Columbia River Research Laboratory
American shad (Alosa sapidissima)
American shad Alosa sapidissima are not native to the Pacific Northwest. Juvenile fish from the Hudson River, New York, were intentionally introduced into the Sacramento River, California in 1871. An anadromous species, they quickly dispersed and created self-sustaining populations in many Pacific Northwest rivers. In the Columbia River, American shad returning to spawn increased greatly between 1960 and 1990, and recently 2-4 million adults have been counted per year at Bonneville Dam. Spawning migrations typically begin in May and peak runs occur in June. Adults are iteroparous and return to the ocean after spawning. Eggs hatch within days and the larvae begin feeding as they migrate downstream. Juvenile fish typically reach estuaries before they are one year old, however, some juvenile American shad remain in Columbia River reservoirs for more than a year. The success of non-native American shad in the Columbia River is likely related to successful passage at the dams, good spawning and rearing habitats, and low exploitation by commercial and sport fishers.