|The new finding has been described as "very worrying".
Whale sharks spotted off the coast of Australia are getting smaller, researchers have said.
In a decade the average size has shrunk from seven metres to five metres.
Whale sharks, the world's largest fish, are caught for food in some east Asian countries and Australian researchers suspect this is causing a decline.
The fish is listed as "vulnerable", and one of the authors of the new study has described the new finding as "a very worrying sign".
The data comes from ecotourism companies which run expeditions to watch whale sharks and swim with them in Ningaloo Marine Park off the north-west coast.
"The eco-tourism industrylogsthe position and size and sex of every shark it swims with," said Mark Meekan of the Australian Institute of Marine Science (Aims).
"We have obtained those datasets and analysed them over time," he said, "and essentially what we have seen in the last decade is a decline in average size of shark from seven metres to five metres.
"Now if you consider that the sharks probably aren't sexually reproductive or mature until they're six or seven metres long - that's a very worrying sign."
Under the IUCN Red List of threatened species, they are categorised as "vulnerable" to extinction.
"Whale sharks, like many other shark species, are highly vulnerable to over-exploitation due to their longlifespanand low reproductive rate," commented Callum Roberts of York University in the UK, who has researched whale sharks extensively in the Caribbean.
There are also indications that the number of sharks visiting Australian waters may be decreasing, which would be additional evidence for a decline prompted by over-fishing.