Escape from the Net-Pen: the Atlantic Salmon

Recent news  has been filled with discussions of Atlantic salmon in the Pacific Northwest. After the unintentional release of over 260,000 Atlantic salmon from net pens near Cypress Island in Puget Sound occurred last August, public outcry caused an increase focus on the controversial use of net-pens in Washington State. Last month, a bill was passed to end Atlantic salmon net-pen farming in Washington by 2025. 

Source: WDFW

The Salmo genus includes European species of salmon and trout, among them includes the familiar Atlantic salmon (S. salar) and brown trout (S. trutta). Although the Atlantic salmon are the biggest Salmo species, they are dwarfed by the native Oncorhynchus species. The exotic Atlantic salmon average 28 to 30 inches in length and 7.9 to 11.9 lbs. While in freshwater, they have blue and red spots; when they reach maturity in marine water, they take on a blue-silver sheen; and during reproduction, the males take on a slight green or red coloration. They are distinguished by large black spots on the gill cover, large spots on the back, and the absence of spots on the tail fin. 

Most Atlantic salmon are anadromous, meaning they rear in freshwater, mature in marine waters, and return to freshwater to spawn; however, the saltwater portion of the lifecycle is not required, and landlocked populations, referred to as ouananiche, exist across the U.S.

Source: William Hartley, USFWS

Scientific evidence prior to 1998 showed no evidence that Atlantic salmon compete with our native species. However, in 1999, naturally-producing Atlantic salmon were discovered in streams on Vancouver Island in British Columbia, showing escaped Atlantic salmon are capable of successfully reproducing in the wild.  

Other potential risks, such as predation, disease transfer, and hybridization are relatively low. There is no evidence of predation by Atlantic salmon in freshwater and most escaped Atlantic salmon investigated were caught with empty stomachs. There is also no evidence that escaped Atlantic salmon transfer diseases to native populations, although there is concern about the potential impact of sea lice (Lepeophtheirus sp.), originating from net-pens of Atlantic salmon. Sea lice can kill juvenile fish, even at low infestation levels. Possible hybridization is also unlikely. Cross-breeding attempts even under laboratory conditions do not often succeed. All this recent data suggests that although Atlantic salmon do not pose a significant risk to native populations; the biological effects of fish farm escapements are still being evaluated, and more evidence is required. 

Native Atlantic salmon occupied the North Atlantic Ocean from Maine in the United States to the Barents Sea, north of Russia. From 1903 to 1935, over 8.6 million Atlantic salmon were intentionally introduced to more than 60 lakes and streams in British Columbia. The intentional introductions began in Washington with 3,821 salmon released in Chamber’s Creek in 1951 and 8,000 Atlantic salmon released in Alexander Creek in 1953. Until 1991, between 140 to nearly 16,000 Atlantic salmon were intentionally released into various lakes in Washington with little success of establishment. In contrast, unintentional releases in saltwater from escaped Atlantic salmon from net pens are the largest pathway of invasion, as shown by the recent escapes due to net-pen failure. For example, Atlantic salmon are still being caught in the Skagit River eight months after the net-pen break in Puget Sound. While most of the stomach contents and condition show they are not feeding regularly and experts do not think this is a sign that they are colonizing, locals still have concerns and are staying vigilant.