Diving duck

Abstract: Invasions of exotic invertebrates have greatly altered many aquatic communities, but impacts on the foraging energetics of predators seldom have been assessed. In San Francisco Bay, California (USA), a major community change occurred with introduction of the Asian clam (Potamocorbula amurensis) in 1986. This species now greatly outnumbers the previous clam prey of a variety of sharks, rays, sturgeon, flatfish, and crabs, as well as several diving duck species for which the bay is the most important wintering area on the U.S. Pacific Coast. P. amurensis also accumulates much higher levels of some contaminants than the formerly dominant prey. Because alteration of the food base or contaminated foods on wintering areas may be factors in the population decline of scaup ducks, effects of this exotic invasion are important to assess. For Lesser Scaup (Aythya affinis), we studied effects of differences in nutrient content, digestibility, crushing resistance of shells, areal density, size, and depth in the sediments on the relative foraging value of exotic P. amurensis vs. the formerly dominant native clam Macoma balthica. P. amurensis, including shells, had higher nitrogen and energy content per clam of the same length class, and higher digestibility of energy, than M. balthica. Gut retention time did not differ between clam species, so their relative profitability for scaup was determined mainly by the intake rate of digestible nutrients during short, costly dives. For scaup foraging in an aquarium 1.8 m deep, intake rates (number of prey per second) of food items buried in sand-filled trays increased with increasing prey density up to at least 4000 prey/m2. For items buried 3 cm deep, intake rates did not differ for prey <6 mm long vs. prey 6–12 mm long; however, intake rates were much lower when prey were deeper in the sediments (6 cm vs. 3 cm). In the field, a much higher percentage of P. amurensis were in the length range most commonly eaten by Lesser Scaup (<12 mm), and unlike M. balthica, almost all P. amurensis were in the top 5 cm of sediments where scaup intake rates are highest. In tensometer measurements, shells of P. amurensis were much harder to crush than shells of M. balthica, which might partly offset the apparent energetic advantages of P. amurensis. In many respects, the exotic P. amurensis appears to be a more valuable food than the native M. balthica for Lesser Scaup. However, because P. amurensis accumulates much higher levels of some contaminants, this exotic invasion increases the risk of toxicity to scaup and a range of other benthic predators.