Great cormorant diet data from the Norwegian coast

Piscivorous wildlife is often perceived as competitors by humans. Great cormorants of the continental subspecies (Phalacrocorax carbo sinensis) in the Baltic and North Sea increase, while local cod (Gadus morhua) stocks decline. In contrast, numbers of the Atlantic subspecies (P. c. carbo), breeding along the Norwegian and Barents Seas have been relatively stable.

We investigated the diet of both great cormorant subspecies in breeding colonies along the Norwegian Coast from Lofoten to the Skagerrak and estimated the biomass of fish consumed annually by great cormorants in Norwegian waters. The birds' consumption was compared with estimated fish stock sizes and fishery catches.

Cod and saithe (Pollachius virens) dominated the diet in the Norwegian Sea, and wrasses  in the North Sea and Skagerrak. Estimated total fish consumption of cod and saithe by great cormorants was < 1.7% of estimated fish stocks and < 9% of that of human catches and therefore considered minor. Cormorant consumption of wrasses amounted to 110% of human catches.

The practice of using wrasses as cleaner fish in the salmon farming industry leads to a conflict with cormorants, and we urge for a better understanding and management of wrasse populations, taking ecosystem functioning and natural predation into account.