Whales in the Southern Ocean are losing weight due to global warming

As the chill of winter sets in during June in the southern hemisphere, the Antarctic Sea solidifies into an icy crust, triggering the northbound migration of right whales. 

One particular congregation of these magnificent creatures frequents the bay near the town of Hermanus, South Africa, opting for the warmer South African waters. This migration pattern has been a long-standing tradition, providing an ideal environment for these whales for mating and rearing newborn calves. 

However, despite its hospitable temperatures, these waters lack the essential sustenance needed by the whales, forcing right whale mothers to deplete their fat reserves to nourish their calves throughout the winter.

The importance of these whales bulking up in the nutritionally rich, albeit colder, Antarctic waters during the summer cannot be overstated. Yet, a worrying trend has emerged, as observed by researchers from Aarhus University. The whales that are now arriving at the South African coasts are noticeably leaner than their predecessors.

Since the inception of their research in the 1980s, the university’s team, led by senior researcher Fredrik Christiansen from the Department of Ecoscience, has noted a disturbing trend. 

“Right whales are 25 per cent thinner than they were in the 1980s. This is bad for the whale population, because it means that the newborn whale calves have a higher risk of dying. Fortunately, the right whales in the Southern Ocean are not endangered, but if this continues, they could become so,” he warns.

Food disappears when the ice melts

When winter descends, these right whales, akin to Antarctic nomads, leave their feeding grounds and embark on a journey north, bracing for several months of scarcity. A period during which they must subsist on the fat reserves accrued during the plentiful summer season.

Summer is a time of bounty for these right whales. They dive beneath the sea ice, opening their enormous mouths to take in seawater teeming with krill and water fleas. Equipped with a unique biological filter system known as the baleen, they sieve out these small animals from the seawater, enabling them to consume colossal amounts of food without expending much energy.

However, Christiansen’s study points to a worrying ecological shift. The voluminous shoals of krill that once represented a feast for the whales are dwindling, compromising the whales’ ability to amass the essential fat stores before winter. 

“The shoals of krill live on phytoplankton, which thrive best in the cold waters around the Antarctic. Here – like plants on land – they transform sunlight into energy. Rising sea temperatures mean there is less phytoplankton, fewer krill and thus less food for the whales,” he explains.

Compelled to seek sustenance elsewhere, the whales venture further north, where they encounter a different, less energy-rich species of krill. Christiansen elaborates, “Further north, there’s less food for these small crustaceans. Therefore, they’re not as big and fat as the animals living beneath the Antarctic sea ice.”

How to weigh a whale

How exactly do scientists ascertain that right whales are becoming thinner over time? One might humorously envision marine biologists wrangling these massive creatures onto an enormous scale. 

However, the reality of this scientific endeavor is even more intriguing, employing the use of cutting-edge technology and the innovative methods devised by Fredrik Christiansen and his team at Aarhus University.

“Right whales like to lie flat on the sea surface. This makes them easy to photograph from above. When the drone has taken some photographs – and we know the height of the drone – we can calculate the size of the animal,” Christiansen explains.

But, calculating the weight of a whale involves more than simply measuring length and width. It requires an understanding of the whale’s volume. This understanding has been meticulously cultivated over years of observing right whales as they roll around on the water’s surface, providing researchers with the opportunity to measure their dimensions.

“We calculate the volume using the drone photographs – and when we know the volume, we more or less know the weight. In this way, we can see that the whales have become thinner over the past 30 years – and that’s serious. The weight of the mothers has a huge impact on their calves,” Christiansen says.

Number of calves has dropped significantly

There’s a stark contrast between the reproductive patterns of the right whales of today and those from thirty to forty years ago. The southern right whale, for instance, used to birth calves every three years on average. This is no longer the case, according to Christiansen.

“In the 1980s, researchers observed that the right whales off the coast of South Africa gave birth to a new calf every three years. But because it’s now difficult for them to fatten up during summer, this has fallen to every five years. This means that the population is growing significantly more slowly,” he reveals.

The impact of the whales’ dwindling fat reserves extends beyond just the increased interval between births. The calves that are being born today are not only less frequent, but they are also smaller and grow more slowly.

