Assessing invasion risk of Didemnum vexillum to Atlantic Canada

Aquatic invasive species are an ongoing economic and ecological problem in Atlantic Canada. To optimize management efforts of high-risk species, we must quantify risk of invasion at scales relevant to management efforts. Here we provide an updated and improved detailed-level risk assessment (DLRA) for Didemnum vexillum that uses new methods and tools to quantify and discriminate risk of invasion to the region. The screening level risk assessment framework CMIST (Canadian Marine Invasive Screening Tool) was used in a novel context to calculate uncertainty-adjusted invasion risk scores for 13 assessment zones in Atlantic Canada. Assessments were informed by 1) environmental niche modelling (MaxENT) to predict areas suitable for establishment; 2) source-based vector analysis to quantify potential for arrival and spread of D. vexillum via high-risk vectors (i.e., commercial vessels, ferries, fishing vessels, and aquaculture transfers); and 3) updated ecological data from the literature. Overall invasion risk, likelihood of invasion, and impact of invasion were highest in Bay of Fundy assessment zones and lowest in the most northern zones (St. Lawrence estuary, northern Gulf and the east coast of Newfoundland). Connectivity with source zones of D. vexillum via both natural (e.g., currents) and anthropogenic vectors (e.g., vessels) is highest in the Bay of Fundy due to proximity to established populations and high levels of vessel traffic. Potential for impacts is highest where vulnerable populations (e.g., scallops) and highly or moderately suitable areas for establishment exist. These areas are in the Minas Basin, Chignecto Bay, southwest New Brunswick, and southwest Nova Scotia with smaller areas in Mahone Bay and offshore on Western Bank and Sable Island Bank. Projections of environmental suitability for 2075 show a northeastward shift, with areas of high suitability retained in the Bay of Fundy and expanding into the Northumberland Strait. To reduce further local spread in the Bay of Fundy, bottom-disturbing activities, such as dredging and trawling where D. vexillum is present, should be addressed. In addition, movement of vessels between source areas and areas of high environmental suitability should be monitored, especially in anomalously warm years when populations are likely to be larger. Targeted monitoring of areas of current and future environmental suitability with high connectivity to source zones of D. vexillum should also be considered to improv