DOI: 10.5061/dryad.b5mkkwhcd
title: Great cormorant diet data from the Norwegian coast

description: 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 &lt; 1.7% of estimated fish stocks and
 &lt; 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.
description: <strong>Data
 collection</strong> 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.   
 <strong>Analyses of diet samples</strong> 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).         
description: 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.