Return to humpback halcyon days?

One of the 21^st century’s greatest ecological success stories, the dramatic recovery of humpback whales, may be leading to ecological complications, scientists say.

Not all that long ago, humpback whales were hunted to near-extinction, their populations plummeting so badly that by the mid-1980s, it was estimated that only 440 remained south of the Equator. Since then, whaling bans have not only saved humpbacks from extinction but allowed them to rebound. Today is thought there are nearly 80,000, worldwide.

It’s one of the world’s great ecological recovery stories… with one caveat. Humpback whales have to eat. And they are big, so they eat a lot. Which means that their recovery may affect marine ecosystems that had previously adapted to their absence – ecosystems that are also being stressed by the mounting effects of climate change.

“The problem is that as whales return to their old feeding and breeding grounds, researchers are unsure how [they] will impact communities that have changed in their absence,” says Emily Zembricki, an undergraduate researcher at Stockton University, New Jersey, US, who studied them in conjunction with Oscar Schofield of Rutgers University in nearby New Brunswick.

In an online presentation at the 2022 Ocean Sciences Meeting, Zembricki examined the effects of the humpbacks’ stunning recovery on krill in the waters offshore from the West Antarctic Peninsula.

Krill are small, shrimp-like crustaceans that form the base of much of the Antarctic food chain, not only for filter-feeding whales like humpbacks, but also for seals, penguins, seals, and fish. (They are also harvested by commercial fishing companies, mostly as feed sources for aquiculture.)

There are a number of species, but in the West Antarctic Peninsula waters, Zembricki was primarily interested in two of them: Euphausia superba, and various species of the genus Thysanoessa.

As the name “superba” might indicate, the former is relatively large, measuring about 60 millimetres long. The latter checks in at a more modest 30 mm.

Prior to 1993, Zembricki says, the big ones were dominant. Then, a warming event reduced the extent of sea ice for several years – important because E. superba (the large ones) are dependent on the ice, attaching to its base in their larval and juvenile stages, and relying on it to shield them from predators. With less ice, E. superba lost its dominance and was supplanted by Thysanoessa, which isn’t as reliant on ice for protection.

When the ice returned in 1999, however, Thysanoessa retained its dominance.

Why didn’t E. superba recover? One possibility, Zembricki says, is that a resurgent humpback whale population was eating all the krill it could find – and not caring what species they were. “Whales don’t pick and choose,” Zembricki says. “They just eat whatever is there.”

The result, she says, is that their increasing presence in West Antarctic waters perpetuated the climate-change-induced shift seen in 1993–98. And because the surviving krill are smaller, and therefore less nutritious, every animal feeding on them now has to spend more energy foraging in order to fulfill its dietary needs. “So, while this wasn’t something I explicitly looked at in my research, I know that the change in krill dominance also affects fish and the [other] animals that eat them,” Zembricki says.