“The amount of fat on the whale mother is directly linked to how much energy she can give to her calf through her milk. When the mother is thin, the calf gets less energy and grows more slowly,” Christiansen says.

The researchers’ findings, published in the journal Scientific Reports, also extend to the northern right whales inhabiting the waters off Canada and the northern United States. These whales are not reaching the sizes they used to, possibly because they are born smaller. The team’s calculations suggest that a whale born in 2019 would, on average, grow to be a meter shorter than a whale born in 1981.

“Small calves have a higher risk of dying. They’re more vulnerable if a killer whale attacks,” Christiansen concludes, highlighting the precarious future these creatures face.

Once hunted to the edge of extinction

The right whales, so named because they were deemed the “right” ones to catch, have had a complicated history with humans. As early as the 14th century, these enormous creatures became a target for hunters in both the northern and southern parts of the Atlantic. 

For hundreds of years, they bore the brunt of human demand for energy, with their fat serving as one of the most critical sources.

Fredrik Christiansen, a senior researcher at the Department of Ecoscience at Aarhus University, explains that the oil derived from the whales’ fat, known as train oil, played a crucial role in lighting up the world. 

“Train oil, which the oil used to be called, became a fuel in lamps – both for indoors and for street lights.” This coveted commodity even drove colonial ambitions, with the quest for train oil being a significant factor behind Denmark’s colonization of Greenland in the 18th century.

The turn of the 20th century saw a seismic shift in the energy sector, with crude oil, often referred to as black gold, emerging as a more efficient energy source. The profitability of whale hunting dwindled, marking a fortunate turn of events for the beleaguered whales.

The southern right whale is one species that has reaped the benefits of this transition. “For more than 100 years, the population has been allowed to grow large and healthy again,” Christiansen states. And this recovery has not only been beneficial for the whales themselves, but also for the entire Southern Ocean ecosystem.

Very important for the marine ecosystem

Christiansen explains that the whales have a vital role in enriching areas of the sea with sparse food resources. The Antarctic seas, where right whales come to feast, are teeming with more life than any other sea on the planet. Despite holding only five per cent of the Earth’s seawater, an astonishing 20 percent of all marine life resides in this region.

This richness of life is attributable to the unique conditions of the Antarctic seas. “The many hours of sunshine in the summer, turbulent sea currents, and the low temperature are perfect for teeming life,” Christiansen says. 

The light fuels the explosive growth of marine algae, which, swirled around by the sea currents, provide a feast for krill and plankton. In some areas, the krill density can reach a staggering 35,000 per cubic meter of water.

Right whales, along with a myriad of other species, gorge themselves on this krill abundance. But unlike many other species, the whales undertake a long migration northward to overwinter.

“The whales are extremely important for the parts of the sea where there is not much food. When the whales die, their huge bodies sink to the bottom. In the depths, they become food for a whole ecosystem of eel, sharks, crabs, lobsters, worms, and microorganisms,” Christiansen explains.

The potential disappearance of these whales, therefore, carries profound implications for the broader marine ecosystem. 

“The whales are an apex predator. When an animal at the top of the food chain disappears, it has a cascade effect. Animals throughout the food chain will be affected if the whales are no longer there. From sharks to bacteria,” Christiansen concludes, underscoring the interconnectedness of life in our oceans and the importance of each link in the chain.

The innovative drone-photography method used by Christiansen and his team provides crucial insights into the right whales’ physical condition and reproduction patterns. These insights underline the urgency of addressing the issues affecting these magnificent creatures and the ecosystems they inhabit.

As the climate continues to change, the ripple effects extend far into the ecosystem. From microscopic phytoplankton to majestic right whales, the intricate food chain is being disrupted, and the long-term consequences could be dire. The lean whales of Hermanus serve as a grim reminder of the broader implications of global climate change and the urgent need to address it.

More about right whales

Right whales are large baleen whales that belong to the Eubalaena genus. They are characterized by their stocky bodies, large heads (which can account for up to a third of their total body length), and long, arching mouths that begin above the eye. 

They have a distinctive V-shaped blow, caused by the position of their two blowholes, and their bodies are mostly black in color, though they can have areas of white.

