Location Matters: Passive and Active Factors Affect the Vertical Distribution of Olympia Oyster (Ostrea lurida) Larvae

Abstract

AbstractDispersal, retention, and population connectivity are impacted by current regime and the behaviors that drive larval distribution, so understanding both is key to informing restoration of native species like the Olympia oyster (Ostrea lurida) across its range in western North America. This study explores the relationships between several factors (temperature, [chl a], larval size, tidal stage, and estimated current speed) and Olympia oyster larval vertical distributions in Fidalgo Bay (48.4828, − 122.5811), a shallow, tidally flushed bay in the Salish Sea. Olympia oyster larvae collected from four depths over the tidal cycle from July 11–14, 2017, were ~ 20% deeper near slack tide and shallower during the faster parts of both ebb and flood, with a threshold for this transition around an estimated 25 cm s−1. This pattern does not suggest tidally timed migrations as has been shown in another population of Olympia oysters, nor can this pattern be totally explained by passive processes. Larvae did not cluster at depths with specific temperatures or [chl a] but there was a difference in larval size between surface and bottom waters, with older, larger larvae more common at the bottom. Fidalgo Bay does not exhibit two-way flow or strong vertical shear, so vertical distribution of larvae likely has little effect on transport in this system but might in other similarly shallow habitat areas with higher stratification that are target restoration sites in the Salish Sea. These results add to the growing number of studies that show location-specific differences in larval vertical distribution and behavior within taxa and underscore the importance of integrating local hydrodynamics into predictions of bivalve larval transport.