‘Yum-yum yellow’: Could your swimwear attract sharks?

escue equipment is the colour it is for a very good reason. A splash of orange or yellow stands out against the blue and black of the open ocean. This flash of colour might be the only chance a floating sailor or air crash survivor has of being spotted from a passing ship or plane.

But what else might be attracted to these bright colours on the water’s surface?

The US Navy finished World War Two with a great deal of eyewitness accounts of shark attacks on sailors and airmen who had found themselves cast into the ocean. These included the sinking of the USS Indianapolis near the war’s end, which resulted in possibly hundreds of survivors being picked off by sharks. The navy’s scientists knew that sharks were attracted by the splashing of survivors, and the blood from injured or dead sailors. But what if the colour of their life preservers played a part as well?

The hunch that the bright colours of the navy’s life-saving equipment attracted sharks led to some dubbing the colour “yum-yum yellow”. The propensity for some colours to attract curious sharks has become a particularly long-lasting myth, resurfaced whenever a spate of shark attacks takes place.

This month there have been reports of a spate of people being bitten off the beaches of New York’s Long Island, raising questions about what might be behind the incidents. The shark species involved – the normally docile and aquarium-friendly sand tiger – mainly eats fish and crustaceans such as lobsters. It is thought that the bites could be the result of mistaken identity – the sharks are confusing the arms and legs of swimmers for the fish it hunts in shallow summer waters – or exploratory behaviour by a curious animal. (Read more about the reasons why sharks attack humans.)

Sharks are blessed with some senses that seem almost supernatural compared to ours; sharks can detect smells at range between one part per 25 million to one part per 10 billion, and can sense movement in the water via a line of sensors which run across their body and the electrical signals from thrashing fish. But what about eyesight?

Hart is an expert in shark senses at Macquarie University in New South Wales, Australia. He has examined not just the arrangement of cones and rods in shark eyes, but also the presence of light sensitive proteins known as opsins that allow animals to distinguish colour. In a review of the most recent scientific research on shark eyesight, he concluded that many, if not all, of the shark species studied to date have monochromatic vision, and appear to have lost the ability to see in colour at a number of points during their evolutionary history.

But this doesn’t mean that sharks have “poor” eyesight. Some bottom dwelling species of shark, for example, appear to have heightened sensitivity to contrast compared to other vertebrate animals, according to Hart’s work.

Water quality can also contribute to this. In clear water with bright sunlight, sharks will be able to discern objects better. Ocean-going sharks like the blue shark and the oceanic whitetip often hunt near the surface where light quality is usually good.

The likelihood is that if you can’t see the shark, Naylor says, it can’t see you. But there are some patterns or combinations of colours much easier to discern underwater.

“If you’ve got some pattern, like a secchi disk [for measuring water turbidity], for example, which is black and white, you have a much better chance of seeing it at a distance than if it was in camo greens and yellows. What Nathan has shown is that it’s not really the yellow, but yellow, when it’s juxtaposed with particular patterns against darker colours,” he says. “So it’s the contrast that they can pick up on.”