There are three species of right whales: the North Atlantic right whale (Eubalaena glacialis), the North Pacific right whale (Eubalaena japonica), and the Southern right whale (Eubalaena australis). Each species is found in the hemisphere its name suggests, with their ranges extending from the subtropical to the polar waters.

Feeding habits and distinctive features

Right whales are filter feeders. They feed by swimming with their mouths open through large swarms of plankton, mainly small crustaceans known as copepods and krill. 

The plankton is filtered from the water by the baleen plates, which act like a sieve. Right whales are known for their slow movements and for generally staying close to coastlines.

These whales are known for their callosities — roughened patches of skin that are infested with small, whitish, cyamid crustaceans commonly called “whale lice.” The pattern of callosities is unique to each individual and helps researchers identify them.

Targets of commercial whaling

Right whales have been the target of commercial whaling since the early whaling days due to their slow speed, coastal habits, and tendency to float when killed. The name “right whale” was even given by whalers who considered them the “right” whales to hunt. 

This hunting severely depleted their populations, and all three species are now considered endangered. Whaling is no longer a significant threat due to international bans, but these whales still face dangers from ship strikes, entanglement in fishing gear, pollution, and climate change.

Many unique aspects compared to other whales

One unique aspect of right whale biology is their long calving intervals. Female right whales give birth to a single calf after a year-long gestation period, but they do not usually give birth again for several years, sometimes as long as a decade. This slow reproduction rate makes it difficult for right whale populations to recover from declines.

Right whales are also known for their spectacular breaching behavior, where they leap out of the water and crash back down. Scientists aren’t entirely sure why whales do this, but it might be a way to communicate, to play, or to remove parasites from their skin. Despite their large size (adults can be up to 60 feet long and weigh up to 100 tons), right whales are acrobats at heart.

Right whales continue to be the focus of conservation efforts worldwide, with researchers working hard to understand their behaviors, track their populations, and reduce human-related threats to these magnificent creatures.

More about climate change and Antarctica

Climate change is having profound impacts on Antarctica and the surrounding Southern Ocean, creating a ripple effect through the area’s delicately balanced ecosystems.

Firstly, warming temperatures are causing a reduction in sea ice coverage. Sea ice plays a crucial role in the Antarctic ecosystem, influencing the distribution and abundance of primary producers like phytoplankton and krill. 

These organisms form the basis of the Antarctic food chain, and changes in their populations can impact the entire ecosystem. Furthermore, sea ice provides important habitat for various species, such as seals and penguins, which use it for resting, breeding, and escaping predators.

Secondly, the melting of Antarctica’s ice sheets and glaciers due to rising temperatures is causing an increase in freshwater input into the Southern Ocean. This freshwater is less dense than seawater and can lead to stratification, reducing the upwelling of nutrient-rich waters from the deep sea. 

This process, known as vertical mixing, is critical for supporting phytoplankton blooms. A reduction in these blooms could significantly impact the food supply for krill and other marine organisms, ultimately affecting higher trophic levels, including fish, seals, and whales.

Thirdly, ocean acidification due to increased absorption of atmospheric carbon dioxide is another major concern. More acidic waters can make it difficult for organisms such as shellfish, certain plankton species, and corals to form their shells or skeletons, which are made from calcium carbonate. This could potentially disrupt the balance of the entire marine food web.

Lastly, warming temperatures can lead to changes in wind patterns and ocean currents, which can further alter the distribution and abundance of species and impact migratory routes and breeding patterns.

As for the specific impact on right whales, these creatures rely heavily on krill as their primary food source, particularly in the Southern Ocean. 

If the populations of krill are negatively affected by the changes in sea ice coverage, water stratification, and acidification, this could result in a scarcity of food for right whales, potentially affecting their ability to build up the necessary fat reserves for migration and reproduction.

Moreover, changes in sea ice conditions and water temperatures might also affect the migration patterns and distribution of right whales, potentially increasing their vulnerability to threats such as ship strikes and entanglement in fishing gear.

In summary, climate change is significantly disrupting the Antarctic and Southern Ocean ecosystems, and protecting these areas requires global efforts to reduce greenhouse gas emissions and mitigate the impacts of climate change.

Image Credit: Fredrik Christiansen

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