Numbers and distribution of the Great Cormorant in Iceland: Limitation at the regional and metapopulation level

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We studied a metapopulation of great cormorant (Phalacrocorax carbo) in Iceland, using complete aerial censuses of nests in 25 years during 1975–2015. Age composition was estimated in 1998–2014 by ground surveys in September and February. Brood size was estimated from aerial photographs in 2007–2015. Weather, food, breeding habitat, and density were considered as explanatory variables when examining numerical and distributional changes in the cormorant metapopulation. In 1975–1990 total nest numbers changed little, very low numbers about 1992 were followed by an annual increase of 3.5% in 1994–2015. Total nest numbers were positively correlated with estimates of spawning stocks of cod and saithe and inversely related to the subpolar gyre index (SPG-I). During the increase in 1994–2015, average colony size at first increased and then declined. Habitat use also changed: the proportion of nests on small rocky islets (skerries) at first declined, from 69% to 44% in 1995–2003 and then increased again to about 58% in 2012–2014. Habitat changes were probably a response to changed patterns of human disturbance. Breeding density, as nests per km 2 sea
This study was supported mainly by the University of Iceland Research Fund by annual grants to AG (1994–2008) and to JEJ (2009–2015). We thank Úlfar Henningsson for piloting the cormorant censuses and brood surveys. For help with local and historical information we are grateful to many colleagues and local naturalists, especially Tryggvi Eyjólfsson (1927–2017), Hafsteinn Guðmundsson, Ari Guðmundur Ívarsson, Steinólfur Lárusson (1928–2012), Ævar Petersen, Kristinn H. Skarphéðinsson, Ástþór Skúlason, Guðbrandur Valdimarsson, Ólafur Þórðarson (1915–2003), and Böðvar Þórisson. For guidance in environmental and biological matters we are especially indebted to Karl Gunnarsson, Sarah Wanless and Thomas Bregnballe. We thank Sigmundur Helgi Brink for preparing the map in Figure 2 and Erling Ólafsson for photos of cormorant in Figure 1. We thank the editors and 2 anonymous reviewers for comments that improved an earlier version of this manuscript.