Stomach content, biomass, abundance and body score of long-tailed ducks (Clangula hyemalis) from south-eastern Baltic Sea

The long-tailed duck (Clangula hyemalis) is a vulnerable and declining species wintering in the Baltic Sea. The introduction of the invasive fish, the round goby (Neogobius melanostomus), dramatically impacted the benthic macrofauna in hard-bottom, while no significant changes occurred in soft-bottom benthic macrofauna. Therefore, we aimed to assess the extent to which the diet of long-tailed duck changed in two different bottom types. We analysed the stomach content of 251 long-tailed ducks bycaught in gillnets from 2016 to 2020 in hard- and soft-bottom habitats and compared these results with those published by Žydelis and Ruškyte (2005). The results show that the long-tailed duck experienced a change in diet in hard-bottom habitats, shifting from the blue mussel to Hediste diversicolor, barnacles and fish. In soft-bottom habitats, their diet remained similar over time and was based on H. diversicolor, a few bivalve species and Saduria entomon. There was no evidence of significant differences in diet neither between sex nor age. Despite the above-mentioned changes in diet, the average body condition of the species did not change neither over time nor between habitats. This confirms that long-tailed ducks have high feeding flexibility and quick species response to changes in prey availability, as they are capable of shifting their diet to new prey.