Spatial and temporal variability in infestation of Oregon oyster farms by shell-boring polychaetes

Shell-boring polychaetes burrow into the shells of cultivated and wild molluscs, leading to the formation of unsightly blisters that fill with mud, detritus, and fecal material. Infestation of cultivated oysters poses economic risks for the Pacific Northwest's shellfish mariculture industry because the blisters reduce the aesthetic quality and market appeal of oysters sold on the half-shell market. To help Oregon's multimillion-dollar mariculture industry develop resilience against this emerging biosecurity threat, we quantified seasonal variability and spatial differences in the infestation of Pacific oysters (Crassostrea gigas) by shell-boring polychaetes, and assessed whether prevalence varied among grow-out methods, seasons, and host traits (shell height and tissue mass). In 2019-2021, we obtained 829 Pacific oysters from seven commercial shellfish farms spanning the Oregon coast (Tillamook Bay, Netarts Bay, Yaquina Bay, and Coos Bay), and observed a mean statewide infestation rate of 11-31% over four sampling seasons. We observed the highest prevalence of infestation by shell-boring polychaetes (72%) in Netarts Bay. Oysters cultivated off-bottom exhibited lower prevalence of infestation than oysters grown on-bottom. Our study also revealed a significant effect of seasonality, with higher rates of infestation during winter. Oyster shell height and tissue mass were not significantly associated with the prevalence of infestation. Our observations identify an infestation hotspot in Netarts Bay, and suggest that off-bottom culture may help reduce infestation rates, mitigating damage caused by shell-boring polychaetes in Oregon bays and estuaries.