The Canadian documentary filmmaker and conservationist Rob Stewart disappeared while diving off Florida's southern coast during the filming of a movie about sharks, the US Coast Guard reported Wednesday.

Stewart, 37, vanished Tuesday afternoon during a dive with three of his crew members at the Alligator Reef coral reef, some 4 nautical miles (seven kilometers) off the island of Islamorada.

Islamorada belongs to the Florida Keys, an archipelago extending from the state's southern tip and ending at Key West.

The US Coast Guard issued a statement saying it had received a report of a missing diver shortly after 5 pm (2200 GMT).

"Coast Guard watch standers issued an Urgent Marine Information Broadcast, launched a Station Islamorada 33-foot Law Enforcement Special Purpose Craft boat crew, launched an Air Station Miami MH-65 Dolphin helicopter crew and diverted the crew of the Coast Guard Cutter Charles Sexton," it said.

The Navy, police and Florida wildlife conservation commission are assisting the search, it added.

The reason for Stewart's disappearance remains unknown. Resurfacing at the end of his dive, he went underwater again before the crew's boat was able to pick him up. The other three divers emerged safely.

The particularly deep and difficult dive was his third of the day, his sister Alexandra Stewart told the state broadcaster CBC, adding that another crew member appears to have temporarily lost consciousness when he resurfaced.

A biologist and underwater photographer, Stewart was filming a sequel to his best-known 2006 documentary "Sharkwater," about shark hunting and its impact on the marine ecosystem. The film won the Directors Guild of Canada award and a dozen others the following year.

"So happy to be shooting #sharkwater2 with the best cameras and equipment in the world," he wrote in his latest Instagram post three days ago. "For the first time I can show you sharks through my eyes."

The filmmaker was capturing "the personality in #shark faces that people that spend their lives with sharks truly understand," he added.

The accompanying picture shows a diver from behind, presumably Stewart, holding a camera on his shoulder.

"The search and rescue continues in the Keys this morning," Stewart's film team wrote in a post on the director's Instagram account on Wednesday morning.

source: AFP