of rectangular aluminum frames for the mouth and tear‐resistant polyester (155 meshes cm −2 ) for the nets (Nagel et al, 2021). For representative sampling, sites were evenly distributed along the course of the bypass, with similar water depths (Table 1).…”