Obstacles to migration constrain nest distribution of Atlantic salmon

– Programmes of habitat restoration usually assume that the targeted populations will readily colonise the newly provided habitat. However, this assumption may not always hold, and the success of restoration may be impaired if the individuals are driven to aggregate in areas of the habitat already available instead of spreading to new ones. We investigated how weirs situated along a river could drive Atlantic salmon (Salmo salar) to aggregate their redds and tested whether aggregation intensified at low fish density. For this, we applied spatial point pattern analysis to a data set consisting on the distribution of salmon redds in a small river over 15 years (1992–2006). Within the habitat suitable for spawning, redds were significantly aggregated, especially in the first 15 morphodynamic units below weirs. Our results suggest that the constraint imposed by obstacles on redd distribution should be considered when conducting habitat restoration.