“You wouldnâ€™t think electric rays that can give you quite a strong electric shock – or stingray barbs – would be a particularly appealing meal but sharks donâ€™t seem to mind them.”
Speaking to the Herald prior to a fatal attack on a surfer by a 3m great white off Kingscliff on the North Coast of NSW, Mr Grainger said understanding what sharks were likely to eat and where could help the public minimise the small chance of an interaction with humans.
People should also avoid dawn, dusk and other times when the water may be murky, such as after rain.
Juvenile great whites – usually less than three-metres – are more likely to be off northern NSW or southern Queensland in winter, nearer Port Stephens for breeding in summer, and as far south as Victoria and northern Tasmania come autumn.
The researchers, including from NSW Department of Primary Industries, found evidence of three dolphins and one hammerhead shark among the stomachs but no seals. The largest of the sharks was a 4.65m mature female.
“From about 2.5 metres [in length], they start to include marine mammals” in their diet, Mr Grainger said. “Itâ€™s definitely not the case of sharks targeting humans [as] what it might have been portrayed – itâ€™s usually a case of mistaken identity.”
Video and other evidence suggest sharks prey on fattier fish when they were preparing for migration. Some would even regurgitate whale blubber and return to eat fattier parts of dead whales, he said.
Vanessa Pirotta, a marine ecologist at Macquarie University, said analyses of stomachs in necropsies offered important knowledge about animal behaviour.
As so-called apex predators, “sharks play an important part of the ecological environment”, Dr Pirotta said.
She noted whales migrated into shark territory as they headed northwards in winter to breed off the Queensland coast before migrating back to the Southern Ocean for summer rather than have the sharks pursue them.
Sharks appear to feed on dead whales by moving their heads from side to side to create a sawing rather than chomping motion to get through the tough blubber. Such a feeding motion would much harder if the whale were alive, Dr Pirotta said.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.