South West dolphin research debunks safety theories

Research conducted in the South West has revealed sharks don’t discriminate based on age or gender when they bite bottlenose dolphins and are more likely to attack them in sheltered waters.

Murdoch University’s Cetacean Research Unit’s Dr Kate Sprogis led a team who tracked shark attacks on dolphins off the South West coast over seven years.

The team embarked on more than 600 boat trips to collect the data from more than 540 square kilometres of open coastal and sheltered waters off Bunbury and Busselton.

“The predatory behavior of sharks, including great whites and tiger sharks, can influence a range of prey species including dolphins,” Dr Sprogis said.

“We were interested in discovering whether any particular group of dolphins were more vulnerable to shark attacks and so we looked for patterns in dolphins grouping based on age and sex, seasonality and annual trends, and location in open versus sheltered waters.

“Although dolphins have anti-predator tactics, lethal shark attacks are still documented and failed attempts can be seen through fresh wounds and scars.

“These wounds and scars can measure the risk from predatory sharks on the population.”

Of the 343 dolphins assessed, nearly 17 per cent of them were found to have had scars and bite wounds arising from encounters with sharks.

Results showed 25 per cent of dolphins in the sheltered waters of Koombana Bay, Leschenault Inlet and Leschenault Estuary were sporting bite wounds compared to just 13 per cent of dolphins spotted in coastal waters.

“This could be due to the fact that the water is more shallow in these sheltered waters, with less space and fewer escape routes, meaning altercations between sharks and dolphins is more likely,” Dr Sprogis said.

“It could also be because the acoustic detection of predators may be more difficult with more underwater noise from boats and ships in these areas - or because the murkier waters make it more difficult for the dolphins to see easily.”

The research challenges the popular Australian theory that the presence of dolphins in an area implies an absence of sharks, or an element of protection for swimmers and surfers.

Dr Sprogis said the incidents of shark attacks on dolphins could be even higher than those documented, as the study could not account for dolphins that may have died from interactions with predatory sharks.

“These are just those dolphins which have survived - with the ones that suffered mortal wounds factored in, this figure could be even higher,” Dr Sprogis said.