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.