Ducks share the aquatic environment with invertebrate-eating fish. Thus, competitive interactions may take place. Fish have been introduced to many formerly fishless lakes, which has profoundly affected the competitive and predatory relations in these waters. In this paper we review recent findings on duck fish competitive interactions in boreal lakes. On a general level, analyses based on presence/absence data of fish have indicated that ducks can be negatively affected by fish. More rigorous studies where fish density has been considered have corroborated the pattern emerging from presence/absence studies. For the Common Goldeneye (Bucephala clangula) and Eurasian Perch (Perca fluviatilis), the effect of competition has been tested experimentally. In general, it appears that diving ducks such as Common Goldeneye, which forage in open water, are the most affected by fish, Common Teal (Arias crecca) is intermediate, whereas Mallard (A. platyrhynchos), which forage among the shore vegetation, is little affected. Likelihood or the strength of competition between ducks and fish may also depend on habitat productivity and structure. Numbers of invertebrates are higher among vegetation where there are less fish preying on them. Duck fish interactions are important to take into account when planning wetland creation and restoration for ducks. There is also an urgent need to mitigate the effects of fish introductions in wetlands.
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