More than 30,000 humpback whales now splash across the western Indian Ocean, according to newly released preliminary data. That’s up from fewer than 600 in the late 1970s, after nearly two centuries of whaling decimated populations in the region.
A team led by marine biologist Chris Wilkinson, technical manager for the Mammal Research Institute at the University of Pretoria, sampled the number of humpback whales in one population by counting them as they swam past Cape Vidal, on South Africa’s east coast. The whales migrate each year from feeding grounds in Antarctic waters to breeding areas near Mozambique. From their 2018 survey and data from previous years, the researchers extrapolated to estimate a population of more than 30,000 humpbacks in the entire western Indian Ocean.
“It’s brilliant to see so many whales,” Wilkinson says. “This is an animal that migrates halfway across the world, and you get to see that and be a part of it.”
The rebound is a promising sign, says Jean Harris, a marine ecologist and executive director of WildOceans, a conservation group that helped coordinate the surveys. “It is saying to us that if we take the right action—even if you catch it when it seems really dire—species and ecosystems can recover.”
Europeans and Americans began whaling in southern Africa as far back as the 1790s. Ship logs and sailors’ journals noted sperm, right, and humpback whales being killed for baleen—which functioned then in some ways like modern plastic—and blubber, which could be rendered into oil for lamps and industrial machinery.
Whale migrations were so predictable that crews returned to hunt in the same bays season after season and simply waited for the animals to appear. Each ship could harvest more than two dozen whales per voyage.
That number grew with the development of local onshore facilities in places like Durban, a city that had six whale processing plants by 1912. And the advent of factory ships, which accomplished the processing work at larger scales while remaining out at sea, further contributed to precipitous population declines in the decades before and after World War II, when thousands of humpbacks across the southern hemisphere were killed each year. (Related: Why are humpbacks dying in unprecedented numbers off the U.S. East Coast?)
In 1979, South Africa took action and banned commercial whaling. By then, the humpback population in the western Indian Ocean was approaching extinction—down to roughly 300 to 600 individuals.
About a decade after the ban, Ken Findlay, a whale biologist at Cape Peninsula University of Technology, began to survey the humpbacks that migrate past Cape Vidal to monitor their recovery. Focusing on a population known as C1, he observed 46 humpback groups, or an estimated 360 whales, during his initial field season in 1988.
By 2002, Findlay recorded 323 groups, or an estimated 1,670 individuals.
Wilkinson picked up from there, with Findlay and Harris as advisors. He
and several volunteer whale spotters climb a pair of towers nestled amid
the canopy of a sand dune forest that overlooks Cape Vidal. The team counts
humpbacks from 7 a.m. to 5 p.m. and works from late June through early
August. (Read about the “super-groups” of humpback whales seen off the coast of South Africa in 2017.)
Overall, the preliminary analysis of the 2018 survey indicates humpback group numbers in the high hundreds, if not the low thousands. That sample passing by Cape Vidal suggests, based on an early extrapolation, that the broader humpback population in the western Indian Ocean has surged to more than 30,000 individual whales.
The estimate is considerably higher than models have predicted as the carrying capacity for the region.
“It’s a surprising number,” Harris says. Scientists still aren’t sure what accounts for the difference between the model predictions and reality.
Harris notes that the rebound might relate to the fact that humans hunted the whales, not their dinner. As a result, humpbacks that survived into the post-whaling era had both an adequate food supply and the gift of time.
Ari Friedlaender, a marine ecologist at University of California, Santa Cruz, who has studied humpbacks in Antarctica, says the ocean optimism isn’t misplaced.
“We’ve seen and measured in other parts of the Southern Ocean populations that are growing at the maximum of their growth rate,” he says. “Humpbacks have a central breeding area, it’s easy to find a mate, and there’s low competition for food. The recipe for them rapidly increasing is right there.”
He also provides some perspective for global ocean conservation.
“There’s no question humpback whales are a success story. We need to celebrate those victories,” he says. “But it’s a species, not an entire ecosystem.”
Check out “Humpback whale song of the summer,” the first episode of Overheard at Nat Geo, National Geographic's new podcast.
Dustin Renwick is a triathlete and a freelance journalist.