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Abstract

This thesis concerns the diet of a piscivorus top predator, the Great Cormorant (Phalacrocorax carbo sinensis), in the Baltic Sea. The Great Cormorant has increased exponentially in numbers during the last decades. This has resulted in a human-cormorant conflict because of its perceived competition with fisheries and negative impact on ecosystems. Information about the diet of cormorant along the Swedish Baltic coast and knowledge about the appropriate methodology to use for investigating potential ecosystem impacts and predatory effects on fish populations is, however, lacking. This thesis therefore examined the diet at two coastal locations, in the Bothnian Sea and in the Baltic Proper. Diets were examined with different methodologies to enable evaluation of its uses and applications. For an overall diet, pellets, regurgitate material and stomach content were considered. By tagging fish, estimates of the predatory pressure could be achieved.

The predatory impact on stocked trout (Salmo trutta) and salmon (Salmo salar) smolt migrating from the river Dalälven was estimated to 2.3 % in 2005 and 2006. This was achieved by tagging smolt with Coded Wire Tags and Carlin tags which were recovered in pellets, faeces and nest material. Common species in the diet of cormorants in Lövstabukten (southern Bothnian Sea) and around Mönsterås (Baltic Proper) were herring (Clupea harengus), European perch (Perca fluviatilis), eelpout (Zoarces viviparus), Cyprinidae and sticklebacks (Gasterosteidae), but the diets varied between locations and over time. One of the investigated areas, Mönsterås, has undergone substantial changes in the fish ecosystem structure over the last two decades, with decreasing numbers of large predatory fish and more sticklebacks and Clupeidae. These changes were reflected in the cormorant diet as this shifted from European perch and Cyprinidae in 1992, towards more herring and eelpout in 2009. Sticklebacks were the most commonly occurring prey in 2009, and this was probably due to the general increase in stickleback numbers. It could also partly be due to a difference in methodology. The study in 1992 was based on pellet material while the study covered in this thesis, in 2009, was based on stomachs, which better represents smaller fish individuals and species than pellets. In studies concerning ecosystem impacts, where it is important to attain the entire size spectra of prey throughout the year, stomach content was concluded the most applicable method.