Seasonal variability in a nascent population of a non-indigenous colonial ascidian (Didemnum vexillum) near Winchester Bay, Oregon

The non-indigenous colonial tunicate Didemnum vexillum plagues many shellfish aquaculture operations around the world by smothering crop and gear and displacing juvenile bivalves. There are two populations of D. vexillum in Oregon, but little is known regarding their spatial extent, seasonal dynamics, annual variability, reproductive biology, or ecology. SCUBA divers monitored the seasonal variation and spatial extent of the nascent D. vexillum population in its current foothold at Umpqua Aquaculture, a longline oyster farm adjacent to Winchester Bay, Oregon. From May 2011 to November 2016, divers performed biannual subtidal surveys to measure the cover of D. vexillum on anchor lines. We found that this population of D. vexillum exhibited extensive fluctuation in colony during this period, especially between fall and spring. With one exception, cover was greater in the fall than the spring, though this fluctuation cannot be explained by our proxies for salinity or temperature. Seasonally, on average, D. vexillum covered the greatest proportion of line between 5.5 and 6.0 m depth in both the spring (37.5 ± 18.8%) and fall (45.1 ± 16.1%). During individual surveys, the depth of maximum percent D. vexillum cover was more variable, spanning ~ 4.5–6.5 m depth. Overall, we document a net increase in D. vexillum cover at Umpqua Aquaculture between the first and last surveys during the 5-year period. This study is the first published multi-year analysis of D. vexillum in Oregon and represents the longest continuous monitoring period of an in situ population of D. vexillum to date.