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By Virginia Morell
When Madison Kosma spied a humpback whale thrashing wildly off the coast of southeast Alaska, she was sure it was hunting salmon. But she was perplexed by the whale’s seemingly uncoordinated movements. To get a better vantage, the graduate student at the University of Alaska in Fairbanks snapped photos from high atop a tower of crab pots—but it wasn’t high enough to capture what was happening. The next year, she took video using a GoPro on a long stick—still not high enough. The third year, she bought a drone and “[it] changed everything.” For the first time, scientists have footage of the whales’ complex fishing routine, which Kosma describes in a new paper: The humpbacks use their pectoral fins to create minicurrents that threaten unsuspecting fish; in response, they mistakenly seek shelter in the whales’ dark, cavelike mouths.
“Bingo!” says Richard Connor, a marine biologist and cetacean expert at the University of Massachusetts in Dartmouth who was not involved in the study. He says researchers have suspected for 90 years that humpbacks use their long fins to fish. But the drone footage is the first solid evidence.
Fishermen and biologists have long puzzled over the size and distinctive shape of a humpback’s pectoral fins, which can grow up to 5 meters in length, and measure from one-quarter to one-third of their body length. (Indeed, the unusual fins gave rise to their scientific name: Megaptera novaeangliae, or “giant wing of New England.”) In contrast, the pectorals of other whales are only one-seventh the length of their bodies. What researchers do know is that even though the winglike extensions don’t help on long journeys, they do help the cetaceans maneuver in shallow water and rapidly accelerate when feeding on small prey such as herring.
From the drone’s images—taken in the spring of 2018, when Baranof Island’s Hidden Falls Hatchery releases some 50 million juvenile salmon—Kosma was able to see exactly how two adult male humpbacks were using their pectoral fins. What had looked like haphazard movements turned out to be very precise maneuvers.
First, she discovered, the humpbacks fished independently. Each swam by itself in a tight circle below the fish and made an individual net of bubbles. (Usually, humpbacks work in teams to fish with bubble nets.) Next, the whale lunged at the trapped prey while using its pectoral fins to prevent the fish from escaping, or to create currents that swept the fish toward its mouth. The white undersides of the whales’ fins likely also confused the fish; sometimes the juveniles swam directly into the whale’s mouth, apparently mistaking it for a place of safety.
Kosma is quick to point out that her observations apply so far only to whales hunting young, hatchery-released salmon. But she expects the technique is widespread, she reports today in Royal Society Open Science. She says it’s likely been missed because it is difficult to see when watching from a boat or dock.
The study shows how new technologies like drones provide insights into the natural world, says Frank Fish, a marine biologist at West Chester University in Pennsylvania who was not involved in this study. That’s particularly true, he adds, “in cases where the creature remains largely hidden.”
Humpbacks use their long fins for many other reasons: communicating with other whales, stunning fish, fighting, and even lifting their newborns to the surface to breathe. But Connor suspects their utility in hunting is what helped these giant appendages evolve. “It’s more likely that [hunting success] is a key selection pressure that favors those whales with long pecs,” he says. “The whales have been fishing like this for a long time. Now, we finally can get aerial views that let us see it.”
*Correction, 15 October, 8 p.m.: An earlier version of this story misstated the number of juvenile salmon released by the Hidden Falls Hatchery.
Source: Science Mag