Salish Sea Ecosystem Conference

Factors that affect the vertical distribution of Olympia oyster larvae in Fidalgo Bay, WA

Restoring viable, self-sustaining populations of the native Olympia oyster (Ostrea lurida) in the Salish Sea is ecologically and socially valuable. Olympia oysters are sessile adults, so they rely entirely upon their free-swimming planktonic larvae for dispersal. Larval dispersal affects the size of local populations and connectivity between metapopulations, so understanding dispersal patterns can help managers prioritize habitat restoration efforts to achieve the ultimate goal of establishing a self-sustaining network of Olympia oyster populations throughout the Salish Sea. Olympia oyster larvae actively control their vertical position in the water column with swimming and sinking behaviors, which can affect which currents carry them and can ultimately determine dispersal and population connectivity. 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 for the species. On four consecutive days in July 2017, we collected, counted, and measured the length of Olympia oyster larvae from four depths over the tidal cycle in combination with salinity, temperature, and chlorophyll-a measurements. In addition, we deployed an acoustic Doppler current profiler to measure current velocities in the bay’s main channel. Mixed effects modelling results indicate that larvae were distributed significantly shallower when current speeds exceeded ~25 cm s^-1 and deeper when current speeds were less than ~25 cm s^-1, but it is unclear whether distribution was due to passive or active larval movement. If larvae were actively controlling their depth, they did not distribute at depth-specific temperature or chlorophyll-a conditions, which was likely due to vertically well-mixed conditions. Results indicated larvae did not perform tidally-timed vertical migrations as predicted and it remains unclear whether O. lurida larvae in Fidalgo Bay exhibit 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. Results from this study 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 areas of likely settlement within the bay and determine how much of the Fidalgo Bay population will self-recruit versus become a source population for the surrounding region.