Human harvesting of krill in the Southern Ocean could threaten the recovery of whale species that were nearly wiped out by industrial whaling in the 20th century, according to a Sept. 10 study in Nature Communications.

The tiny, shrimp-like crustaceans known as krill are the essential food source for baleen whales such as blues and humpbacks. To feed, these giant marine mammals take in great gulps of ocean water, filtering krill through bristly mouth structures. Booming demand for krill as fish meal and omega-3 fatty acid nutritional supplements, however, could leave whales without enough victuals to sustain even their diminished numbers.

“Our calculations suggest an alarming possibility that we might harvest krill to the point where we do real damage to recovering whale populations,” said lead study author Matthew Savoca, a research scientist in the lab of Jeremy Goldbogen, associate professor of oceans in the Stanford Doerr School of Sustainability.

The results highlight a need for scientists, regulators, and industry to carefully assess the impacts of krill harvesting in the Southern Ocean at current levels before expanding. “With this study, we want to draw attention to how there likely isn’t enough krill to support fully recovered whale populations, and now on top of that, we’re harvesting krill and plan to harvest more krill in the near future,” said Goldbogen, the study’s senior author. 

Counting on krill

The new research grew out of a prior Stanford study documenting how baleen whales gobble up significantly more krill than scientists had previously estimated. A paradoxical finding of that study was that, as whale populations plummeted by roughly 90% in the Southern Ocean during whaling’s grim heyday, so, too, did krill populations.

The researchers worked out that baleen whales effectively fertilize the ocean through their prodigious droppings, providing nutrients for the phytoplankton that krill eat. The upshot: The krill population must have been much larger, perhaps five times greater, than it is currently to have sustained the pre-whaling whale populations in the early 20th century.

“Krill is the foundation of the entire Southern Ocean ecosystem. They’re really the only thing that large whales eat down there,” Savoca said.

In the nearly 40 years since a global whaling moratorium went into place in 1986, some Southern Ocean species – particularly humpbacks – have made an impressive comeback. Yet this recovery has taken place against increasing competition with humans for the whales’ critical food source; over the past 30 years, the krill catch has quadrupled to around 400,000 tons annually and is set to expand further. 

Savoca and colleagues calculated how much krill is left in the Southern Ocean for baleen whales, seabirds, and other predators to eat after industrial krill harvesting at current rates, compared to the estimated amount of krill available before industrial whaling began. “The basic math makes it pretty clear that the current krill biomass cannot support both an expanding krill fishery and the recovery of whale populations to pre-whaling size,” said Savoca. 

Krill for all?

The study offers suggestions for how the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), an international organization established in 1980 that manages Southern Ocean fisheries, can help avert disaster. 

Regulation of the areas and times of year when commercial krill boats can operate to minimize competition with whales is a starting point. The most important such region, according to the study, is a fairly small area (16,000 square kilometers) to the west of Coronation Island in the South Orkney group, a prime fin whale feeding ground where about 30% of all krill has been fished since 2000. Krill fishing directly overlaps with whale feeding in this region, driving welfare concerns for the whales. The researchers also suggest expanding the use of marine mammal exclusion devices to prevent accidental bycatch of whales entangled in krill nets, as has occurred recently with the documented deaths of at least four humpback whales across the 2021 to 2022 seasons. 

As for the krill themselves, improved monitoring to bring in more data on krill egg and larvae hotspots could identify zones to ban or limit fishing. Additionally, fishing vessels could regularly sample krill swarms to avoid harvesting those in the midst of critical spawning periods.

The researchers hope their study will prompt more rigorous accounting and deeper investigations into krill predators and the krill themselves, ultimately forging a path ahead that preserves the delicate ecological balance in the Southern Ocean. 

“It’s not a foregone conclusion that whale populations have to suffer because of higher levels of krill harvesting,” said Savoca. “With more research informing careful management, this story of saving the whales can continue to be a conservation success.”

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