Background & Aims

Great cormorants (Phalacrocorax carbo) are a species of piscivorous seabirds with a widespread global distribution [1]. They are generalist predators, feeding in a range of aquatic habitats [1] and temporal and spatial shifts in cormorant diet are well documented [2-5]. Due to its opportunistic feeding strategy, the species is considered a competitor for fish stocks as a result of its predation on commercially important species such as cod (Gadus morhua), herring (Clupea harengus), and sprat (Sprattus sprattus), thus creating a human-wildlife conflict in various countries [6].

After coming back from the brink of extinction following European Union level protective legislation [7, 8], cormorant numbers have increased rapidly in the continent, including in my study area of Västra Götaland in Sweden [9, 10]. This region, and more specifically the 8-fjords area, has suffered economically following the collapse of codfish stocks due to overfishing in the 1980s [11-13]. Despite the creation of a fish protection area forbidding the fishing of cod, haddock (Melanogrammus aeglefinus), and pollack (Pollachius pollachius) within the fjords [14], codfish recovery has not been forthcoming [15-17] and thus a “protective” hunt was created in order to shoot cormorants perceived as a threat to these protected species [18].

Cormorant diet may be assessed in several ways: through feeding observations, pellet (mucus covered indigestible prey material which is expelled around once a day) and regurgitate collection and analysis, and stomach contents analysis [1]. Stomach samples provide a direct assessment of consumed prey items but may be subject to biases related to digestion. Previous studies in the 8-fjords worked only with pellet data [19, 20], while this study focuses almost exclusively on stomach samples.

The aim of this study is to determine the diet composition of cormorants in the 8-fjords area to ascertain their importance as predators of protected fish species.

More specifically the aims of this study are to (1) increase knowledge on cormorant diet composition and contribution of protected fish species to the diet by comparing data from different fjords over multiple years and seasons, (2) compare the information obtained from pellet and stomach analysis, (3) evaluate the differences between fish caught by cormorants and those caught by the monitoring surveys.