Getting up close and personal with the sharks

IN 1943, Jacques Cousteau co-invented the aqualung, the self-contained breathing apparatus which revolutionised underwater exploration, writes Richard Collins.

His book, The Silent World, appeared in 1956. Three years later, a documentary film, Le Monde du Silence, co-directed by Cousteau and a young Louis Malle, was released. It won the Palme d’Or that year and an Academy award the following one.

In those days, sharks were thought to be mindless killing machines. The book and the film challenged that perception. Divers were photographed swimming with sharks and interacting with them; the marine monsters were not so lethal after all. Research, since then, has transformed our understanding of them.

Evan Byrnes, who examined their behaviour at Australia’s Macquarie University, claims that sharks have “individual personalities”, an attribute once thought appropriate only to humans. Studies over the past few decades show that nearly 200 species of animal have “personality”, so it’s “deeply engrained in our evolutionary past,” says Byrnes. Do sharks belong to the personality club? Testing their reactions to new situations and how they behaved when subjected to handling stress, she showed that they do; some individuals, like their human counterparts, are consistently more adventurous than others.

Although Cousteau’s book and film were groundbreaking, their title was misleading. The undersea world isn’t silent. It’s actually very noisy, but air in the diver’s ear canals suppresses and muffles sound. The vocalisations of whales rival those of birds; the half-hour long ‘songs’ of humpbacks are among the most complex and mysterious in nature. But do fish ‘sing’? Is there an undersea equivalent of the dawn chorus?

Sharks have extraordinarily powers of taste and smell. They can detect the minute electrical fields generated by fish muscles. Sound, travelling faster and farther in water than in air, is an ideal sensory medium in the murky dark world where sharks live. They must surely avail of it. Evidence that they do so first became apparent during cage-diving activities. Food is thrown to sharks to lure them to cages so that they can be observed up close and personal. Some sharks were seen to approach boats before the smelly ‘chum’ was released into the water; they had learned to associate the throb of boat engines with food. But how good are sharks at discriminating between sounds?

A paper, just published in Animal Cognition, confirms that sharks can learn to identify particular sounds. According to lead author Catarina Vila Pouca, of Macquarie, they have ‘really big brains’ and ‘are generally underestimated when it comes to learning abilities’. She and her colleagues played jazz to eight juvenile Port Jackson sharks in a pool at the university and offered them food at the same time. This was always provided at a particular corner of the pool. Five of the fish soon learned to associate the music with the food; on hearing it they would swim to that corner.

Then the team played classical music to the sharks. This time, however, the food was offered at the opposite end of the tank. The fish would have to distinguish between the two types of music to know where to go for the food. The result was confusion; the sharks could not figure out which corner they should visit. Their ’response to the original jazz stimulus’ soon ‘declined to chance levels’. The fish were unable to discriminate between the two music genres. Sharks are sophisticated, but only up to a point.

Catarina Vila Pouca & Culum Brown. Food approach conditioning and discrimination learning using sound cues in benthic sharks. Animal Cognition. 2018.