The southern right whale, South Africa’s charismatic poster child for conservation, has been well on the road to recovery since commercial whaling finally ceased in the 1960s. Sadly, this conservation success story now hangs in the balance as they face a new and deeply concerning threat to their recovery.

The ‘right’ whale to hunt

Southern right whales were so named by early whalers as they were the "right" whale to hunt. They are generally slow-moving, float when dead, and have a large layer of oil-rich blubber. These traits meant that southern right whales were heavily exploited by the commercial whaling industry in the 18th and 19th century. A study published in Molecular Ecology has estimated that, prior to whaling, there were about 100,000 southern right whales cruising the high seas of the Southern Hemisphere. However, by the year 1920 it is estimated that only 300 of these whales remained.

Fortunately, the plight of the southern right whale came to an end when legal protection was afforded them in the early 20th century. What followed is what has been called a true conservation success story. This legal protection allowed southern right whale populations in Brazil, Argentina, South Africa, Australia and New Zealand to recover dramatically.

While their populations globally have bounced back spectacularly since the whaling days, their numbers are only at about 13% of the total population size before whaling, and their story is far from over.

Early signs of problems for South Africa's southern right whales

Annual aerial surveys flown by the Mammal Research Institute (MRI) Whale Unit have revealed that, since 1972, the South African population of southern right whales has been increasing steadily.

However, this long-term increase has seen some recent and abrupt changes.

The first indication that things were not quite right came in 2009. During that year's aerial survey, 254 non-calving southern right whales (adults without calves) were counted – this was nearly half the total of the previous year! Since then, non-calving southern right whales have virtually stopped migrating to our coastline.

Perhaps even more concerning, in 2016 only 55 cow-calf pairs were counted along the entire southern coast of South Africa, making it one of the lowest counts ever recorded. In that same year, only 9 non-calving whales were counted - a shocking 3% of what was expected for the year.

Then, two years later, there was an incredible "baby-boom", with over 500 new calves being born, followed by yet another massive dip in the numbers of cow-calf pairs visiting our shore - only 95 in 2019, and 67 in 2020.

A clue to explain these extreme fluctuations is found in the ‘calving interval’ of southern right whales. Typically, a healthy female will produce a calf every three years. But since 2009, female southern right whales in South Africa are waiting longer to reproduce. 3-year intervals are no longer the norm, instead, 4- and 5-year intervals are becoming more frequent.

This is no trivial shift.

These longer intervals imply calving failure, which means that females may not be falling pregnant or that they are losing their calves early on in their pregnancy. The impact of these increasing calving intervals is evident in the dramatic fluctuations in the numbers of cow-calf pairs visiting our shores.

Big is beautiful

To understand the reasons behind an increasing calving interval, we need to delve deeper into what makes things tick for a female southern right whale. What is it exactly that controls how often she will produce a calf?

Southern right whales are called 'capital breeders'. This means that females use stored energy from the food they eat in summer to sustain their reproduction in the winter. While on our coastline, pregnant females seldom feed, so it is vital that they have eaten sufficiently during the summer. This means that the number of cow-calf pairs that we see on our coastline each year is strongly related to the "feeding success" of the whales during the previous summers.

Our rapidly changing planet

Since the Industrial Revolution, greenhouse gas emissions and human activities have been causing our planet to warm at an unprecedented rate. The concentration of carbon dioxide in our atmosphere is the highest it has been in 800,000 years and, if things don't change soon, by the end of the century we'll be looking at the highest it has been in about 50 million years.

There is global scientific consensus that our climate is changing. Human activities are the main cause of this, and it is happening faster than ever before. According to the Intergovernmental Panel on Climate Change’s (IPCC) fifth assessment report, our emissions are resulting in a host of deeply concerning impacts, including warmer temperatures, melting sea ice, ocean acidification, sea-level rise, and an increase in frequency and severity of extreme weather events such as heat waves.

Scientists warn that the impacts of climate change on marine mammals will mostly be felt through changes in the distribution and abundance of their food sources. This is of grave concern for animals like southern right whales that desperately need enough food at the right time.

Antarctic krill, a favourite food item of southern right whales, are heavily impacted by climate change. In the Southern Ocean they are declining in density and their ranges are shifting radically due to warming waters.

Life in the fast lane: coping with climate change

Plants and animals have coped with changing environments before, however, the speed of change is unprecedented in recent eras, and the natural world is battling to keep up.

Humanity will need to adapt by using technology, but the rest of the natural world does not have this option. Some species may be able to adapt behaviourally, through social learning, but most species will only adapt through genetic adaptation via the process of natural selection from one generation to the next. Long-lived species like southern right whales are unlikely to adapt genetically, so they will need to alter their behaviours in an attempt to keep up with climate change.

The increasing calving intervals seen in South African southern right whales are likely as a result of insufficient feeding during the summer. This raises the question as to whether the population can, or is trying to, alter its feeding behaviours in response to a rapidly changing ocean.

The right whale at the wrong time

A new publication in Global Change Biology highlights that even though southern right whales are in fact changing where, and on what, they are feeding, it still is not enough to keep their recovery from whaling on track.

The work, led by researchers from the MRI Whale Unit and the University of Auckland, New Zealand, focused on southern right whales that migrate to South Africa’s coastline in winter. By using microchemical markers (stable isotopes) found in small samples of the whales’ skin, the researchers were able to garner information on the whales’ feeding behaviour. Stable isotopes vary predictably across the Southern Ocean, meaning that information can be uncovered on where feeding occurs.

The study reveals that in the 1990s, while the South African population was growing steadily, the whales fed predominantly on krill from the island of South Georgia/Islas Georgia del Sur. However, during the late 2010s, when southern right whales were calving far less often, the stable isotope data show that the whales were feeding much further north.

This finding tells us that the South African population is attempting to adapt to changing ocean conditions but is struggling to keep pace with the changes. Even though they have changed their feeding behaviours in response to declining and shifting prey resources, they are still unable to maintain their typical 3-year calving intervals.

Despite our success in protecting whales from being hunted, our actions are once again impacting southern right whale populations, this time through climate change, and the entire ecosystems that they inhabit are at risk.

Gideon van den Berg (MRI Whale Unit)

Need to get in touch? Email Gideon here.

A special word of thanks to Dr Emma Carroll and Dr Els Vermeulen, as well as Meredith Thornton, for their assistance with this article.