A massive search for survivors has ended off Canada's Vancouver Island after a whale-watching boat with 27 people on board sank, killing five people.

The 20-metre Leviathan II, operated by Jamie's Whaling Station, made a mayday call late on Sunday afternoon, local time, on a clear and sunny day in the tourist community that is a popular destination for whale watchers on the country's west coast.

Lieutenant Commander Desmond James, a spokesman for the Joint Rescue Co-ordination Centre, staffed by Canadian military and coastguard members, said the agency's search concluded with five dead, 21 rescued and one person missing.

The Royal Canadian Mounted Police will take over the search for the missing person.

The boat was partially submerged eight nautical miles west of Tofino.

Tofino's mayor Josie Osborne described the mood in the town as tense, but praised residents for their aid in the rescue effort.

"Everybody's heart is just breaking for what's going on here and wanting to be as helpful as possible," she said.

Valerie Wilson, a spokeswoman for the Island Health hospital agency, said 18 people were taken to Tofino General Hospital and said to be in a stable condition.

Three were transferred to other hospitals and some of the 15 who remained in Tofino had already been discharged, she said.

Coastguard vessels and search and rescue aircraft were searching for people on the boat who were missing. The helicopter and aircraft being used in the waters off Tofino has equipment to search in the darkness.

Boats from the nearby Ahoushat First Nation arrived first on the scene, said aboriginal councillor Tom Campbell, who watched from the waterfront as rescuers brought several survivors ashore.

"Their looks tell the whole story," he said. "You can't describe looks on people that are lost. They look totally lost - shocked and lost."

Mr Campbell said his cousin pulled at least eight people from the water into a rescue boat.

John Forde, who runs The Whale Centre, another whale-watching operation, responded to the call for help. He said he did not know how the Leviathan II could have sunk.

"Over the course of a season and years we take out thousands and thousands of people on these trips in conditions similar today. I have no idea what the issue was or what actually happened," he said.

Jamie's Whaling Station was one of the first of its kind off Vancouver Island and had been around for many years, Mr Forde said.

Joe Martin, a member of the Tal-o-qui-aht First Nation, was near the dock when rescue boats began returning to Tofino.

He said two people were brought in on a Zodiac boat and workers tried in vain to resuscitate them.

His brother and nephew were halibut fishing when they saw the overturned boat and tried to help. Instead, the men pulled in three bodies, Mr Martin said.

The ship was on the far side of Vargas Island in Clayoquot Sound, an area that Mr Martin said could get very rough. But the weather was fine on Sunday.

"It wasn't even blowing hard," he said. "This is the largest boat in Tofino and I was really surprised that it went down."

It is not the first fatality on Jamie's Whaling Station's record. In 1998 one of its vessels capsized during an excursion, sending all four people on board into the water. The operator and a passenger died.

Asked how many were dead off Tofino, British Columbia Coroner spokeswoman Barb McLintock said earlier: "Multiple but we don't yet have a firm number. Still a very fluid situation so we really are not sure yet."

Jenn Hamilton, a spokeswoman for British Columbia Emergency Health Services, said five ambulances were dispatched and several off-duty paramedics went to the dock to help.

The Transportation Safety Board is investigating the incident.

Brandon Hilbert from Tofino Water Taxi said local companies pitched in to help the rescue effort.

Tofino fishing guide Lance Desilets arrived after being told rescue boats were needed. At least 12 boats were already out and more were going in as he returned.

"I saw a lot of personal belongings, a long diesel slick and the top 10 feet of the Leviathan II sticking out of the water," he said. "It's a sad day for our community."

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