The Olympia oyster (Ostrea lurida)‡ was historically abundant in Willapa Bay, WA, but populations were decimated by overexploitation in the mid to late-1800s and have failed to recover. We investigated the potential role of two introduced predatory gastropods, the Japanese drill (Ocinebrina inornata) and the eastern drill (Urosalpinx cinerea), in limiting Olympia oyster recovery. We quantified the bay-wide distribution, local abundance, and per capita effects of drills, and asked how each of these three components of total invasion impact might be influenced by another dominant introduced species, the Pacific oyster (Crassostrea gigas). Bay-wide sampling revealed differences in spatial distribution of the two drill species, with U. cinerea more abundant toward the head of the estuary and O. inornata more abundant toward the mouth. Individual feeding trials indicated that both drill species preferred Pacific oysters to Olympia oysters of similar size, and preferentially attacked smaller oysters. We used field enclosures to quantify the direct effects of Japanese drill predation on Olympia and Pacific oysters, intra and interspecific competition, and indirect effects mediated by the shared predator. Predation reduced the survival of both oyster species, but the per capita impact of Japanese drills declined with increasing density of either Olympia or Pacific oysters, consistent with a type II functional response. This positive indirect effect of Pacific oysters on Olympia oysters was offset by asymmetric competition, in which Pacific oysters reduced Olympia oyster growth and survival but not vice versa. Despite the large drill impacts seen in these experiments, Olympia oysters transplanted to intertidal sites throughout the bay experienced low and variable rates of drill predation compared with other mortality sources. Introduced drills may be only one of a suite of factors that prevent rebuilding of Olympia oysters in the intertidal zone in Willapa Bay.
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