A Horizon Scan watchlist of freshwater non-indigenous species in the maritime provinces of Canada

There is a growing in-flux of non-indigenous species (NIS) brought to new locations via anthropogenic pathways, some of which are harmful to ecosystems, economies, and social and cultural health of receiving environments.In order to protect native ecosystems from invasion by novel NIS, watchlists of potentially problematic species are required but this task can feel overwhelming when considering the vastness of global biodiversity.To date in Canada, the Great Lakes are the only jurisdiction to have generated a freshwater NIS watchlist and published the associated methods.Here we compiled a working list of 4,446 species (plants, invertebrates, vertebrates) which was refined to 221 species based on taxa of interest (i.e.freshwater species), climate matching, invasion status and history, and associated vector presence, to focus on species relevant to the Maritime Provinces of Canada (i.e.Nova Scotia, New Brunswick, and Prince Edward Island).Using a rapid-level risk assessment tool applied to the refined species list, we identified 130 NIS as high-risk invaders to the Maritimes.We also identified numerous species with knowledge gaps that require further research.Common gaps included species' ecological thresholds, summaries of direct and indirect ecosystem impacts, and socio-economic impacts of species.We also noted the vectors associated with each of the 221 species.Although this work is specific to the Maritimes, the methodology and species list (including risk assessments) could easily be adapted to other regions of Canada or elsewhere.This project represents the first Canadian-led multi-province, multi-taxon, large scale horizon scan aimed at identifying species for management action, including listing in legislation.