Baleen isotopes tell whale of a tale

Baleen whales in the southern hemisphere are likely to face challenging conditions as climate change continues, new research from the University of New South Wales suggests.

The study used stable isotope data preserved in whale baleen – a bristly feeding apparatus made of keratin – to unlock information about feeding and migration patterns of humpback whales and southern right whales going back 60 years.

The researchers matched the isotope information with climate data to understand how climate cycles have been affecting the whales over the study period.

Their results indicate that La Niña events, such as the southern hemisphere has been experiencing this summer, can make life much more difficult for whales.

What a whale wants, what a whale needs

Both humpback and southern right whales are baleen whales – meaning that instead of teeth, their mouths contain a structure called baleen. It’s made of keratin, the same material as our hair and fingernails, and acts a bit like a tea strainer.

As the whale lunges through the water with an open mouth, the baleen traps its main prey – krill – and filters the water back out.

“Baleen whales gave up on teeth,” explains Tracey Rogers, a professor in marine ecology at UNSW and senior author on the study. “They strain their food with this feathery material, baleen.”

Both species of whale spend their summers feeding on large schools of krill in the Southern Ocean around Antarctica. In the southern hemisphere winter, they migrate north to tropical temperate waters to breed.

Whales can’t feed very much on the journey north, so they rely on gorging themselves in Antarctica to build up the reserves that will sustain them during migration.

“[Baleen] has really worked for them in the past, because there were these massive predictable schools of prey, in the Southern Ocean, which allowed them to really boom and become enormous,” says Rogers.

How climate change is impacting baleen whales

However, due to human-induced climate change, these schools of krill are getting less reliable – and that’s bad news for the whales.

“Krill rely on sea ice for survival in the Southern Ocean, and sea ice is driven by different climates conditions that unfortunately, due to climate change, are becoming more difficult to predict,” explains Adelaide Dedden, a PhD student at UNSW and lead author on the study.

Rogers explains that sea ice helps krill survive the dark Antarctic winter, during which their normal prey – photosynthetic phytoplankton – can’t grow due to lack of sunlight.  

Instead, the krill will feed on bacteria found on the under-surface of sea ice in winter. They also use brine channels in the ice to hide from predators.

The new study looked at climate data from multiple climate cycles, including the Southern Annular Mode in the Southern Ocean, the Indian Ocean Dipole, and the El Niño Southern Oscillation (ENSO), which includes El Niño and La Niña events. Their findings confirmed earlier evidence that La Niña can negatively impact baleen whales.  

After a La Niña event, sea ice is less concentrated in the whales’ Antarctic feeding areas. Less ice means less krill and so less food for the whales.

“Our colleagues have shown humpbacks are leaner – a sign they’re experiencing poor feeding conditions – and have a higher chance of stranding in the years following La Niña events,” says Rogers.

The study found that whales migrating up the east coast of Australia were particularly vulnerable, while whales based off the continent’s west coast seemed to fare better during La Niña periods. That’s probably because ENSO impacts the Pacific Ocean more strongly than the Indian Ocean.

It’s a poignant issue, all the more so because humpback whales have only recently been removed from Australia’s threatened species list, as their populations have slowly recovered since bans on commercial whaling were enacted in the 1960s.

“But they do still face probably one of the biggest threats of all, which is climate change,” Dedden says.

Isotope insights

The researchers gained their insights into the whales’ feeding and migration by analysing stable isotopes preserved in the whales’ baleen.

These isotopes come from the whales’ diet and are integrated into their tissues, including baleen.

“It’s basically a form of natural tagging,” Dedden says. “Stable isotopes within marine consumers like baleen whales reflect the isotope signals of the prey that they eat, and they can also tell you [about the] general region that they feed in.”

The baleen continues to grow throughout the whale’s life. Dedden says that a baleen ‘plate’ about one metre long contains isotopes from about four years of the whale’s life.

Carbon isotopes can tell scientists where the whales were feeding, because waters around Australia and Antarctica contain markedly different carbon levels.

“Carbon varies latitudinally, so you see lower carbon levels in Antarctic regions and higher carbon values as you get closer to temperate and tropical regions,” Dedden explains.

Nitrogen, meanwhile, can indicate when and how much whales have been feeding or fasting. That’s because heavier nitrogen isotopes tend to accumulate up the food chain.

“When krill are feeding on the algae and bacteria in the environment, they take in the stable isotope signatures from the algae,” says Rogers.

“They excrete lighter stable isotopes and hold the heavier elements in their body tissues. Then the whale comes along and eats that krill and it does the same thing.”

When humpback whales are fasting on their migrations, they actually begin to break down their own bodies to get energy, which leads to even higher nitrogen isotope values.

“They’re essentially eating themselves when they’re fasting,” Dedden says.

What does the future hold for whales – and us?

Dedden says that climate change is likely to impact other predators who rely on krill, like certain seal species. However, the whales are particularly vulnerable due to their migratory behaviour, which requires them to build up reserves within a relatively short feeding period.

She hopes that this clear evidence linking the whales’ feeding patterns to climate cycles in the past may help to predict how they will fare under different future climate scenarios.

For Rogers, the results are just another reason to add to the list of why we need to work to limit further climate change. She points to the stress that increased La Niña events have put on humans as well, referencing the recent floods in eastern Australia.

“It’s all driven by the oceans being too warm and the atmosphere being too warm so there’s more heat for us and more moisture being held in the atmosphere,” she says.

“In actually doing something for the whales, we’re doing something for ourselves.”