Flood mitigation structures transform tidal creeks from nurseries for native fish to non-native hotspots

Habitat connectivity is important for ecosystem processes, yet globally is altered by anthropogenic structures. Anthropogenic barriers are common in coastal aquatic ecosystems, yet the effects of small-scale barriers such as floodgates have received relatively little study. The lower Fraser River in British Columbia, Canada, is a large river-floodplain ecosystem with numerous dikes and floodgates protecting valuable human infrastructure and developments. Here we assess fish communities in ten tributaries of the lower Fraser River, five with floodgates and at five reference sites, located primarily in agricultural areas. Floodgate presence was associated with reduced dissolved oxygen concentrations, increased abundance of three invasive fish species, and decreased abundances of five native fish species including two salmon species. These findings provide evidence that floodgates decrease suitable habitat for native fishes, becoming hotspots for non-native fishes. Given sea-level rise and aging infrastructure, there is an opportunity to incorporate biodiversity considerations into restoration of this infrastructure.