Being a dolphin might seem like a fun life – lots of frolicking in the ocean and not so much with the office work. However, the oceans also contain sharks, and a study of dolphins off Western Australia has shown just how threatening these apex predators can be, as well as where the dangers are greatest.
Dr Kate Sprogis of Murdoch University's Cetacean Research Unit observed 343 dolphins for signs of shark attack and found one in six bear the scars. In the Journal of Experimental and Marine Biology and Ecology Sprogis reports dolphins were in more danger in sheltered waters, and when temperatures rise.
Off Bunbury, southwest Australia, in locations like Koombana Bay and Leschenault Estuary a quarter of the dolphins bore bite wounds, but this was just 13 percent in coastal waters. The bites ranged from relatively shallow, to some so extreme it is amazing the victim survived.
“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 are more likely,” Dr Sprogis said in a statement. “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.”
Sprogis told IFLScience no one knows how often attacks are fatal, particularly on dolphin calves. She added we also know little about how dolphins escape once sharks have bitten them – fast swimming and leaping out of the water can be useful evasion tactics, but probably aren't much use when a shark already has you by the flipper. Perhaps if we knew how dolphins respond to shark attacks, we could adopt some of their approaches
The study was conducted from 2007 to 2013, and attacks increased after 2011, which Sprogis attributes to the warmer conditions at the time. Ocean conditions off Western Australia are very dependent on the Leeuwin current, the longest current in the world. During La Niña years, the Leeuwin strengthens, bringing warmer waters from Indonesia, and perhaps tiger sharks come with it, which may up the attack rate on dolphins.
Surfers delight in sharing the waves with dolphins, and many believe dolphins and sharks don't co-exist, so dolphins' presence means a beach is safe. However, these bites on the dolphins do show they are also prey, so the waters in which they live are unlikely to be safe havens.