Ecological effects of re-introduction of salmonid spawning gravel in lowland Danish streams

During the past centuries natural spawning gravel was removed during the regulation of many streams in the Northern Hemisphere and wild fish stocks subsequently suffered due to this severe habitat degradation. To improve spawning conditions, projects involving reintroduction of spawning gravel have recently been conducted in many streams and rivers. However, systematic monitoring of these spawning gravel restoration projects is limited.

The overall aim of this paper was to evaluate gravel reintroduction as a long-term salmonid rehabilitation method in 32 lowland streams. Displacement of gravel and abundance of original gravel was assessed on each site along with spawning activity and density of fry. In three streams, Gels Å, Stensbæk and Ryds Å, intensive studies of spawning activity, nursery habitat development and densities of trout fry and young-of-the-year (YOY), were performed the following year, including both restored reaches and upstream control reaches.

Downstream displacement of gravel was most common at sites where gravel was reintroduced without further improvement, although these sites exhibited the highest density of YOY brown trout (Salmo trutta), evidencing that the remaining gravel is still functional. The intensive study of three streams showed that spawning was enhanced by the introduction of spawning gravel at the restored sites compared to control sites and that habitat quality generally were improved. Our results also suggest complex interactions exist between spawning activity, migrating trout and the emergence and survival of fry and YOY trout and the habitat conditions provided in regulated lowland Danish streams.

Our results clearly indicate that reintroduction of spawning gravel to restore fish populations in river systems warrants consideration of the differences in habitat quality that may occur among the reaches within a river system. Furthermore, spawning and nursery habitats should be present in all reaches to enhance survival of trout on re-instated gravel. To increase our knowledge of spawning gravel restoration systematic monitoring and assessment out of projects are required following project implementation.