Our Burning Planet

SEA CHANGE

Recovering southern right whales face new challenge as crucial marginal ice diminishes in foraging grounds

Recovering southern right whales face new challenge as crucial marginal ice diminishes in foraging grounds
A southern right whale mother and calf. Research shows that South African female southern right whales are taking longer to reproduce and have become skinnier. (Photo: UP - Mammal Research Institute Whale Unit)

While formerly critically endangered southern right whales are on the way to recovery thanks to legal bans on whaling, female southern right whales off South Africa are becoming skinnier, taking longer to breed, and swimming further to find food – all of which coincides with a changing climate.

Today there are 80% fewer southern right whales than there were in the 18th century when whaling began. The species is starting to recover, but researchers in South Africa have noticed that a changing climate is coinciding with the whales’ ability to reproduce and their foraging and migration behaviour.

Dr Els Vermeulen, the research manager of the Mammal Research Institute Whale Unit at the University of Pretoria, explained that the name of the species came about because they are the “right” whales to hunt – they are slow-swimming, swim close to the shore and their behaviour is predictable.

“I find them now the ‘right’ whales to study for the same exact reasons,”  Vermeulen said at the second Plett Marine Science Symposium earlier this month.

Right whales were the first whales to be hunted by whalers. Between the late 18th century and the start of the 20th century, more than 1,500 southern right whales were killed by open-boat whaling off the shores of southern Africa.

southern right whale

A breaching southern right whale in Hermanus. (Photo: UP – Mammal Research Institute Whale Unit)

Between 1908 and 1975, during the era of commercial whaling, only 105 southern right whales were killed – when the population was near extinction, with only an estimated 60 reproducing females left in the world.

The species was saved when whales were legally protected in 1935 by the International Whaling Commission, but because of their slow reproduction cycle (and illegal Soviet whaling between the 1950s and 1970s), there was a 100-year delay in their recovery.

“This is really a conservation success story. It really showed that internationally, we protected these whales and the populations are thriving now,” said Vermeulen.

The global population of southern rights is increasing by about 7% a year, which is the maximum biologically possible for the species. The latest population estimate, which is outdated (new global estimates are in the works), is approximately 15,000 animals – which is only 20% of the pre-exploitation levels.

And it seems the threats to the species are not over yet.

‘Site fidelity’

“Generally speaking, the animals go back to the same areas where they have been feeding, basically what their moms told them to do or taught them to do,” Vermeulen said.

Researchers believe these whales typically have a “site fidelity” to their foraging grounds, which are in the Southern Ocean. Every year, they return from their foraging grounds in the Southern Ocean to their breeding grounds, which include South Africa, Brazil, Argentina, Australia and New Zealand.

Southern right whales have a slow reproduction cycle, and take three years to produce a healthy calf (one year for pregnancy, one for nursing and one to rest).

Vermeulen said that these trajectories are not that normal – it’s odd behaviour that Cyclopia (green line) has swum all the way to South America, and strange that the other whales are staying in the mid-latitudes. They originally went South (typical foraging grounds), but swam back up to the mid-latitudes, be a more stable region, but maybe not as productive as more Southern regions.

But Vermeulen and her Whale Unit have found that in the past decade, southern right whales have had longer breeding cycles, are returning to the breeding ground skinnier and are not going to the Southern Ocean to forage for food.

Population tracking

Researchers have been tracking and documenting whale populations in South Africa for more than 4o years.

Peter Best, the founding father of whale research in Africa, started counting southern right whales in 1969 from an aeroplane, a method he used for a decade. 

The first helicopter survey of the whales in South Africa was in 1979 and it’s the mode of aerial survey that researchers still use today.

southern right whale

Dr Els Vermeulen, research manager of the Mammal Research Institute Whale Unit at the University of Pretoria, during her unit’s annual aerial survey to catalogue southern right whale species in SA. (Photo: UP – Mammal Research Institute Whale Unit)

Every year in October, Vermeulen spends a week flying in a helicopter between Nature’s Valley in the southern Cape and Muizenberg in Cape Town, taking photos of every southern right whale she sees (focusing on mothers and calves).

