Five people have died and one person remains missing after a whale watching boat with 27 people on board sank off Vancouver Island, Canadian authorities said.

The vessel made a mayday call late Sunday afternoon on a calm, clear and sunny day off the tourist community of Tofino, a popular destination for whale watchers, the Joint Rescue Coordination Centre said. The cause of the sinking remains a mystery.

A search by the rescue agency staffed by Canadian military and coastguard personnel concluded late Sunday with five people dead, 21 rescued and one person missing.

The Royal Canadian Mounted Police (RCMP) have taken over the search for the person who remains missing.

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