At least four passengers of a whale-watching boat died after the tour vessel, which carried 27 people on board, capsized off the coast of Vancouver Island in Canada.Authorities warned that the number of deaths could rise as the situation unfolded throughout Sunday evening.The Canadian Coast Guard's Joint Rescue Coordination Centre said there were 27 people on board when the boat sank off the coast of Vargas Island.A British Columbia Emergency Health Services spokeswoman told Xinhua that at least four people died in the sinking, which occurred at 4.10 p.m. local time.She said four others were initially taken to a local hospital for treatment and five others were taken in by local residents, adding that it was unclear whether all the passengers had been accounted for.The cause of the sinking remained unknown as local fishermen and other boat owners joined emergency rescue teams in the search for survivors, according to local media reports.