In the waters off the northern tip of the world's southernmost continent, one of the most important creatures is also the most profitable: pinky-length Antarctic krill.

These swarming, translucent, shrimp-like creatures are eaten by almost everything here—fish, penguins, seals, and whales. But krill also support a multimillion-dollar global fishing industry. They get sucked into nets and ground into meal to feed aquarium fish or farm-raised salmon and get squeezed for their oil, which is used in pharmaceuticals, including in the United States.

Now, with climate change rearranging life along the western Antarctic Peninsula, scientists and marine advocates have been warning that wildlife—particularly penguins—are under far too much stress. Krill fishing, they say, could be making things worse.

Monday, after years of negotiations, a