Effects of native macroalgae and predators on survival, condition and growth of non-indigenous Pacific oysters (Crassostrea gigas)

Biological invasions by non-indigenous species are causing widespread environmental changes and can threaten biodiversity and ecosystem services. Biotic interactions can play a key role in promoting or inhibiting spread of non-indigenous species. The Pacific oyster (Crassostrea gigas Thunberg, 1793) is commonly found on intertidal shores with hard substrata, but is negatively associated with macroalgae and may be influenced by the macroalgae itself or by predators associated with it. The influence of macroalgae and predation on growth, condition and survival of juvenile Pacific oysters were tested in a field experiment at two intertidal boulder shores using hatchery-reared juvenile oysters deployed on ceramic tiles. After 10 months, condition of oysters was not significantly decreased in the presence of macroalgal canopy. At one site growth was reduced by macroalgae and at the other, it tended to be enhanced, but the effect was not significant. An effect of predation could not be detected. Although spatial variation in survival and growth of juvenile oysters was found, the results suggest that pre-settlement processes and/or post-settlement processes acting upon juvenile oysters at sizes not tested in this study might better explain variation in abundance of Pacific oysters in intertidal habitats.