Changes in the size of whales can help predict if their populations are at risk of sharp declines decades down the track, scientists have discovered.
The average body size of four species of whales - sperm, blue, fin and sei - shrunk rapidly during the second half of the 20th century after 200 years of commercial whaling, researchers from Australia and Switzerland found.
The biggest decline was experienced by the giant sperm whale, whose average size shrank by four metres between the early 1900s and the 1980s.
Blue, fin and sei whales remain listed as endangered species while sperm whales, which can measure up to 18m, are considered vulnerable after their populations were decimated by commercial whaling before it was banned in 1986.
Using data from the International Whaling Commission based on the size and number of whales caught before the ban, the scientists discovered the animals reduced in size about 40 years before their populations collapsed as a result of overfishing.
One of the study's co-authors, Professor Mark Hindell, from the University of Tasmania's Institute for Marine and Antarctic Studies, said the finding suggested that changes in an animal's size could serve as an early warning signal that a species is under threat and action is needed to protect them.
"The real world application for this would be in fisheries where you think there's potential for collapse," he told AAP.
"So if you're monitoring things like not just the abundance of an animal but also their body size then you can look for these change points where you can see a marked change."
Prof Hindell said the change in the size of the whales was partly because many of the largest ones would have been caught by whalers.
There were also biological changes in the remaining whales because more food was available for them, allowing them to breed earlier.
Whaling in Australia began in the 18th century, with the export of products including whale oil becoming one of the country's first major primary industries.
As technology developed in the 19th century, greater numbers of whales were killed and many species came close to extinction, which then led to the collapse of the whaling industry.
An estimated 16,000 sperm whales, which can weigh up to 70 tonnes and live for 60 years, were caught in Australian waters between 1952 and the end of commercial whaling in 1978.
A year later Australia introduced an anti-whaling policy, well ahead of the IWC moratorium.
However despite the protections in place, the IWC estimates that the number of blue and fin whales in the southern hemisphere remain a fraction of the 200,000 or so that roamed the oceans before commercial whaling.
The number of sei and sperm whales is unknown.
The study was published in the journal Nature Ecology & Evolution on Friday.
