Farmed salmon kept in net pens like these can escape if the nets tear, as they did near northern Vancouver Island this week. (CBC)

About 40,000 Atlantic salmon have escaped from a fish farm on the B.C. coast.

Marine Harvest Canada says the fish escaped Wednesday from its farm at Port Elizabeth, on the Pacific Coast about 400 kilometres northwest of Vancouver.

The company says divers discovered several holes in two pens at the farm and efforts are being made to prevent more escapes from the pens, which still hold thousands of fish.

"Through the night Wednesday we had people working at the farm site to stop up the holes and reverse any ongoing problem with the escape," said Clare Backman, Marine Harvest's environmental compliance officer.

The Living Oceans Society environmental group says hundreds of thousands of farmed salmon escape every year.

"This demonstrates once again the urgent need to transition all open net-cage farms to closed containment systems," said Will Soltau, local co-ordinator of the society's salmon farm campaign told the Campbell River Mirror.

"This will be a major financial loss to the company and another blow to the health of our marine ecosystems and wild salmon populations,' said Soltau.

Conservationists say escaped farm salmon can spread disease and sea lice to wild salmon on the B.C. coast.

Marine Harvest is the biggest aquaculture company in B.C., according to the company's website.