Cormorant impact on trout (Salmo trutta) and salmon (Salmo salar) migrating from the river Dalälven emerging in the Baltic Sea

Abstract

Cormorant (Phalacrocorax carbo sinensis) population increases and the resulting predation, can lead to severe effects on fish stocks. In the Dalälven River in Sweden, it was observed that the number of trout and salmon returning to spawn decreased, while the number of cormorants along the coast had increased simultaneously. This study estimated the predatory impact that cormorants had on stocked trout (Salmo trutta) and salmon (Salmo salar) populations in 2005 and 2006 by using Coded Wire Tags and Carlin tags recovered in cormorant pellets. Predatory impact on salmon was found to be low, as no tags were recovered. Impact on trout was estimated 1.9%. Additional inspections of material within and around nests for tags gave similar estimates within a low range 0.8–2.3%.