DOI: 10.5061/dryad.fxpnvx0th
title: Stomach content, biomass, abundance and body score of long-tailed ducks (Clangula hyemalis) from south-eastern Baltic Sea

description: 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.
description: The stomach content of 267 long-tailed duck individuals
 accidently bycaught in fishermen’s gillnets between 2016 and 2020 from
 November to May.<br>Before the collection of stomach samples,
 long-tailed ducks were weighted and dissected for the determination of
 sex, age and body index.<br>Stomach content samples were frozen at
 -20 °C after individuals were removed from the bycatch until the
 analysis.<br>Before the analysis, stomach samples were defrosted and
 rinsed with clean water.<br>The prey items were removed and sorted
 for taxonomic identification to the lowest possible taxonomic unit and for
 the determination of abundance and biomass.<br>The ability to count
 prey individuals depended on the level of digestion. The intact prey items
 were counted and weighted.<br>Bivalve shells, fish otoliths and
 polychaete jaws, were paired and used to estimate the preys’ abundance as
 explained in Camphuysen and Leopold (2007).<br>All taxonomically
 identified prey fragments were weighted and used for characterisation of
 the biomass of a given prey taxa.<br>The biomass was measured as wet
 weight with an accuracy of ± 0.0001 g after excess water was removed using
 filter paper.<br>This approach ensured comparability of our results
 with those obained in the study of Žydelis and Ruškyte
 (2005).<br>The body index was estimated by evaluating and summing
 intestinal fat, subcutaneous fat and the condition of the pectoral
 muscle. <br>Fat deposits were scored from 0 (no fat) to 3 (very
 fat).<br>The same procedure was used to determine the condition of
 pectoral muscle, where 0 was a strongly emaciated pectoral muscle and 3
 was a muscle in good condition.
description: Few data on body condition are missing. ReadMe file is available
 with all the information about the dataset.