Salad on the menu for whale sharks off WA

Australian researchers have made a surprising discovery about the feeding habits of whale sharks, giving the biggest fish in the sea another world title.

It turns out that the giant creatures regularly enjoy a salad with big helpings of krill, which means they’ve officially dethroned the Kodiak bear as the world’s largest omnivore.

Scientists made the discovery while studying whale sharks off Ningaloo Reef in Western Australia and say it’s a reason to rethink what really sustains the large-bodied species.

“Everything we thought we knew may not be true,” said fish biologist Mark Meekan, from the Australian Institute of Marine Science.

“We saw them coming to Ningaloo and we saw them feeding on krill and we were like, ‘Boom, here’s the answer.’

“But with sophisticated methods that examine the microchemistry of these animals, this story becomes much, much more complex.”

Scientists carefully analyzed possible food sources, ranging from tiny plankton to large algae, for amino and fatty acids.

Next, they examined what was present in samples of whale shark skin.

“This study suggests that they feed on a lot of plant material, more in fact than krill,” Dr Meekan said.

He believes the size of the whale shark caused an evolutionary response that effectively turned bycatch – such as the brown sargassum seaweed common at Ningaloo – into food.

“They are very big animals and when you are a huge animal you need a lot of food,” he said.

“But it takes a lot of energy to push their mouths – open like a huge net – through the water. When you have a gut full of food but there is also a lot of algae, what do you do ?

“Are you throwing it up? Energetically, it’s a very expensive thing to do because you just spent all that energy collecting it.

“Whale sharks have just gotten around that in an evolutionary sense by being able to digest algae. They turn bycatch into part of their diet.

Another part of the study involved collecting and testing whale shark poo, with the results showing that they certainly ate krill – but didn’t metabolize much of it.

“They are much less efficient than we would expect if they had evolved just to eat krill,” Dr Meekan said.

The study was published in the journal Ecology.

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