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

Abstract

Hydropower use of rivers can exert multiple effects on aquatic species and habitats. Due to limitations of conservation projects in the main channels of hydropower-affected rivers, there has been an increasing focus on tributaries, side channels, and fish passes as target areas for conservation and restoration. However, some of these side channels require frequent dewatering for their maintenance, and the ecological effects of such measures remain largely unknown. In this study, we used two dewatering events in a side channel of the River Inn as an opportunity to assess the effects of these common measures on fish. All stranded fish were collected after the two dewatering events in remaining puddles of formerly restored bank habitats, determined to species level, and measured. The fish community was compared by electrofishing before and seven weeks after the dewatering in a subset of the same habitats. The dewatering created one to three remaining puddles in the bank habitats, covering 3% of the assessed bank habitat area. In these remaining puddles, 184 stranded fish from 12 species were found, including species strictly protected under national and international law. In relation to their relative abundance, smaller and less mobile species such as Cottus gobio were mostly affected by stranding in contrast to larger and open-water-oriented species such as Chondrostoma nasus. The dewatering also caused drying out of important nursery zones, resulting in a distinctly lower recruitment success of endangered riverine fish species in the summer following the dewatering. The evidence about the negative ecological effects documented herein should be transferred into policy measures to reduce the impairment of dewatering to a minimum and to contribute to the fulfilment to national and international legal requirements. This can be achieved by reducing the extent and frequency of periodical dewatering to a minimum, by slowing down the dewatering speed, by selecting the least critical time of the year, as well as by compensation measures.

Variation in hydropeaking‐induced stranding of Barbus barbus L. and Chondrostoma nasus L. larvae: Assessing the impact of daytime and down‐ramping rates

“…Recent studies underline that cyprinids like barbel or nase exhibit species‐specific responses to artificial flow down‐ramping (e.g., Hayes, Schülting, et al, 2022; Pander et al, 2022). Moreover, initial stranding experiments conducted with nase larvae indicate a significant difference in stranding between earlier (III–IV) and later (V) larval stages (sensu Peňáz, 1974), with the highest effects at faster down‐ramping during the night (Führer et al, 2022).…”

Characterizing the effects of morphological microstructures and hydropeaks on fish stranding in rivers

“…Consequently, individuals are much less vulnerable when reaching a threshold length, reported as 25 mm for cyprinids (Young et al, 2011), 40 mm for steelhead trout ( Oncorhynchus mykiss ) or 50–60 mm for chinook salmons ( Oncorhynchus tshawytscha ) (Hunter, 1992). Smaller, less mobile species may be more affected than larger, open‐water species (Pander et al, 2022).…”

Effects of episodic stream dewatering on brook trout spatial population structure

“…Isolated pools also restrict movements necessary for fishes to escape locally unsuitable conditions and access remote resources (e.g., spawning and foraging areas; Schlosser, 1991), and fish mobility may be particularly important to escape pools during initial stages of dewatering (Archdeacon et al, 2022;Vander Vorste et al, 2020). For instance, an experimental drawdown study found that less mobile fish species were stranded within isolated pools at a higher rate than more mobile species (Pander et al, 2022). Likewise, Perkin et al (2015) suggested the importance of vagility given observed declines of pelagic fishes with increasing fragmentation due to isolated pools in Great Plains streams of North America.…”