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

Authors: Dumbauld, Brett; DU, XIUNING - Oregon State University; HUNSICKER, MARY - National Oceanic & Atmospheric Administration (NOAA); FORSTER, ZACHARY - Washington Department Of Fish & Wildlife

Submitted to: Journal of Sea Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2023
Publication Date: 5/5/2023
Citation: Dumbauld, B.R., Du, X., Hunsicker, M., Forster, Z. 2023. Multi-decade changes in the condition index of adult Pacific oysters (Crassostrea gigas) in response to climate in a US west coast estuary. Journal of Sea Research. 193. Article 102383. https://doi.org/10.1016/j.seares.2023.102383.
DOI: https://doi.org/10.1016/j.seares.2023.102383

Interpretive Summary: Pacific oysters (Crassostrea gigas) were introduced to the US west coast in the early 1900’s and have become the mainstay for the shellfish aquaculture industry on this coast. Pacific oysters only regularly spawned and became “naturalized” in several discrete estuarine locations like Willapa Bay where conditions allowed for adult oyster gametogenesis and spawning as well as larval survival, retention and settlement. Oyster industry participants have long recognized that the condition or marketability of their oysters was related to proximity of their growing areas to the ocean in this estuary. This prompted resource managers to begin routinely collecting data and examine trends in oyster condition index (CI) which is a measure of oyster meat weight relative to shell weight in 1954. A new analysis of this almost seven decade long CI record revealed consistent trends across four sampling locations in Willapa Bay with 50-70% of the variability across years explained by a single component. This variability was positively correlated to that in a separate index of upwelling intensity measured in the coastal ocean during the summer. A negative relationship with the Pacific Decadal Oscillation (PDO), a broader basin-scale index of temperature in the Pacific Ocean was also significant, but weaker. Dramatic shifts or changes in the long-term CI record were identified in 1977/78 and 1999/2000. The shift in 1977/78 corresponds with a previously identified shift in the basin scale PDO, but the more recent shift appeared to be correlated with local fluctuations in both ocean temperature and upwelling intensity. These shifts are likely tied to seasonal timing of temperature and phytoplankton as oyster food which have been shown to control gametogenesis and spawning and therefore further investigation could guide efforts to mitigate these effects given an uncertain future climate.

Technical 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.