Humpback whales have been captured on film herding shoals of fish into their cavernous mouths with their oversized pectoral fins.

Marine biologists recorded the extraordinary feeding behaviour for the first time off the coast of Alaska, where the whales lurk around salmon hatcheries that release juvenile fish into the sea.

Humpbacks are known to corral fish into meal-sized shoals by surrounding them in streams of bubbles released from their blowholes. But the footage shows that some have taken their foraging methods to another level.

In footage recorded off Baranof Island in south-east Alaska, whales corral young salmon in “bubble nets”, then rise up from beneath and swish the fish into their mouths with elaborate movements of their pectoral fins.

“The first time I saw this behaviour it was from boat level and it looked chaotic,” said Madison Kosma at the University of Alaska in Fairbanks. “But when I got a bird’s eye view with a drone, I was ecstatic. It wasn’t chaotic, it was actually graceful, intentional and calculated.”

Humpback whales have exceptionally long pectoral fins. Though slender, the flippers can reach to a third of their body length, making them up to 5 metres (16ft) long in adult whales. The fins are primarily used for manoeuvring and providing sudden bursts of acceleration. When a humpback lunges at a shoal of fish, the whale’s baleen, a large comb-like filter in its mouth, sieves the food from the inrush of water.

Kosma noticed the whales’ unusual feeding strategy while working for a master’s degree. Fascinated and perplexed by what she had seen, she got hold of a small video camera and strapped it to a pole with cable ties. With this she recorded some of the first footage of the whales using their pectoral fins to round up and capture prey. She later saved up to buy a drone and recorded from directly above the feeding whales.

Analysis of the footage revealed that the whales used their pectoral fins in at least two ways. During “horizontal pectoral herding”, the whale creates a bubble net around the fish it is hunting and then rises up at an angle, using one pectoral fin to swish the fish into its mouth. In “vertical pectoral herding”, the whale again creates a bubble net but then lunges at the shoal from beneath with its fins pushed upwards. The light colouring of the insides of the fins may scare the prey into the dark refuge of the whale’s mouth, the scientists believe.

Writing in the journal Royal Society Open Science, the researchers explain how the herding strategy may make feeding more efficient by squeezing more prey into the volume of water the whales take in. “We believe that the whales are getting more bang for their buck by using pectoral herding techniques,” Kosma said. Further studies in other parts of the world are needed to confirm whether the trick has been learned by whales elsewhere, she said.