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A one-of-a-kind killer whale population appears to be threatened by human appetites for Antarctic toothfish, better known to restaurant-goers as Chilean Sea Bass.

As fishing fleets patrol their waters, catching what was their primary source of food, the whales are vanishing. It’s not certain whether they’ve only moved on, or are dying out, or both. But something is happening, with potentially dark implications for Earth’s last pristine ecosystem.

“There’s been a dramatic disappearance of the whales,” said biologist David Ainley of ecological consulting firm H.T. Harvey and Associates, and co-author of a March Aquatic Mammals article on the whales’ disappearance. “We think they’re having a harder time trying to find food. Whether that leads to population decrease, hopefully we won’t find out. But we will find out, if it continues.”

Antarctic killer whales form two types of populations, known to researchers as ecotype-B and ecotype-C. While the former resemble killer whales found elsewhere, ecotype-C whales are much smaller, with different markings and a tendency to gather in especially large groups. Many researchers now consider them a distinct species.

Dubbed Ross Sea killer whales, ecotype-C whales are found only in the Ross Sea, an expanse of water off Antarctica’s southern coast, flanking the France-sized Ross Ice Shelf. Many scientists consider the region to be the last pristine ecosystem on Earth, the only remaining piece of pre-industrial nature.

The Ross Sea, however, isn’t what it used to be. About 25 years ago, North American diners discovered the Chilean Sea Bass, the market-friendly name of the Patagonian toothfish. It is a large, codlike Southern Ocean fish that lives for a half century, breeds infrequently and is both tasty and easy to cook, and its populations were soon devastated. Fishing fleets moved into the Ross Sea, searching for its close relative, the Antarctic toothfish.

Antarctic toothfish are now called Chilean Sea Bass, too. They’re thought to be the primary food of Ross Sea killer whales, which were described as common by the first Antarctic explorers and subsequent visitors. Just a few years ago, boats headed into the McMurdo research station on Ross Island were “literally surrounded by killer whales out toward the horizon,” write the Aquatic Mammal researchers. Not any more.

Though ecotype-B whale sightings have remained steady, Ross Sea killer whale sightings are down by two-thirds in the last five years, and big groups no longer gather.

“We don’t know for sure what this means. But we do know that they eat the toothfish, and we know that the toothfish industry has taken off in the last 10 years,” said study co-author Grant Ballard, a Point Reyes Bird Observatory biologist.

If the whales have moved elsewhere in search of food, there is no guarantee of success. Other, smaller fish can be harder to catch, making them an inefficient source of nourishment. Even if other food is available, the whales may not eat it. Hunting is a behavioral tradition — even, arguably, a culture — for these highly social animals, and not easily changed.

In a possibly analogous situation from the northeast Pacific, a population of historically salmon-eating killer whales appears doomed by the fishes’ decline, though seals and sea lions are an abundant alternative source of prey.

After more than a decade of studying penguins, Ballard said he’s yet to see a Ross Sea killer whale eat one.

“I was hoping I’d see them eating one, and it never happened. There are plenty of penguins around for them to eat,” he said. “The arrows point at this type of killer whale being a toothfish eater, and not knowing how to change.”

How long Antarctic toothfish can survive human pressures is an open question. After just a couple decades of modern fishing, Patagonian toothfish were mostly gone. They bred too slowly to keep up with losses. Even in areas where fishing has ceased, the Patagonian toothfish, and other local deep-sea species fished during the 20th century’s latter half, have not come back. Like the cod of North Atlantic, they appear to have hit some sort of tipping point beyond which recovery may not be possible.

“The fact that these stocks haven’t recovered suggests that some ecological mechanism has been turned off, that the ocean has changed in the meantime, to the extent that the fish can’t recover,” Ainley said.

Should Antarctic toothfish and Ross Sea killer whales vanish, the ecological impacts could be profound. Existing at the center of marine food webs, such high-level predators are important to regulating ecosystems. In their absence, food webs take different shapes. That’s what appears to have happened in the western North Atlantic. With cod fished to near-extinction, it’s now dominated by small fishes and crabs.

As a stopgap solution, Ainley and Ballard want diners to avoid Chilean Sea Bass, though that strategy did not save the Patagonian toothfish. Despite attempts to educate the public, Chilean Sea Bass remained a popular menu item in upscale American restaurants.

Their greater wish is for the Commission for the Conservation of Antarctic Marine Living Resources, the international body charged with protecting the continent, to declare the Ross Sea a protected area, off-limits to all fishing.

That’s not just sentimental enviromentalism, but economic practicality, Ainley said. “If you have areas with no fishing, it ensures that there will still be fishes caught around the edges of the reserve. Protecting the Ross Sea could probably ensure the continuation of that fishery. Otherwise, it’s going to go economically extinct,” he said.

But Ballard is more idealistic. “We’re talking about the last pristine ecosystem. It’s important to have one of them left,” he said. “Going forward, people won’t have reference points to what we used to have. We’ll get used to a more and more degraded Earth. And I think we’re running into that here. It’s the last stand.”