VANCOUVER -- Most of the 21,000 Atlantic salmon that were in a Vancouver Island fish farm pen damaged by fire have escaped, the company who operates the farm confirmed Sunday.

The breach happened on Friday, but on Saturday the company, Mowi, was still inspecting the damage and couldn't yet confirm the number of fish that had made their way out of the pen and into the ocean near Port Hardy.

The news that most of the fish did indeed escape has confirmed the worst fears of wild salmon advocate Tavish Campbell, who flew over the fish farm site on Saturday and took photos and video.

"My heart just sank," Campbell said of the moment when he saw the collapsed pen.

Dr. Diane Morrison, managing director of Mowi Canada West, said she and the company are profoundly sorry the incident happened, because the public is so concerned about farmed Atlantic salmon getting into the ocean.

But, she said, studies suggest there is a low risk of Atlantic salmon swimming into B.C. rivers to spawning grounds in large numbers, and "an even lower risk of them establishing successful populations."

Scientists and First Nations have become increasingly concerned about the sharp decline in B.C.'s wild salmon populations. This year, DFO forecast 4.7 million salmon would return to the Fraser River; just 600,000 – or 13 per cent of the original forecast – showed up.

Campbell said that with wild B.C. salmon struggling with sharply declining populations, the escape of thousands of non-native salmon is a concern.

"These are an exotic species that don't belong in Pacific waters," he said.

The farmed Atlantic salmon could compete for habitat with wild salmon, Campbell said. He also fears the farmed fish could spread a sickness called piscine reovirus to wild salmon.

"Based on the (fish farm) industry's own numbers, over 80 per cent of all the farmed Atlantic salmon grown on the British Columbia coast are infected with the piscine reovirus," Campbell said.

"It's incredibly problematic to be growing these fish in open net pens, and then to have them escape."

Kwakwabalas (Ernest Albert) of 'Namgis Nation echoed those fears.

"The Nimpkish River salmon have crashed," Kwakwabalas told CTV News, referring to the wild salmon in the 'Namgis' traditional territory on the north coast of Vancouver Island. "They are in such steep decline. And now we have the threat of these Atlantic (salmon) being dumped into the water, and potentially moving up the river."

Morrison said Mowi tests all fish for PRV before they are transferred from the hatchery to ocean water pens, and all the fish tested negative at that point. But she could not provide numbers on how many of the escaped fish, which were ready for harvesting, had PRV.

Morrison said the company is working with DFO and local First Nations on a plan to recapture the escaped salmon. That effort needs to start as soon as possible, Campbell said.

In a press release, Mowi said that the few remaining fish left in the damaged pen have been secured, and once they're removed, the pen will be towed to shore and the company will investigate what caused the fire.

The pens are built with an electric system running through them to power underwater cameras to monitor the fish, Morrison said, and a fire could be caused if there is an electrical short in that system.

Campbell said the incident shows why the province's fish farm industry needs to move to land-based fish farming, instead of raising non-native salmon species in open-pen nets in the ocean. Non-native fish have escaped in the past, and will do so again, he warned.

Campbell pointed out that Prime Minister Justin Trudeau recently directed Bernadette Jordan, the federal fisheries minister, to "work with the province of British Columbia and Indigenous communities to create a responsible plan to transition from open net-pen salmon farming in coastal British Columbia waters by 2025."

In August 2017, net pens operated by Cooke Aquaculture Pacific, a Canadian company, collapsed in northwest Washington and tens of thousands of non-native fish escaped into the ocean.

In response, Washington State introduced legislation to phase out marine farming of Atlantic salmon and other non-native fish by 2022.

But Morrison said both open-water and land-based fish farming both have pros and cons when it comes to environmental impact, and there needs to be open discussion of the risks and benefits of both systems.

With files from the Associated Press.