PERTH, Australia >> A man was killed by a 13-foot shark on Saturday while diving with his brother off a beach in southwestern Australia, authorities said. It’s the fourth fatal shark attack in Australia since September, all of them off the continent’s southwest corner.

Businessman Peter Kurmann, 33, and his brother Gian Kurmann, 34, were diving from a boat off Stratham Beach, about 230 kilometers (140 miles) south of Perth, the Western Australia state capital, when the younger brother was attacked midmorning, state police spokeswoman Sgt. Naomi Smith said.

She said the victim, from the town of Vasse near the bay where the attack took place, was apparently killed instantly. His brother was unharmed.

The Fisheries Department has launched an investigation, which will include an examination of bite marks on the victim’s body to determine the shark species.

Department manager Tony Cappelluti said Gian Kurmann reported seeing “a four-meter (13-foot) dark, shark-like shape” in the water. Authorities won’t speculate on the species.

The body was recovered by his brother helped by the crew of a nearby boat and taken south to the town of Busselton, Cappelluti said.

An airplane surveillance team had later spotted a shark in the area and a government boat equipped with shark capture gear was en route to the scene, Cappelluti said.

Experts have been unable to explain the spate of attacks in Australia’s southwest, but agree that different sharks are likely responsible for each fatality.

The last fatal attack in Australia was American George Thomas Wainwright, 32, who was taken on Oct. 22 by a 3-meter (10-foot) great white while diving solo off a boat near Rottnest Island, 18 kilometers (11 miles) west of Perth.

A great white of the same size is believed to have taken 64-year-old Australian swimmer Bryn Martin off Perth’s premier Cottesloe Beach on Oct. 10.

The attacks followed the Sept. 4 death of bodyboarder Kyle Burden, 21, who was killed by a shark described as 4.5 meters (15 feet) long at a beach south of Perth. Witnesses were unsure of the type of shark.

Australia averages little more than one fatal attack a year along an expansive 35,000-kilometer (22,000-mile) coast, although attacks have become more common in recent decades as more people take part in ocean recreation.