Escapes from open-net cage salmon farms are not uncommon. An estimated 3 million farmed salmon escape annually throughout the globe. Environmental advocates urge the use of closed pens, or conatinment tanks, for salmon farming.
Farmed salmon may be a concern wherever they escape, but in the Pacific Northwest the major concern is their impact on threatened and endangered salmon stocks in the area.
Wild Pacific salmon and steelhead face a number of threats, including potential impacts of hatchery-reared specimens, that complicate legal mechanisms for conserving these populations. Development pressure, dams and other human demands on the ecosystem make complete resoration of the runs virtually impossible. Escaped Atlantic salmon add additional difficulties: parasites & potential competition.
Farmed salmon are more likely to be infested with potentially fatal sea lice than wild fish. There is some evidence that wild Pacific salmon acquire sea lice from merely swimming near aquaculture pens -- escape may make infestation of wild stocks all the more likely.
Perhaps more importantly, the escaped fish may compete with native salmon for food or fresh water spawning grounds, for example, and may prey on Pacific fry. Environmentalists emphasize the risk, the aquaculture industry downplays it. Governmental assessments of the risk posed by escapees vary (see for example, WDFW and USNFS), but given the precarious condition of many wild Pacific salmon populations it is wise to be very cautious.
Marine Harvest Canada, which owns and operates the farm from which the fish escaped, appears to give geniune attention to environmental issues and has recently entered into an agreement with WWF-Norway. On the other hand, an ongoing investigation is not the first time that Canadian officials have targeted the company for possible liability over escapes.