Researchers recognise whales through their unique velocity patterns. They have catalogued 2,665 southern right whales in South Africa.

In 2022, the South African population was estimated to be 6,470 individuals, growing by about 6.5% per year.

southern right whale

The first helicopter survey started in South Africa in 1979 and it’s the mode of aerial survey researchers still use to catalogue whales today. (Photo: UP – Mammal Research Institute Whale Unit)

“The growing population sizes have impacts on the ecosystem, but also the removal of hundreds of thousands of baleen whales had an enormous impact on marine ecosystems,” Vermeulen said.

Going somewhere else to find food

To look at how the southern right whales’ feeding location and diet have changed over time, Vermuelen and her whale unit analysed stable isotopes in skin samples from the whales.

“Stable isotopes in the skin are basically little microchemical markers that show you where a whale has been feeding and what it’s been feeding on,” Vermeulen explained.

“And the results have shown that whales are feeding more north than they used to – we’re not seeing as many whales feeding in the southern region.”

Vermeulen said that the whales are going westward to find food.

The researchers used satellite tracking to find where the whales were foraging for food, and in 2021, put satellite transmitters on four southern right whales, two of which went to unexpected places.

“They went all the way to the southwest Atlantic,” Vermeulen said. “One even went to the Patagonian Shelf, which we didn’t know they would do. And that’s all females with a calf. So these are distances they haven’t done before, or at least we didn’t know them.”

This map shows the live location of 4 adult female southern right whales which were tagged in Walker Bay in October 2021.

While they still need more data, Vermeulen said that the original foraging grounds around Bouvet Island in the Southern Ocean are not being used as much. “And it kind of coincides with some of the data we have out of the skin, that their foraging location is changed,” Vermeulen.

Taking longer to reproduce

Vermuelen said while the animals are adopting different behaviours in response to changing conditions, they are reproducing at a slower rate, with the data since 2009 showing that the calving interval is increasing from a three-year cycle to four to five years for them to reproduce successfully.

“So even though they’re adapting, maybe it’s not enough,” Vermeulen said.

“If they can eat well, they can reproduce well,” which is not happening. 

Getting skinnier 

Using drones to take photos, Vermeulen and her team of researchers compared the body condition of the whales today to that of the whales in the 1980s, based on photos taken by Peter Best.

They determined that there had been a 24% reduction in the body condition of the mothers (lactating females).

“These females come here with already a quarter less of the body condition, the energy reserves, than they used to [have] in the late ’80s,” Vermeulen said.

“We also did a global comparison with the populations in Australia and Argentina, and South African mothers are the skinniest of them all.”

Changes coincide with environmental changes

Vermeulen and her team noticed that the area of marginal sea ice in the area around the whales’ previous foraging ground of Bouvet Island had shrunk significantly, diminishing the population of krill (small crustaceans that the whales feed on).

Vermeulen explained that marginal sea ice is ice between the continuous ice cover and the open ocean. “It’s ice that is very important… [for] krill larvae to grow.”

The decrease of this marginal ice in the past decade coincides with the changes the researchers have seen in the whale population.

Climate change?

While researchers can’t attribute it to climate change or global warming at this stage, Vermeulen said they’re looking into whether climate change is responsible.

“The aspect of the sea ice is the first thing we’ve discovered now. And we need to go deeper into that, like what is causing that sea ice to be less?”

But to look at the impact on climate, you need decades of data.

“Often people are not always keen on monitoring,” Vermeulen said. “But it really shows you that this long term is so critical to understand changes over time. We can’t look at climate change if we don’t have a long database of demographics.” DM

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This story first appeared in our weekly Daily Maverick 168 newspaper, which is available countrywide for R29.

To read all about Daily Maverick’s recent The Gathering: Earth Edition, click here.

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