IMPROVED RESOLUTION OF Filtration services FROM OCEANOGRAPHIC AND PHYSIOLOGICAL STUDIES of the native Olympia oyster (Ostrea lurida) and the introduced Pacific oyster (Crassostrea gigas)

Worldwide, many native oyster populations have been severely depleted. In the Pacific Northwest (PNW), large-scale restoration efforts for the native Olympia oyster (Ostrea lurida) are currently underway to augment stocks in order to regain their ecosystem services, particularly those related to filter-feeding activity. An important limitation in estimating the potential impact of these restoration efforts, however, is that little information is available on the feeding physiology of O. lurida. Preliminary models of oyster filter-feeding services, which were based on laboratory experiments, predicted that this species likely had little impact on reducing concentration of suspended particulate matter in estuaries due to their low clearance rates, relatively cold environments and short residence times of PNW estuaries. Additionally, the non-native Pacific oyster (Crassostrea gigas), which is now the dominant oyster species in the PNW, was found to have much greater filtration rates and would more likely contribute to improving water clarity compared to O. lurida.

To improve filtration service estimates for both species, we conducted in situ feeding experiments in Yaquina Bay, Oregon, to better understand their feeding behavior under natural environmental conditions during the dry (high salinity) and wet (low salinity) seasons. From these data, we constructed models to predict the clearance rates of each species within and between seasons. We also incorporated results from a highly-resolved oceanographic model suggesting that residence times were likely to be much longer than previously assumed, with considerable spatial and seasonal variation.

We found that both species fed at much lower rates under natural compared with laboratory conditions. Filtration services were specific to species and seasonal environmental conditions. Our data, along with results from previous investigations, suggest that in situ studies are required to accurately determine the ecosystem services and ecological role of a filter-feeding oysters in a given environment.  Combining in situ feeding models and more highly defined oceanographic data for Yaquina Bay, yields more accurate estimates of potential filtration services of populations of O. lurida and C. gigas within this PNW estuary.