30 years for Humpback whales to recover - scientists

The Humpback whale. File / APN

It will take another three decades for the Southern Hemisphere's humpback whale population to recover from the slaughter of the whaling era, scientists say.

The first ever comprehensive assessment of humpbacks in the vast region has found their population has gradually bounced back in the 50 years since commercial whaling was banned, but a New Zealand researcher says more can be done to help them.

The International Whaling Commission's scientific committee has been working with the South Pacific Whale Research Consortium to get a precise picture of humpback numbers.

Using photo records and genetic samples from 2000 whales, the researchers provided the first estimates of abundance, descriptions of genetic differentiation and documentation of migratory movement for humpback whales throughout Oceania.

With this, they were able to reconstruct the history of decline and slow recovery of the species from 20th century whaling.

The committee concluded that Oceania humpbacks, a favourite with whale watching ventures, once numbered more than 14,000, but by the time of the whaling ban in 1966, had been reduced to less than one per cent of that number.

Since that time, numbers had increased slowly to about 5000, or about 37 per cent of their pre-exploitation numbers.

Given the present rate of increase, it could be another three decades before this population was fully recovered.

"All in all, they are recovering slowly, but at least the population numbers are heading in the right direction," consortium member and University of Auckland scientist Dr Rochelle Constantine told the Herald.

The assessment, she said, had confirmed just how close the population of Oceania came to extinction.

"There may have been fewer than 40 mature females surviving in this vast region, after the catastrophic programme of illegal Soviet whaling in the early 1960s."

Dr Constantine said the species still faced a lesser threat from fishing - humpbacks were being accidentally caught in equipment - and potentially also from climate change.

Oceania whales had been tracked migrating down to the Bellingshausen Sea, off the West Antarctic Ice Sheet, where large-scale, climate-driven changes were being observed.

Less ice, Dr Constantine said, set off changes down the food chain that meant less krill for the whales to feed on.

"We also need to be careful with activities like whale watching, especially around vulnerable mothers with calves on their breeding grounds."

Next year, she plans to travel to Raoul Island, in the Kermadec Island, in the hope of satellite-tagging humpbacks to better track their migration to Antarctic feeding grounds.

Humpback whales - By the numbers

* 80,000 - humpback whales estimated worldwide.
* 37% - increase in humpback numbers in Oceania, to 5000, since commercial whaling was banned in 1966.
* 40 - the number of females the population may have dwindled to in the region following illegal Soviet whaling in the early 1960s.
* 15m - upper-end length of an adult humpback whale.