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By Mike McRae Mike McRae 23 August 2017 3 min read

Humpback whale. Image: Diego Cotterle ©  DiegoCotterle(C)2017

A global ban on hunting most marine mammals might have saved the largest mammals on Earth from extinction. But with widespread climate change on the horizon, researchers are now asking how quickly we can expect different species of whale to bounce back.

University of Queensland and CSIRO PhD student Vivitskaia Tulloch knows better than most how whaling affected some of the world’s biggest cetaceans. Her research on the health of hunted whale populations has delivered a concerning report card on a number of species that came within a whisker of extinction.

“None of the whale populations that were whaled extensively during the 19th and 20th century has recovered,” Tulloch explains, claiming slower reproductive rates mean some are slower to bounce back than others.

“We estimate southern right whales (Eubalaena australis) globally remain at less than 11 percent of their pre-whaling numbers,” says Tulloch.

Fewer than 17,000 fin whales (Balaenoptera physalus) swim the waters of the Atlantic and Indian oceans, which is less than four percent of their previous population size. For that majestic titan the Antarctic blue whale (Balaenoptera musculus intermedia), a return to glory is even slower, where at an estimated 1,890 individuals their numbers are barely 1/100th of the way back to their previous size.

The fin and the blue whales are unlikely to be even half their pre-exploitation numbers by 2100, meaning they have a long, slow road to recovery. Things would be far worse if not for the restrictions placed on commercial whaling. The southern right whale was protected in Australia in 1935, though it took until 1978 for the national whaling industry to come to a complete halt.

Following a moratorium on commercial whaling decreed by the International Whaling Commission in 1983, most countries have given up the hunt. Some whale harvesting still occurs, however the pre-moratorium catches of tens of thousands of whales each year can now be counted in tens of hundreds.

Image: Karl Malakoff

Fortunately, not all whales are finding it quite as hard to repopulate.

“One species that has bounced back relatively well after whaling stopped in the late 20th century is humpbacks,” says Tulloch.

Since the dawn of commercial whaling in Australia and New Zealand in the late 18th century, more than 200,000 humpback whales (Megaptera novaeangliae) have been harvested from Southern Hemisphere waters. Today their population is estimated to be about a third of the size it was prior to the days of industrial scale whale harvests. While it might seem low, by 2050 we could expect the population to have recovered completely.

“We show their numbers are increasing rapidly across the Southern Hemisphere, which is good news for whale watchers currently enjoying watching them on their migration towards the tropics.”

Of MICE and whales

For their research, Tulloch and her colleagues used a big process with a cute name. A relatively novel approach called MICE (Model of Intermediate Complexity for Ecosystem Assessments) integrates important features of ocean and biological systems to create a more holistic model of an ecosystem, allowing the scientists to take diverse factors into account when determining the trajectory of population growth.

The researchers simulated whale populations based on what they knew about the species’ life histories and then subtracted historical catch numbers from between 1890 and today.

“We also coupled predator-prey information from our MICE model to global climate models, which allowed us to predict how whales are going to respond given expected future changes in primary productivity in the Southern Hemisphere,” says Tulloch.

Baleen whales depend heavily on a diet of krill, a key link in the food chain of the southern oceans surrounding Antarctica. While warmer oceans could be expected to make these waters more productive, the long-term impact on these tiny crustaceans and the role their abundance plays in the growth of whale populations hasn’t been spelled out before.

As far as the food chain goes, global warming could be good news for certain whale species.

“Minke whale numbers are seen to increase in response to increases in primary productivity,” says Tulloch.

“This is explained by the increasing primary productivity predicted around Antarctica where minkes are found, which may drive increased survival and breeding success for the whales.”

Global warming influences a variety of ecological factors, however, not all are expected to be quite so benevolent. Sea ice change and increases in surface temperature could cost some species dearly.

“The next step of this project will look at how ongoing climate changes are influencing food availability such as krill and what this will mean for whale recovery into future,” says Tulloch.

The researchers are intending to focus more on these other variables in future studies, further fine-tuning their estimates on the pace and variability of growth for whale populations over the next century.

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