Whale-come news: Why these ocean giants are on the rebound

Scientists have reported "hundreds of hundreds" of humpback whales around South Georgia Island. Photo / Emma Carroll / BAS South Georgia Whale Team

A new survey has revealed promising numbers of the world's largest animal, the critically endangered blue whale, along with "hundreds and hundreds" of humpbacks - signalling a comeback decades in the making.

In the first multi-year survey at South Georgia Island in the southwest Atlantic, where hunting drove whales almost to extinction, a multi-national team of scientists report some whale populations may even be close to full recovery.

University of Auckland whale researcher Dr Emma Carroll, who has co-led the survey with Dr Jen Jackson from the British Antarctic Survey, said the latest data from South Georgia showed protection of whales has worked.

"South Georgia has similar latitude to New Zealand's own sub-Antarctic islands and, as with our own populations of southern right whale near the Auckland Islands, we knew populations were increasing but these latest results are fantastic," Carroll said.

"It's also particularly significant because whales were slaughtered in their tens of thousands at South Georgia and to see them return in such numbers is just an absolute thrill."

Surveying humpback, blue and southern right whales, leading scientists from around the world report humpbacks are now a common sight in coastal waters at South Georgia, with 790 reported during 21 days of surveying this season.

A preliminary estimate suggests more than 20,000 humpbacks are now feeding there in the summer months before migrating to colder waters in the sub-Antarctic to breed.

The rare and critically endangered blue whale, the largest animal that has ever lived, was sighted just once during the first year of the survey, in 2018. This year blue whales were sighted or acoustically recorded 55 times.

"The sightings of Antarctic blue whales were pretty amazing. There used to be more than 200,000 of them before whaling, and they probably got down to a couple of hundred at one point," Carroll said.

"While we still don't have a strong handle on how many there are, we've recorded them 36 times already during 2020 – so for such a short period of time, that's really encouraging."

Jackson said continued protection and monitoring would be needed to see if this unprecedented number of blue whales sightings was a long-term trend, as had been seen in humpbacks.

The observations came after Kiwi scientists last year reported how southern right whales, or Tohorā, could be rediscovering ancient migratory pathways as its population grew back.

While few of these whales were seen in 2019 – unlike 2018 – that could be just because they preferred to feed elsewhere last year, Jackson said.

Genetic monitoring work pioneered in Tohorā would also be used to understand how southern right whales feeding around South Georgia are connected to wintering grounds around South America.

"What is clear is that protection from whaling has worked with densities of humpbacks in particular similar to those of a century ago and we are thrilled to see them at South Georgia again," Jackson said.

Carroll said the area appeared to be a crucial place for the whales' recovery.

"It may take our life time to see a change, but it can work. We just have to give them space and time."

However, she said populations could face a new challenge in climate change – and scientists were still trying to understand what large-scale shifts in our oceans would mean for factors like prey distribution.