The Ecological Role of Invasive American Shad in the Columbia River

American shad are a prolific invasive anadromous fish along the Pacific coast of North America, and are of increasing conservation concern. The species was introduced to the Sacramento River in 1871, but rapidly dispersed and colonized additional rivers along the Pacific coast. Although the spread of aquatic invasive species is a global concern, the specific impacts of invasive American shad in Pacific coastal ecosystems are equivocal. American shad now constitute the single largest spawning run of any anadromous fish in the Columbia River, and until recently outnumbered all native salmon species of both wild and hatchery origin combined. Despite their presence in the region for well over a century, American shad persists in relative obscurity, having not been the subject of much scientific investigation, and remains ecologically overlooked. This synthesis reviews the factors responsible for the rapid dispersal and colonization of American shad along the Pacific coast and within the Columbia River basin, and identifies some of the possible negative effects, but also some unrecognized potential benefits, of this non-native anadromous clupeid.