The escape was one of the biggest for Marine Harvest Canada, the largest aquaculture business in the province.

Provincial officials are investigating the incident, which happened early Tuesday, and the Marine Harvest Canada may face charges. Environmental groups say the mass escape demonstrates the dangers fish farms pose to wild salmon.

Strong ocean currents shifted a net holding 30,000 salmon in Marine Harvest's Frederick Arm farm site near Campbell River, pulling down a corner of the cage and allowing the fish to swim free, said Marine Harvest environmental compliance director Clare Backman. The company is not sure whether any fish were left in the pen, but it's possible all 30,000 escaped.

“One of the anchor lines … apparently slipped to a low spot on the ocean floor and in so doing pulled down the corner of the cage so much the fish were able to swim out, which is really unusual,” he said.

Jennifer Lash, executive director of the Living Oceans Society, said if the Atlantic salmon survive and breed, they'll compete with wild salmon, whose stocks have already fallen sharply.

“You get juvenile Atlantics, they're not indigenous to the coast and they start competing with the wild salmon and they start putting the wild salmon at risk. Everything has to be done to stop having those Atlantic salmon in the ocean,” she said. “Any time you bring in an invasive species or a non-indigenous species … it poses a threat to the existing biological diversity.”

The only reliable way to prevent escapes like these is to farm fish in closed-off pens, Ms. Lash said. Environmental groups and Marine Harvest itself have called on the provincial government to finance research into closed-containment salmon farming. 

Read the full article in The Globe and Mail or the Times Colonist.