Shark movements a mystery
New detection data from a great white shark suspected of mauling a surfer off Esperance three weeks ago shows the creature has spent the past two years tracking up and down the coast from Perth to South Australia.
The 3m female, tagged WA017, was killed on a drum line set by Fisheries Department officers trying to catch the shark that attacked Sean Pollard in Wylie Bay on October 2.
Rory McAuley, Fisheries' principal shark research scientist, said WA017's movements showed she was passing through, rather than lurking in the Esperance area, as some feared. "She would have been following whatever instinct drives these kinds of movements," he said. "She could have been coming further west or even heading back to South Australia when she was killed.
HOW FIVE STRANGERS SAVED SEAN POLLARD
"But the one thing I'm pretty confident in saying is, like in the other regions that we've detected this shark and other sharks, she was moving through." WA017 was the first of four great white sharks to be fitted with an internal and external transmitter in Cockburn Sound on October 5, 2012. Dr McAuley said WA017 headed south to Albany three weeks after being tagged, while others headed north and were later detected in Ningaloo Reef.
He said this disproved a common theory about shark movements along the WA coast.
"There's this lingering theory that sharks are all passing by Perth in the October, November time of the year because they're following the whale migration, but the data we're collecting now doesn't support that at all," Dr McAuley said. "In fact, they're moving in opposite directions coming through Perth. So they're obviously not all doing the same thing and what the motivations are of individual sharks is not really something we can tell with the data we've collected."
After being detected in Cockburn Sound again on October 30, WA017 embarked on a mammoth trip south. She travelled at least 650km in 10 days to be detected at Bald Island, east of Albany, on November 10.
"This is one of the really staggering findings from our work," Dr McAuley said "These fish, when they get up and go, they can really get up and go. It's moving but it's certainly not the fastest we've seen. We've seen sharks we've tracked go even larger distances and averaged straight line speeds of 100km a day." Dr McAuley said this showed that while it was a sensible precaution to close beaches because of shark sightings, it was risky to be too "beach specific".
Sean Pollard and girlfriend Claire Oakford.
"What we're seeing is that just because a shark is seen on day one, it doesn't mean they are going to be anywhere near or close on day two," he said.
Because of sharks' high mobility, it is possible that WA017 and the other shark caught and killed in Wylie Bay hours after Mr Pollard was bitten were not the animals responsible for the attack.
Dr McAuley said analysis of Mr Pollard's surfboard showed the bite marks "unequivocally" came from a great white shark but that there was no evidence linking the two caught sharks to the bite marks.
WA017 was an under-16 juvenile female when she was killed. Her movements over the past two years are consistent with the high variability of other tagged sharks' movements in WA.
Dr McAuley said that sharks seemed to aggregate in SA, then travel to WA for a few months, before heading back to SA. "What they're doing over here, we don't know, but it's probably not food-related or related to predating seals as there are plenty of those where they came from," he said. "There's no reason for them to make the trip west."
Next month, Dr McAuley and his team plan to expand Esperance's shark monitoring network in the Recherche Archipelago with the deployment of about 15 acoustic receivers to detect the presence of tagged sharks.
Dr McAuley said they hoped to find how long sharks spent in particular areas to identify risky spots, how far off the coast and when they travelled, and whether there were particular times of year when sharks were more commonly travelling past the WA coastline.
