Assessment of the Risk of Invasion of National Forest Streams in the Pacific Northwest by Farmed Atlantic Salmon

Bisson, Peter A. 2006. Assessment of the risk of invasion of national forest streams in the Pacific Northwest by farmed Atlantic salmon. Gen. Tech. Rep. PNWGTR-697. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 28 p. This report describes the evidence for invasion of Pacific Northwest streams by Atlantic salmon (Salmo salar) that have escaped from marine salmon farms, and assesses the potential impact of farmed salmon invasion on native fishes inhabiting streams on National Forest System lands. The current risk to streams on National Forest lands in the Pacific Northwest from Atlantic salmon invasions appears to be low and is limited to a few areas in northwest Washington and southeast Alaska. However, long-term risks may be substantial if fish continue to escape from marine rearing pens or freshwater hatcheries. The two greatest threats appear to be that (1) Atlantic salmon could transmit a serious disease or parasite to native fishes, and (2) escaped salmon could eventually adapt to local conditions, leading to selfsustaining populations. If Atlantic salmon populations are eventually established, this species’ preference for swiftly flowing stream habitats could facilitate competition with currently at-risk species such as steelhead (Oncorhynchus mykiss). This could result in a pattern of expansion similar to that observed in other nonnative aquatic plants and animals, in which a prolonged early colonization period is followed by a rapid phase of exponential growth as breeding populations adapt to local conditions.