Up and down or how to stay in the bay: Retentive strategies of Olympia oyster larvae in a shallow estuary

Abstract

The decline of Olympia oysters along the US west coast has prompted interest in population recovery and therefore the larval biology of the species. Olympia oysters are estuarine dependent as adults. Larvae must be retained in or return to the estuary for successful recruitment. We examined larval abundance and tidally timed vertical migration for Ostrea lurida in Coos Bay, Oregon. Weekly zooplankton tows and CTD casts were conducted from June to October 2010 in alternating rising and falling tides. All larval stages were represented, confirming that larvae are retained in the estuary during their entire development. Possible mechanisms to increase retention are the timing of larval release and the behaviour of the larvae. No Olympia larvae were observed until mid-July, when water temperature rose over 16°C. Above this temperature, the probability of presence increased directly with temperature and indirectly with stratification (generalized additive model; R2 = 0.89); larvae were most abundant during the dry season (August to September), when low river inflow leads to long water residence times in the upper bay, where the water is warm (>16°C), salty (> 25) and weakly stratified. Even in well-mixed conditions, Olympia oyster larvae performed tidally timed vertical migrations, moving deeper during falling tides (mean depth 7.22 ± 0.43 and 3.79 ± 0.69 m for falling and rising tides, respectively). High current speeds (>0.5 m s-1) overcame their swimming capability, preventing vertical migration and limiting the retentive effectiveness of this behaviour. Matching larval release with predictable hydrographical features (dry season) that favour larval retention in the bay may be the main factor determining population sustainability in Coos Bay.