A type of toxic algae is killing off farmed salmon in a section of Clayoquot Sound on the west coast of Vancouver Island.
The Millar Channel and Ross Passage salmon farms, operated by the company Cermaq, have been affected by the harmful bloom over the past three weeks.
Members of the group Clayoquot Action have been monitoring the situation.
"They have got bio-waste trailers on site and they are sucking the dead fish out of the fish farm pens and moving them in through Tofino," said Bonny Glambeck of Clayoquot Action.
The hardest hit site has lost about 10 percent of its fish to the toxic algae, Cermaq said.
The company has dealt with toxic algae blooms in the past, but spokesperson Grant Warkentin said the current bloom is too large to deal with using the usual mitigation measures.
"We would put down tarps around the outside of the farm, and then we would pump water up from below the bloom, but unfortunately in this case the bloom is so deep, it's deeper than the pens, so there's not a whole lot we can do," he said.
"We are just hoping that the weather will change and maybe rain, and that would dissipate the problem,"
The die-off shows the pens should to moved onto land to reduce their impact on the ecosystem in the area, which is designated as a UNESCO Biosphere Reserve, Glambeck said.
But Warkentin said that is not currently part of the company's business plan.
Cermaq will continue to monitor the situation and remove dead fish from the pens, he said.
Algal blooms are a common problem for fish farms on the B.C. coast, said Nicky Haigh, who manages the Harmful Algae Monitoring Program, based at Vancouver Island University in Nanaimo.
The program, which is funded by salmon farming companies, has been monitoring the impact of harmful algae on fish since 1999.
Various types of algal blooms are naturally occurring on the B.C. coast and little can be done to prevent them, Haigh said.
"B.C. is an upwelling zone. We have a lot of nutrients that feed algal blooms, she said. "Sometimes conditions are just right for them."