Making up the bed: Gravel cleaning as a contribution to nase (Chondrostoma nasus L.) spawning and recruitment success

Spawning substrate quality is a major factor influencing the early ontogeny of European nase (Chondrostoma nasus), a target species of conservation. Analogous to findings from salmonids, restoration of spawning grounds was hypothesized to enhance spawning, development and thus recruitment success of nase, by improving the substrate quality, and subsequently spawning site use, egg infiltration and protection of larvae in the interstitial zones before emergence. These assumptions were tested using a comparative approach by cleaning 50% of the area of each spawning ground in two Bavarian rivers. Substrate cleaning resulted in an immediate reduction of ~70% fine sediment content with improvements still detectable 2 months later. Spawning nase used the restored areas of spawning grounds preferentially, which was evident in the number of spawning fish and the significantly higher number of eggs laid. Infiltration of eggs into the interstitial zone was distinctly more successful in the opened interstices of the cleaned spawning substrate, where they were found down to a depth of 20 cm. The same was true for larvae, which could be found down to 30 cm and up to 13 days after hatching. Moreover, higher peaks in the drift density of emerging larvae from the restored spawning substrate were detected (2.5 compared with 1.7 larvae m⁻³ discharge for the River Mangfall and 0.3 compared with 0.03 larvae m⁻³ for the River Sims). These results clearly indicate that gravel cleaning is a successful short-term restoration tool for nase spawning grounds. It is a quick, cheap and effective method for the conservation management of nase, which may also be applicable to other riverine species with a similiar ecology and incubation time, such as Barbus barbus, Squalius cephalus, Leuciscus leuciscus and Phoxinus phoxinus. This especially holds true if streams lack internal dynamics and suffer from high loads of fine sediment and colmation.