Diving whale sharks surface to warm up
Whale sharks and other deep-diving fish return to the surface to warm up from the cold water, new research says.
ONE OF THE MANY mysteries about the world's largest fish seems to have been solved: just why whale sharks frequently come to the surface.
New Australian research shows that whale sharks and other fish rise to the surface to warm up after a bout of deep-sea swimming.
Many large fish, including tuna, swordfish, marlin, as well as a number of sharks, are known to spend sometimes long periods at or near the surface following a descent to deep water, presumably to feed, but it hadn't been clear why; after all, they're gill-breathers, so, unlike whales, they don't need to surface for air.
Whale sharks surface to warm up
In a bid to find out more, Michele Thums at the University of Western Australian Ocean Institute in Crawley, and colleagues fitted tags to four whale sharks - three found at Ningaloo Reef in WA, and one at Christmas Island.
The tags were designed to gather regular information on the shark's depth, and the light level and temperature of the water. The sharks made three main types of dives: day and night 'bounce' dives, in which they spent only about 10 or 20 minutes at depth, and a third type, which had never been reported before: very long, very deep dives lasting more than two hours, followed by a long period at the surface.
The water temperature varied from about 28C at the surface, to about 14C for the deepest dives, below 300m from the surface.
And the team found a clear pattern: the warmer the water encountered during the dive, the less time the whale sharks spent at the surface afterwards. After the deep, cold dives, they spent much more time at the surface than after a relatively shallow, warmer dive.
So it seems that, as one theory had predicted, the surface swims are necessary for the sharks to return their body temperature to the optimum level for normal biological processes.
Whale sharks hard to research
Very little is known about whale shark biology. "Their massive size and their migratory behaviour make it very difficult," says Michele. As they spend so much time in the open ocean, it's hard to keep track of them, and tags do often fall off. Just getting the tags on these four fish required a massive effort, she adds.
"We find the sharks with a spotter plane, the plane directs us in our boat to where the whale shark is, then we jump in. The shark's skin is incredibly thick and tough so it's not that easy to get the tag in."
It's important to know whale sharks, and presumably other large fish, spend time at the surface, because it provides an insight into what controls their movements, says Michele. "And it will assist in predicting their responses to environmental change and ultimately developing effective conservation and management strategies," she says.
The research was published this week in the Journal of the Royal Society Interface.
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