Juvenile salmonid abundance in a diamictic semi-fluvial stream in Norway—does stream bed shelter beat large woody debris?

This study investigates the effect of large woody debris (LWD) on the abundance of juvenile Atlantic salmon (Salmo salar, L.) and anadromous brown trout (Salmo trutta, L.) in semi-alluvial side channels of the river Aurlandselva (Norway) using point electrofishing and microhabitat mapping. Not the presence of LWD, but stream bed shelter availability and the distance to spawning grounds affected the fish abundance (fish/point), independent of other habitat components. LWD showed only an effect on fish abundance when in interaction with other habitat components. This discrepancy can be explained by the availability of cavities in the shelter-rich coarse substrate which provide sufficient cover and territory for juvenile fish at the given carrying capacity of river Aurlandselva. Whilst LWD may be most effective to provide shelter in lowland streams (bed slope <0.005), maintaining or restoring shelter-rich coarse substrates should be considered a key priority in steeper salmonid rivers and associated semi-fluvial streams.