Study shows dolphins don't deter sharks in the surf
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The theory that dolphins keep surfers safe from sharks has been blown out of the water with a Western Australian study.
Researchers from WA's Murdoch University Cetacean Research Unit monitored dolphin populations off the coast of Bunbury in the state's south west and Shark Bay in the north, and found shark bites increased each year from 2009 to 2013 — in line with rising ocean temperatures.
Dolphins in sheltered waters were "significantly" more likely to be bitten than populations in deep coastal waters. The research also saw a spike in attacks over the summer months of 2012–13.
Researcher Kate Sprogis said great whites and tiger sharks were believed to be largely responsible for the attacks in Western Australia.
"In the case of great whites, for example, once they get beyond a length of about 2.7 metres, they need marine mammals in their diet, so they will shift from fish to predating on seals, sea lions and they will also try for dolphins," Dr Sprogis said.
A dolphin bearing a substantial bite mark, thought to be from either a tiger or great white shark (Kate Sprogis/Murdoch University)
"We attempted to identify each bite mark, but often they get quite messy as the shark will come in from a side angle. We were able to identify a few that were from tiger sharks if the bite mark was broad, blunt or square."
Dr Sprogis said the research also concluded there was little difference when it came to the sex or age of dolphins but lethal bite marks tended to come from "large predatory sharks".
"In the two areas the research was carried out, there was a 74 per cent bite rate off Shark Bay while in Bunbury it was 17 per cent," she said. "But that is just the dolphins we do see; many more would have sunk to the bottom so we could not include those that had incurred lethal bites."
Research indicates shark predation on dolphins is on the rise (Kate Sprogis/Murdoch University)
Dr Sprogis said the research team had initially hypothesised the incidence of sharks attacking dolphins were unlikely to increase over the years, but between 2007 and 2013 predation increased as ocean temperatures also rose in line with the La Niña weather pattern.
"The Leeuwin current [a warm ocean current found off the west coast of Australia] was bringing the warmer waters down with it," Dr Sprogis said. "What we did see was an increase in shark bites on the dolphins. We also think that drew more tiger sharks into the areas with the warmer waters."
Dr Sprogis said there was no available research to indicate dolphins in turn predated on sharks but there was evidence they did "try to escape".
"They will leap out of the water and alert other dolphins of the presence of sharks," Dr Sprogis said. "But if the water is shallow, they often can't turn quickly enough or they don't acoustically hear or detect sharks in proximity, so it might be harder for them to react and that would be a large reason in why they get bitten."
Dr Sprogis, also a keen surfer, said the research put paid to a long-held theory for surfers relying on the presence of dolphins to deter sharks.
"Growing up, I also thought that every time I saw a dolphin out in the surf everything was fine," she said. "Seeing these shark bites on dolphins though, I now know sharks do predate on dolphins and you shouldn't rely on that theory when you're out in the surf."
// ANTHONY PANCIA
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(Homepage photo of Pete Tomlinson by Shane Smith / Salt Eyre)