Since the European population of great cormorants ( Phalacrocorax carbo sinensis ) rapidly increased 30 years ago, Denmark has been one of the core breeding areas for this colonial water bird. Following a 10-year period with stable breeding numbers in Denmark, the population of great cormorants decreased. At the same time, a combination of cold winters and low availability of coastal prey fish apparently triggered birds to seek new foraging areas. Thus, cormorants began to appear in rivers and streams coinciding with an observed massive decline of fish, mainly brown trout ( Salmo trutta ) and grayling ( Thymallus thymallus ). In this paper, we present the results from studies using radio-telemetry, PIT-tagging, and traditional fish surveys to estimate the impact of predation in Danish lowland rivers. Recovery of PIT-tags revealed that an estimated 30% of wild trout and 72% of wild grayling tagged in a small river were eaten by cormorants. In another medium-sized river, 79% of radio-tagged adult grayling were removed, presumably by cormorants during winter. Thus, predation from cormorants appears to be at a level that explains the observed collapse of grayling and brown trout populations in many Danish streams.