Australasian Science: Australia's authority on science since 1938

Whale Sharks Need to Bask

Whale shark

Whale sharks’ tendency to stay near the surface for long periods of time may indicate they need warmth.

By Stephen Luntz

Whale sharks stay underwater for more than 2 hours on deep dives but appear to pay a price, resting near the surface for long periods afterwards. The first people to track this behaviour in detail believe that they need the warmth to recover from their time in the chilly depths.

Whale sharks have been known to dive to around 100 metres and return to the surface fairly quickly in a manner resembling whales or penguins. However, unlike mammals and birds, whale sharks do not need air to breathe so this behaviour has been something of a mystery. One theory is that it is a response to their negative buoyancy, preventing them from getting trapped at too great depths.

Dr Michele Thums of the University of Western Australia’s Oceans Institute and A/Prof Mark Meekan of the Australian Institute of Marine Science attached tracking devices to four whale sharks and revealed in the Journal of the Royal Society that they dive deeper and for longer than anyone realised. These deep dives averaged depths of 340 metres lasting 169 minutes.

“We imagine they are foraging, but even if we got a camera on them it is too dark to see down there,” says Thums. Afterwards the sharks spent roughly 2.5 hours at the surface, with the length of their stay correlating with the depth to which they had gone.

“Whale sharks, like many other fish, are ectothermic, which means that their body temperature is similar to the surrounding water temperature and they can’t regulate their body temperature through internal physiological processes. So, behavioural mechanisms such as spending time in the warmer surface waters are needed to warm them, similar to a reptile basking in the sun to warm up and then moving under a rock once sufficiently warmed,” says Thums.

Smaller-bodied creatures should warm and cool faster than the gigantic whale sharks, and Thums says she is seeking funding to do similar studies on species of other sizes. This may provide a check of the competing theory that low levels of oxygen at depth drive the time spent at the surface.