Great cormorant (Phalacrocorax carbo sinensis) predation on adult anadromous brown trout (Salmo trutta)

The increase in abundance of great cormorants (Phalacrocorax carbo sinensis) over the last decades has caused concern for the sustainability of fish populations. Cormorants are opportunistic piscivorous birds that eat fish from a wide range of species and sizes, in marine, lacustrine and riverine habitats. It has been documented that juvenile salmonids are under significant predation pressure by great cormorants, although knowledge on the predation of adult salmonids by cormorants is limited. In this study, adult anadromous brown trout (Salmo trutta) were tagged over multiple years with PIT tags during their spawning migration in a Danish lowland river. Two nearby cormorant colonies were subsequently scanned for PIT tags to estimate predation rate. It was estimated that by minimum 15.4% of tagged adult sea trout were predated by cormorants. The majority of the individuals were predated in the river, and females were more likely to be predated than males. Moreover, length had a significant effect on predation probability: individuals between 35 and 43 cm had the highest likelihood to be predated, while smaller and larger individuals were less likely to be predated. Our findings challenge the assumption that cormorants do not prey on reproductively mature salmonids. Furthermore, predation of mature individuals may have implications for the recruitment and sustainability of the population.