The Great cormorant, Phalacrocorax carbo, as a predator of the invasive Round goby

Sammanfattning: Dispersal of invasive species poses a significant threat to biodiversity loss globally and is one of the major threats to the ecological condition in the Baltic Sea. The Baltic Sea is facing potential problems caused by the Round goby, Neogobius melanostomus, which was presumably introduced via ballast water. Since its first discovery in 1990, the Round goby has continued to expand and is currently established along certain parts of the Swedish coast. An invasive species can play a substantial ecological role by serving as prey for native species. An abundancy of prey can alter diets and foraging behaviors of native predators, such as the Great cormorant, Phalacrocorax carbo. Since the Round goby might have negative impacts on local ecosystems, cormorants can play a beneficial role by regulating Round gobys' population growth. The objective of this study was to examine predation on Round goby by the Great cormorant in the Karlskrona archipelago, in the South-eastern Swedish coast. The aim of the study was to investigate prey selectivity on round gobies by cormorants throughout years and seasons. Monitoring-and catch data from the same area during the years 2015-2018 was analysed to see the role Round goby play as a prey for cormorants. The diet composition of cormorants was determined by analysing stomach contents of 182 cormorants. Identification of species in the stomach content was possible due to examination of prey and otoliths. My analyses detected an indication of active selection of round gobies in June-September 2015 and 2017, whereas inaccessibility or non-selection of Round goby was observed in 2018. Selection of round gobies during summer months might be a result of seasonal changes in diet. To better understand the dynamics between cormorants and round gobies in the archipelago in Karlskrona, further sampling and research is required.