Ecological Interactions Between Non-Native American Shad and Pacific Salmon: the Columbia River Case Study

Many ecosystems have been dramatically affected by non-native species, but not all such species have strong deleterious effects. American shad, Alosa sapidissima, a fish species native to the Atlantic coast of North America, was transplanted to the Pacific coast in the late nineteenth century and quickly colonized many US rivers. Their increased abundance in the Columbia River coincided with declines in native anadromous Pacific salmon and trout, and adult American shad now greatly outnumber returning salmonids. This paper reviews evidence for possible ecological interactions between salmon and American shad across their life histories and habitats. Despite the great abundance of American shad and their apparent overlap in use of Columbia River mainstem habitats, harmful effects on salmon are neither clear from empirical studies nor from ecological principles. Rather, the life histories and habitat use patterns tend to separate spawning adult salmon and their offspring from American shad in space and time. Currently available evidence indicates that this separation results in weak, neutral, uncertain, or offsetting effects on salmon (i.e., a mix of positive and negative interactions). Given the limited research on shad in Pacific ecosystems, several lines of investigation are warranted to advance understanding of their ecology and scope for interactions with native fishes and to support a clearer scientific basis for management decisions regarding American shad.