Reconstructing the Ecological History of Shell‐Boring Polychaetes in the Salish Sea to Inform Conservation and Restoration Strategies for Native Oyster <scp> <i>Ostrea lurida</i> </scp>

ABSTRACT The Olympia oyster ( Ostrea lurida ) is the only oyster native to the west coast of the United States. Its populations are less than 5% of what they were historically, and although restoration efforts have re‐established some populations, the long‐term viability of the species will depend on its capacity to resist environmental threats. Shell‐boring polychaetes have recently been reported for the first time in the Salish Sea, infesting both Pacific ( Magallana gigas ) and native oysters. These parasitic worms are pervasive aquaculture pests that can jeopardize oyster health and growth. It remains unknown whether these polychaetes are new to the Salish Sea ecosystem, or if they have long been present but were previously unreported. We take advantage of the fact that traces of infestation are preserved in oyster shells to generate a timeline for shell‐boring polychaetes in the region. We collected 996 native oyster shells from four populations (Modern, Archaeological, Recent Fossil, and Fossil) across three different time periods (Modern, Holocene and Pleistocene). Infestation prevalence ranged from 9% to 55% in the past populations, and from 18% to 96% in the Modern populations, suggesting that prevalence is higher today than it was in the past. Radiocarbon dating and prior studies allowed us to determine that shell‐boring polychaetes have coexisted with native oysters for at least 80,000 years. We end by providing some recommendations for managers and practitioners on how to monitor for this parasite if it were to pose threats for native oyster populations undergoing restoration.