Multi-decade changes in the condition index of adult Pacific oysters (Crassostrea gigas) in response to climate in a US west coast estuary

Abstract

Pacific oysters (Crassostrea gigas) were introduced to the US west coast in the early 1900's, but only regularly spawned in several discrete estuarine locations like Willapa Bay where conditions allowed for adult oyster gametogenesis as well as larval survival, retention, and settlement. Oyster industry participants have long recognized that the condition of marketable oysters was related to proximity of their growing areas to the ocean. This prompted resource managers to routinely collect data and identify trends in oyster condition index (CI) in this estuary. An analysis of this almost seven- decade long CI record revealed consistent trends across four sampling locations in Willapa Bay where 50–70% of the variability in CI was explained by a single component. This component of variability in CI was related most strongly to variability in the upwelling index (UI) which was positively correlated with CI during the summer upwelling season. A relationship with the Pacific Decadal Oscillation (PDO), a broader basin-scale index of temperature was also significant, but weaker and PDO was negatively correlated with CI. Shifts or breaks in the long-term CI record occurred in 1977/78 and 1999/2000 with another shift related to the second component of variation in CI that occurred in 1988/1989. The change in 1977/78 has previously been shown to correspond with a shift in the basin scale PDO, but more recent shifts appear to be correlated with local fluctuations in both temperature and upwelling intensity. Further investigation into shifts in the seasonal timing of temperature and phytoplankton as variables that control the oyster gametogenic cycle seem warranted.