Masters Thesis

High predation may hinder native oyster (Ostrea lurida Carpenter, 1864) restoration in North Humboldt Bay, California

Oyster reefs serve an important role in estuarine ecosystems. Their shells create a complex habitat that serves as a home for future generations of oysters, other sessile invertebrates, and fishes. Native oysters, Ostrea lurida, are thought to have been historically abundant in Humboldt Bay. Populations of this species have failed to rebound from over harvesting and harvest practices that broke up existing reef structures. Early attempts to farm native oysters were unsuccessful, which lead to attempts to farm Crassostrea virginica in Humboldt Bay. Importing C. virginica is thought to have introduced the predatory Atlantic oyster drill, Urosalpinx cinerea. In order to determine why populations of O. lurida have not rebounded, I examined predation as a potential limiting factor on native oysters in north Humboldt Bay, California. In 2008 and 2009, juvenile oysters were cultured at the Telonicher Marine Laboratory following spawning of adults from Mad River Slough, North Humboldt Bay, California. After rearing the oyster larvae released from these adults through settlement and metamorphosis to juvenile stage (approximately 8 weeks), plastic tiles seeded with juvenile O. lurida were assigned to one of three treatments: (1) a treatment open to predation, (2) a caged treatment in which settlement panels were surrounded with stainless steel mesh to exclude predators, or (3) a fenced control, which allowed predators access while controlling for possible cage effects (with a strip of stainless steel mesh only along the top and two sides of the settlement panels which allowed predators access through a central opening). Results showed that mean survivorship of juvenile O. lurida in caged treatments was significantly greater than treatments open to predation (open and fenced “cage control”). The predominant predator at one site was U. cinerea, which was abundant at the site. In 2010, I repeated the study using one-year old oysters. As with juvenile oysters, survival was significantly greater in treatments protected from predation. This study suggests that mortality from predation is a significant factor which leads to high mortality of juvenile and one-year old oysters in Humboldt Bay. Predation pressure may therefore be a major hurdle to overcome before successful restoration efforts can be achieved in Humboldt Bay, California.

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