The bottom feeding behavior of humpback whales
If you go on a whale watching tour, you might be lucky enough to catch humpback whales feeding. Sometimes they slap their tail flukes on the surface, frightening their fishy prey so they school, making the fish easier to eat. Or the whales might create a net of bubbles to gather fish together before lunging forward to gobble up a meal. But humpbacks living off the coast of New England have been spotted going deep to catch themselves a midnight snack.
A team of researchers studying humpbacks in the southern Gulf of Maine, mostly the Stellwagen Bank National Marine Sanctuary and Great South Channel, used tags to track the whales’ motion and sound underwater. They also attached a Crittercam to one whale. The scientists then used that data to create three-dimensional maps of the animals’ paths. Their study appears in Marine Mammal Science.
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Humpbacks survive on a diet of tiny fish such as herring and sand lance, which they eat by opening up their mouths wide, taking in a huge amount of fish-laced water — two-thirds or more of their body mass — and then squirting the water back out through their mouths, filtering out the food with bristly baleen plates inside their mouths. This usually happens through lunge feeding, in which the whale makes several powerful strokes with its tail to get up to speed, then quickly slowing when it opens its mouth.
But the whales in the southern Gulf of Maine also feed through a different method, the new research found. These whales employ a technique named “bottom side-rolling,” which the research team describes as “a flat-bottomed dive with repetitive sustained rolls.” These rolls happened most often between 11 p.m. and 4 a.m., and the whales would perform these slow rolls at the rate of 30 per hour, repeatedly diving to just a meter or two from the seafloor.
The Crittercam video showed that sometimes a whale’s rostrum (a snoutlike projection) came in contact with the seafloor (which explains scarring seen on some whales), kicking up sand in clouds, and that the whales’ ventral pleats expanded, indicating that they were feeding. The video also showed what the whales might be after – sand lance. At night, sand lance burrow into the seabed or form horizontal schools just above the seafloor. The whales don’t perform bottom side-rolls in another area of the Gulf of Maine where the dominant humpback prey is herring, further suggesting that the late-night acrobatics are for catching sand lance.
Several times, whales worked together in coordinated bottom side-rolls, which the researchers speculate might help to cluster fish or at least prevent them from escaping. That’s pretty impressive considering that this action happens in the middle of the night at the bottom of the ocean. “It is very likely that such coordinated feeding behaviors require practice and knowledge associated with a long-term relationship between animals,” the researchers write. “What keys the animals might use to time coordinated movement when at the seafloor, which would lack light at many depths, is…unknown.”
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Bottom side-rolling is more than just an interesting bit of whale behavior. This “behavior puts humpbacks at jeopardy from bottom-set fishing gear such as gillnets and trap fisheries, which is heavily used in our study area and is a known risk to humpback whales,” the researchers note. By going after the tasty sand lance, it appears that humpbacks may be putting themselves in danger of getting tangled up in the gear humans are using to catch their own tasty meals.