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.

Data collection

Diet samples were collected between 2001 and 2016 from breeding colonies at five different sites spread along the Norwegian coastline (Table 1, Fig. 1): Røst (67.5° N, 12.0° E), Sklinna (65.2°N, 10.9° E), Frøya (63.8° N, 8.5°E), Rauna (58.1° N, 6.7° E) and Øra (59.2° N, 11.0° E). Røst, Sklinna and Frøya are breeding sites of the Atlantic subspecies, whereas Rauna and Øra hold the continental subspecies. Diet samples consisted of pellets, i.e. indigestible material that is regurgitated daily as a natural part of the digestive process. All samples were collected from the vicinity of nests, thus presumed to originate from breeding adults either directly or via their chicks. After collection, they were stored frozen until being analysed. 

Analyses of diet samples

The treatment of the pellets followed previous work by Hillersøy and Lorentsen (2012). Soft parts were digested in a saturated solution of biological washing powder (Bio-tex®) kept at 50°C in an oven for 1−2 days. Fish otoliths were removed and identified to the lowest possible taxonomic level using descriptions in Härkönen (1986), Camphuysen and Henderson (2017) and a reference collection. The length and width of each otolith were measured to the nearest 0.1 mm using a binocular microscope and mm paper. Note that fragments of invertebrates as well as fish fragments other than otoliths were not systematically recorded for all sites and years (see main article for details). 

The attached data are organised per site, year and pellet number. We have further added a column with the information about which cormorant subspecies the pellets were obtained from. Sampling date is given when noted. For samples collected at Øra in 2011, only the week of the year were noted, not the actual date.

Note that fragments of invertebrates as well as fish fragments other than otoliths were not systematically recorded for all sites and years (see main article for details). 

Please refer to the "Explanations" sheet for additional details.