Relative Foraging Value to Lesser Scaup Ducks of Native and Exotic Clams From San Francisco Bay

Abstract

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/m 2 . For items buried 3 cm deep, intake rates did not differ for prey Ͻ6 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 (Ͻ12 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.

A Conceptual Model of the Aquatic Food Web of the Upper San Francisco Estuary

“…In deeper water, phytoplankton may circulate below the critical depth and lose carbon through respiration, unless stratification occurs. Biomass accumulation in shallow water may be preserved by a trophic cascade in which foraging sturgeon, splittail, and diving ducks limit clam abundance and release grazing pressure on phytoplankton (Richman and Lovvorn 2004) (Linkage 1.52). Shallow water production that is exported to deeper channels may subsidize those lower-production, heterotrophic environments (Lucas et al 2002;Lopez et al 2006).…”

“…The second mechanism of release occurs by trophic cascade, when predation by waterfowl, sturgeon, and splittail reducesthe clam population (Poulton et al 2002;Richman and Lovvorn 2004;Kogut 2008). Waterfowl predation explain the low abundance of clams in some shallow water areas; and it is here that potentially productive blooms may form and be "exported" to adjacent habitat (Cloern 2007).…”

Effects of clam species dominance on nutrient and energy acquisition by spectacled eiders in the Bering Sea

“…A type 2 functional response, in which intake rate increases with increasing prey density up to an asymptote where intake is limited by handling time, often typifies diving duck foraging on benthic foods (Giles 1990, Lovvorn & Gillingham 1996. Prey depth in the sediments has important effects on the functional responses of crabs and shorebirds eating clams (Wanink & Zwarts 1985, Lipcius & Hines 1986, Seitz et al 2001), but effects of prey depth have seldom been studied for diving ducks (Carbone 1995, Richman 2002. Nutrient and energy content, digestibility, and crushing resistance of shells can vary with both species and size of bivalves (Beukema & de Bruin 1977, Jorde & Owen 1988, Bustnes & Erikstad 1990, Piersma et al 1993, and various studies have shown size selection of bivalves by diving ducks (Draulans 1982, Bustnes & Erikstad 1990, de Leeuw & van Eerden 1992, Bustnes 1998, Hamilton et al 1999.…”

U.S. Geological Survey science strategy for highly pathogenic avian influenza in wildlife and the environment (2016–2020)

“…USGS field surveillance has also detected AI in a sentinel oyster (Densmore and others, 2016). Because bivalves are an important prey species for diving ducks (Richman and Lovvorn, 2004), it is also important to investigate the possibility of AI transmission from invertebrate prey to water bird species.…”