From June until October, southern right whales give birth and nurse their calves in protected bays along the Southern Cape coast of South Africa.
From June until October, southern right whales give birth and nurse their calves in protected bays along the Southern Cape coast of South Africa.
The first preliminary count of aerial surveys conducted for 2023 on 28 August by the Mammal Research Institute’s (MRI) whale unit at the University of Pretoria, revealed 556 females with calves — or 1 112 individuals — and 24 unaccompanied adults (males and non-calving females) between Hermanus and Witsand.
This is about double what was seen last year around the same time and marks a second “bumper” year after 2018, re-confirming a five-year cycle, according to Els Vermeulen, a whale specialist from the whale unit.
But the numbers are regrettably misleading. “I can guarantee that next year won’t be good because the whales are calving every four to five years instead of every three.”
Southern right whales feed in the vast Southern Ocean of the Antarctic, thousands of kilometres away from the South African coast.
Since the late 1980s, a reduction and fluctuation in krill populations in these feeding grounds has meant that the breeding females have dropped 23% in body weight. Their main diet is the tiny crustacean, which is what the southern right whales feed on in the summer in that region.
This has led to worrying changes in their reproduction and body condition, particularly over the past 13 years, Vermeulen said. It appears that ocean warming decreases suitable habitat for krill to reproduce, which correlates with changes in sea ice conditions because of climate change. This, however, requires further research, she said.
Els Vermeulen, a whale specialist from the whale unit.
Capital breeders
As they feed one time of year — summer — and fast another time of year — winter,
when they have their calves — scientists call southern right whales “capital breeders”, which means that their successful migration and calving are dependent on them eating enough during their summer feeding season in the Southern Ocean.
“In other words, they need to have an adequate body condition to be able to migrate and calve successfully. However, a study published earlier this year indicated our right whale mothers have decreased 23% in body condition (or fatness) since the late 1980s, indicating clearly that their feeding has become less successful.”
Additional research has indicated that “our whales have changed the location at which they feed as well as the prey they target in the past decades”.
This shows that these whales are responding to environmental changes in their Southern Ocean feeding grounds, which “we believe lies at the basis of the increased calving interval … and a reduced migration towards our shores”.
Fluctuating numbers
Sightings of females with calves decreased in 2015, 2016 and 2017, increased above normal levels in 2018, and then dropped substantially in 2019, 2020, 2021 and 2022.
Sightings of unaccompanied adults fell in 2010 and have not yet returned to normal levels.
This, Vermeulen said, suggests that only pregnant females about to give birth complete their migration to the South African coast to increase their calf’s survival chances, whereas most of the others are not undertaking their migration to the full extent.
The South African southern right whale population, which occupies seasonal calving and nursery grounds in the nearshore and protected waters of the Southern Cape coast, is the largest breeding stock of species worldwide, comprising some 6 500 individuals.
Vital food source
According to Vermeulen, it is essential that the whales get their fill of krill as research indicates that conception is between February and April in Antarctica at the end of the feeding season.
If they haven’t gained enough weight, a number of females won’t conceive. And, while they do also feed on other tiny crustaceans like more-abundant copepod populations, which occur at mid-latitude, just south of South Africa, this is a relatively small part of their diet compared to krill.
The drop in krill has further resulted in pregnant females and mothers not carrying sufficient fat or energy reserves to see them through the nursing season as they don’t feed in our waters.
“Because of this, it appears that the cow-calf pairs are leaving our waters earlier for Antarctica because the females cannot sustain three months of lactating with their reduced body weight,” Vermeulen said, noting that the demand on the mothers is significant as the calves drink up to 600 litres of milk a day and grow 3cm a day.
Els Vermeulen photographs Southern Right Whales.
Changing oceans
Vermeulen pointed out that while there is krill fishing around the Antarctic peninsula, it’s “probably much less to where our right whales feed” but still, such threats don’t help their plight.
“I think the biggest culprit is climate change and sea ice reductions … Preliminary results are showing some of the strong anomalies in sea ice around the areas that are known as the major feeding ground for South African right whales.
“We also see in our tagging data, it’s the region they don’t go to at this point, which is a region they usually go that’s around Bouvet Island and more south, so really south of South Africa.
“The strong anomalies in sea ice conditions reduce the habitat that’s suitable for krill development.” Vermeulen said they were busy with those analyses but it’s certainly something that’s coming out.
The whales, too, are making unprecedented journeys in their search for krill. “Some of the migration patterns we’re seeing on the whales we’ve tagged are quite unseen before, like the distances they’re travelling. We looked at, for example distances, normally between 6 000km and 8 000km and we’re looking at distances of 15 000km, 20 000km now, which for these whales is massive.”
This is probably one of the reasons they reduce their body condition, she said. She also said that it’s possibly not so much about finding food, they might be finding enough food but have to travel further to get it.
For Vermeulen, the concern is that relatively little is known about whale behaviour because of the complexity and expense of researching these mammals. “Scientists are trying to do as much research as possible into climate change and the krill reserves as this is a major problem for all krill predators, including humpback whales, penguins and seals.”
Sentinels of ocean health
Since 1969, the whale unit has conducted its annual October survey, counting and photographing every single southern right whale, in one of the longest database monitoring programmes in the world on any marine mammal. This is done to monitor the recovery of this population post-whaling and its overall health.
In 2022, satellite transmitters were placed on 11 adult southern right whales, and “results have shown us incredible journeys so far, with three tags still being active to date”.
On 30 September, the whale unit will start its 54th annual aerial survey of southern right whales to monitor the health of the population. As of 12 October, it will be conducting boat-based surveys in Walker Bay (south west of the Western Cape) to deploy satellite transmitters on 14 adult southern right whales to study their migration and feeding ecology.
To better understand these changes in migration patterns and feeding ecology, they will be deploying another 14 satellite transmitters (SPOT and SPLASH tags) on adult southern right whales between 12 and 5 November. These tags will provide the scientists with information on the location of each individual (SPOT tags) as well as data including dive depth, duration, temperature (SPLASH tags) for up to one-year maximum.
As the deployment of these tags is of “sensitive nature”, the team will work with two specialist scientists, Amy Kennedy and Alexandre Zerbini from the National Oceanic and Atmospheric Administration, the University of Washington’s Cooperative Institute for Climate, Ocean and Ecosystem Studies, and the Marine Ecology and Telemetry Research.
The results of the tagging work are processed in conjunction with similar work around the Southern Hemisphere for a species-wide approach to understanding the effects of climate change on global southern right whale population growth, Vermeulen said, explaining this is done under the umbrella of the Southern Right Whale Consortium. The South African Polar Research Infrastructure provided funding for this year’s tags “to whom we are extremely grateful”.
For Vermeulen, the southern right whales are “flagships” of an important message about large-scale environmental changes in the vast Southern Ocean, which is also affecting many other marine species. “They need our attention and help to ensure they can continue to thrive into the future in an ever-changing ocean.”