The importance of stream interstitial conditions for the early-life-stage development of the European nase (Chondrostoma nasus L.)

Rheophilic cyprinid populations are in decline in many European rivers and have become target species of conservation and river restoration. This is especially true for the European nase (Chondrostoma nasus), a lithophilic species for which the early life stages pose the first bottlenecks in successful population development. In this study, egg dispersal at the substrate surface as well as within the stream interstitial was evaluated and the development of nase eggs was tested at three spawning grounds in the German Alpine Foreland. New incubation systems for cyprinid eggs as well as reference incubation boxes were developed. A significantly higher number of nase eggs (95.5%) were deposited within the stream interstitial compared to eggs sticking to the substrate surface (4.5%). Most eggs successfully developed within the stream interstitial, and hatched larvae moved deeper into the stream substrate. Higher fine sediment accumulation rates in incubation systems had a negative impact on hatching rates. Our results suggest that a permeable, well-oxygenated stream interstitial at spawning grounds can greatly enhance hatching rates and successful development of nase, as previously found for other taxa such as salmonids. Furthermore, movements of hatched larvae into deeper layers of the stream interstitial underline its importance not only for egg development, but also as shelter for early life stages. Future conservation and restoration efforts should thus consider more intensively stream interstitial conditions on spawning grounds, including evaluating physical characteristics, fine sediment accumulation as well as bioindication with suitable incubation systems for cyprinids.