The call came in via high-frequency marine radio at about 8:30 p.m. Tuesday, claiming a boat had overturned and several people were in the water near Baie D'Urfe, on Montreal's West Island.
The Coast Guard dispatched three air units while the Montreal fire department sent 15 firefighters on three boats to scour the area.
"It wasn't found," Claude Deschuymer of the Montreal fire department said of the boat.
"The [story] wasn't very solid. We missed a lot of information. That's why the Coast Guard figured it was a false call."
About two hours later, the male caller contacted authorities again, this time saying a sailboat had capsized in the Lachine rapids with six people on board, according to Stacy Dufour of the Canadian Coast Guard Maritime Rescue in Quebec City.
Another boat told authorities it was answering the mayday and reported itself as helping and recovering people in the rapids.
"When we got on scene, we quickly realized there was nobody there," he said.
Officials tried to contact the person who initially made the call and the second boat that reportedly responded to the scene, but were unsuccessful.
"The more time was going by, the more we were suspecting that this whole incident was a hoax," he said.
"But, until we are sure that there is no one in danger, we remain on scene and keep searching."
Rescue crews officially declared it a false alarm after two hours.
Investigators listened to recordings of the two calls and were able to determine the same person called in both incidents.
Industry Canada is now investigating.