A population of humpback whales off the coast of South America and Antarctica has bounced back from near-extinction, the latest estimate shows, in an example of the powerful impact of conservation measures.

Hundreds of thousands of humpback whales were killed worldwide between the late 1700s and the mid-1900s, with the humpbacks in the western South Atlantic coming under assault by steam-powered ships early in the twentieth century.

Numbering about 25,000 in 1905, fewer than 1,000 remained 15 years later.

Hunting continued, and by the mid-1950s fewer than 500 were left.

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Whatsapp Where'd all the whales go? Estimated population trajectory of western South Atlantic humpback whales.

The population probably began to recover in the 1970s, after they had been placed under protection, but the strength of the recovery wasn't appreciated until the turn of the century, when a study estimated humpbacks in general were doing relatively well - the ones in the South West Atlantic had returned to about 30 per cent of their pre-whaling number.

The new assessment, published in the Royal Society journal Open Science, estimates they're doing even better - the population is now at 93 per cent of its pre-exploitation levels.

Dr Alex Zerbini, a senior research scientist with the Alaska Fisheries Science Centre and co-author of the study, told Hack the dramatic rebound showed the success of conservation measures.

"If you protect them, they will recover," he said.

If you stop killing whales, they will survive. They will reproduce, and they'll come back. And I think this is a clear example of that.

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Whatsapp A humpback whale.

Why are these humpbacks doing better than thought?

Since they're mostly underwater and out of sight, figuring out how many whales are in a population isn't easy. The International Whaling Commission's (IWC) effort in the mid-2000s involved spotting them from planes, but the authors of the new study say the planes moved too fast.

"The plane moves so fast, you will miss animals," Dr Zerbini said.

Instead, the researchers relied on another assessment: ship surveys.

"Because the ship travels much slower than the airplane, the probability that you're going to miss animals is much lower and the estimate is more accurate."

As a result, the authors believe the IWC underestimated the number of humpbacks in the western South Atlantic in the mid-2000s.

"When the IWC estimated 30 per cent, we estimated 50 per cent with this new study."

Does this mean there are more whales around in general?

Unfortunately, probably not.

Humpback whales reproduce at a faster rate than other whale species - the western South Atlantic humpback population is growing at 9 per cent per year.

"That's a high rate of population increase for a whale," Dr Zerbini said.

Blue Whales or Right Whales - they cannot grow that fast. And because they cannot grow that fast, they cannot recover as fast.

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Whatsapp A whale on the flensing plan at Grytviken, South Georgia - a major whaling base in the western South Atlantic.

Humpbacks are generally a good news story. The population that migrates along the east coast of Australia is growing at about 10.5 per cent per year, while, according to a recent study, the number of this species in the western Indian Ocean has gone from 600 in the late 1970s to more than 30,000.

Other whale species, however, aren't doing so well.

A 2017 study by the CSIRO and University of Queensland estimated the Antarctic Blue, Southern Right and Fin Whales will not have reached half their pre-whaling numbers by 2100 because of slow growth rates.

As an added irony, the recovery of humpbacks may actually reduce the population of other species that compete with them for their main food source: krill.

"In the past, these whales were feeding far away from land," Dr Zerbini said.

"But now that they are increasing in their large numbers, there's some evidence that they are getting close to shore. And if they're getting close to shore, they are going to overlap in distribution with other krill predators, such as seals and penguins."

Despite the example of whale conservation, ocean-faring governments around the world are increasing financial support for fishing practices that decimate marine life.

A 2018 study found excessive fishing has caused stocks of large Australian fish, like bream or snapper, to decline by a third over 10 years, while in no-take zones stocks remained stable. The Federal Government released a plan last year that would effectively strip 35 million hectares of no-take ocean from Australian's marine parks, according to analysis by conservation groups.