Fine-scale habitat factors linked to density but not distribution of an invasive estuarine predator

The Atlantic oyster drill Urosalpinx cinerea is an introduced predatory gastropod that has negatively impacted Olympia oyster restoration in multiple estuaries on the west coast of the United States. In San Francisco Bay, California, Atlantic oyster drills have a patchy spatial pattern of presence and absence and occur in a range of densities where they are present. This variable population distribution and a limited understanding of their local dispersal history poses a challenge to oyster restoration site selection. To address this dilemma, we evaluated five abiotic habitat factors as potential determinants of drill distribution. In 2017 and 2018, we compared quarterly drill abundance data to substrate composition, elevation, water temperature, salinity, and inundation at eight sites in Richardson Bay, a small embayment in San Francisco Bay. Using generalized linear mixed effects models, we found that amount of coarse substrate and elevation were positively and negatively, respectively, associated with drill population density at the four sites where drills were present. None of the five habitat factors, however, explained the absence of drills from the other four sites. These findings suggest ways for oyster restoration practitioners to select sites that optimize the chances oyster and drill co-existence or minimize the risk of drill invasion and point to the need for extreme caution against accidental introductions of drills to novel areas with suitable habitat. We recommend extensive drill population surveys in regions where Olympia oyster conservation is taking place coupled with additional fine-scale environmental data to better understand Atlantic oyster drill biogeography and to improve the odds of success of future Olympia oyster restoration work.