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COPYRIGHT 2005 Wilson Ornithological Society
The Long-tailed Duck (Clangula hyemalis) is the most abundant sea duck wintering in the Baltic Sea, where estimated numbers exceed 4 million. Wintering Long-tailed Ducks inhabit a variety of coastal habitats and shallow offshore banks (Durinck et al. 1994). Diet composition varies widely throughout their Holarctic range (Madsen 1954, Peterson and Ellarson 1977, Vermeer and Levings 1977, Goudie and Ankney 1986, Stempniewicz 1995, Bustnes and Systad 2001, Jamieson et al. 2001). However, few attempts have been made to relate feeding habits of Long-tailed Ducks to attributes of their local environment (Nilsson 1972, Stott and Olson 1973, Kube 1996). Long-tailed Ducks are recognized as opportunistic feeders (Peterson and Ellarson 1977, Goudie and Ankney 1986, Bustnes and Systad 2001), but ecological factors related to use of different habitats have received little study. We investigated food choice of Long-tailed Ducks wintering in two distinct marine habitats in nearshore waters of the Baltic Sea off the coast of Lithuania. Our objectives were to determine whether there were differences in diet choice of Long-tailed Ducks in the two winter habitats, and whether body condition of the ducks varied between the habitat types.
METHODS
Study area.--The Lithuanian coast can be characterized as an exposed, sandy coast, typical of the southern and eastern Baltic Sea (Oleninas et al. 1996). The sea floor is dominated by sand, gravel, or boulders. Sandy-bottom substrates predominated in the southern half of our study area along the Curonian Spit coast. The northern half of the Lithuanian nearshore zone is characterized by a mosaic of sediments of sand, gravel, and boulders (Oleninas et al. 1996). The sublitoral slope is gentle, with the 10-m isobath extending 7002,000 m and the 20-m isobath extending 1,500-4,000 m from the shore. Water salinity along the Lithuanian coast is low, ranging from 6 to 8%0, which results in relatively poor faunal and floral diversity, as well as in low productivity.
Two main types of macrofaunal communities can be distinguished in the Lithuanian coastal zone: the Mytilus edulis community of sessile, epifaunal filter-feeders, and the Macoma baltica community of mobile, infaunal surface-deposit feeders (Olenin 1996, Oleninas et al. 1996). The M. edulis community dominates in the northern half of the Lithuanian coastal zone, occurring on hard bottoms covered by stones and boulders at depths between 5 and 30 m. This community has the highest biodiversity (up to 50 macrozoobenthos species) and biomass (mean = ~1,750 g/ [m.sup.2], maximum = ~2,500 g/[m.sup.2] wet weight). M. edulis makes up ~93% of the total biomass; Balanus improvisus (a barnacle) and all the remaining species contribute ~7 and <1% of total wet weight, respectively. In some places, stony substrates at depths from 4 to 14 m are overgrown by the red algae, Furcellaria lumbricalis, which serves as an important spawning ground for Baltic herring (Clupea harengus membras; Olenin 1996, Oleninas et al. 1996).
The M. baltica community is associated with soft bottoms, and extends along the coast of the Curonian Spit at depths from 5 to 30 m. This habitat can be characterized as a homogenous, benthic biotope with up to 40 macrozoobenthos species dwelling in a sandy bottom. M. baltica and Mya arenaria are the dominant infaunal bivalves, composing 61 and 12% of the total biomass, respectively. Polychaetes (Nereis diversicolor, Pygospio elegans) and crustaceans (Saduria entomon and Corophium spp.) also are abundant. Mean zoobenthos biomass is ~150 g/[m.sup.2] wet weight, with a maximum of ~300 g/[m.sup.2] (Olenin 1996, Oleninas et al. 1996).
Based on spatial distribution and dominance of benthic communities (Olenin 1996; S. Olenin unpubl. data), three zones have been distinguished along the Lithuanian coast: a hard-bottom benthic community zone, an intermediate zone, and a soft-bottom community zone (Fig. 1). We present data on Long-tailed Duck foraging in hard-bottom and soft-bottom benthic community zones, but not the intermediate zone.
Data collection and analysis.--Birds accidentally drowned in fishing nets were collected for diet analysis during winters of 1997/ 1998 through 2000/2001. Nets were set at depths ranging from 1.5 to 20 m. In total, 326 Long-tailed Ducks were collected: 181 from habitats with hard-bottom and 145 from areas with soft-bottom substrates. Sex-age cohorts of collected birds in hard- and soft-bottom habitats, respectively, were as...
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