There has been a mass drowning of 51 California sea lions that got trapped in the nets of a fish farm on the west coast of Vancouver Island.

The sea lions got caught between nets at the Creative Salmon fish farm in Tofino Inlet this month, company general manager Spencer Evans said.

He said he thinks the sea lions chewed through the predator net, which surrounds the fish farm, and then got through the shark guard attached to the bottom of the grower net that enclosed the farmed salmon — but then couldn't get back out again.

"It appears the sea lions were swimming up from underneath and pushing the shark guard up against the flow of the grower net.

"And then as they try and suck on the fish in the grower net, they create small holes in the shark guard, and then they eventually get into that space between the floor of the grower net and the shark guard."

Evans called it an unfortunate incident, saying it's the first time in 16 years of operation that Creative Salmon has experienced anything like it.

He also noted there has been a large increase in the sea lion population in the area over the past 18 months.

The company has reported 110 drowned sea lions so far this year, up from 46 last year.

Environmentalists have been calling for years for fish farm operators to move away from open-ocean net cages to closed-containment systems.

With files from the Canadian Press