Long-standing fishermen's tales of huge Zambezi sharks in the Breede River have been confirmed - and huge means just that.

Last week, researchers on an expedition to confirm the presence of this species - also known as bull sharks - not only caught one of these sharks, but the catch proved to be a world record at 4m and weighing more than half a ton.

Another record: it represented the most south-westerly distribution of bull sharks in Africa.

The shark has been fitted with three tracking devices and researchers are excited by some of its behaviour that they have uncovered as a result of data from these tags.

They have been surprised to learn that it spends much of the night going from fishing boat to fishing boat, and that when dawn breaks and the boats return to the shore, the shark turns its attention to lines of shore anglers.

"She's seemingly looking for a soft prey, and appears to be attracted by sound and vibrations," said Meaghen McCord, managing director of the South African Shark Conservancy who was among researchers that visited the Breede last week.

Others taking part were fish scientist Dr Steve Lamberth and his team from the marine and coastal management branch of the Environment Department, Hennie Papenfuss from Big Fish Safari, and members of the Lower Breede River Conservancy, and three researchers from the shark conservancy.

They fished unsuccessfully for three days, but were rewarded on the fourth when Papenfuss caught a bull shark from his boat about 5km upriver from the mouth.

"After an hour-and-a-half struggle during which the fish towed him 2 189;km further upstream, Hennie managed to tire her enough to bring her close to shore for landing.

"Our team then brought her to the shore, where we were able to collect all the required data."

The female shark tipped the scales between 550kg and 600kg - and was fitted with three tags.

The shark was the largest known to science, with the largest previous maximum size thought to be about 3.5m.

"We gathered genetic samples to determine whether bull sharks in the Breede River represent a distinct population from those found elsewhere in South Africa.

"We also suspect she was pregnant and may very well be using the Breede as a pupping ground."

The team had tracked the shark continuously for 43 hours, and found that it had spent most of this time in the actual estuary, with only a few hours in the surf zone just outside the river mouth.

"Her behaviour during the 43 hours was fascinating," McCord said.

"To speak quite frankly, it was the most incredible experience of my life and I am so excited about the research we will continue to do."

It was vitally important for the researchers to return to the Breede within the next fortnight to continue tracking the shark, but funding was a problem, she said.