Vertical Distribution of Olympia oyster (Ostrea lurida) larvae in Fidalgo Bay, WA

Restoring populations of native Olympia oyster (Ostrea lurida) in the Salish Sea is ecologically and socially valuable. Olympia oysters disperse as planktonic larvae that control their depth with swimming and sinking behaviors, which can affect the currents that carry them and ultimately determine dispersal. Understanding larval dispersal patterns can help prioritize restoration efforts and establish a self-sustaining network of oyster populations in the region. The purpose of this study was to determine which factors (temperature, chlorophyll-a, larval size, current speed, tidal stage) influence the vertical distribution of Olympia oyster larvae in Fidalgo Bay, which is a Washington state priority restoration area. On four consecutive days in July 2017, we collected and analyzed Olympia oyster larvae from four depths over the tidal cycle in combination with temperature and chlorophyll-a measurements. We also measured current velocities profiles in the main channel. Mixed effects modelling results indicate that larvae were distributed significantly shallower when current speeds exceeded ~25 cm s-1 and were deeper when currents were below ~25 cm s-1, but it is unclear whether distribution was due to passive or active larval movement. If larvae were behaviorally controlling their depth, they did not distribute at depth-specific temperature or chlorophyll-a conditions. Larvae did not perform tidally-timed vertical migrations and it remains unclear whether larvae exhibited an ontogenetic vertical migration strategy. Fidalgo Bay does not exhibit a two-way flow or strong vertical shear, so Olympia oyster larval vertical distribution likely has little to no effect on their transport through the main channel of the bay. These results should not be generalized to other restoration areas due to the unique conditions of this location and the possibility of larval behavioral plasticity between distinct populations of Olympia oysters. Results can inform a Fidalgo Bay larval transport model to predict dispersal patterns and prioritize Olympia oyster restoration efforts.