Bottom-Feeding Observed In Gulf Of Maine Humpback Whales
October 31, 2013

Bottom-Feeding Observed In Gulf Of Maine Humpback Whales

April Flowers for - Your Universe Online

Humpback whales have exhibited a complex set of feeding techniques, including "trapping" krill and other prey within bubble nets produced by the whales and gulping up to two-thirds their weight in prey-laden water.

A new study, published in Marine Mammal Science, confirms that humpback whales in the southern Gulf of Maine are spending more feeding time on the ocean floor than in any of these other typical feeding behaviors. These findings have implications for the use of bottom-set gear in the fishing industry because entanglement in such gear is a major risk to humpbacks.

“Humpbacks have not been known as bottom-feeders, yet this is their dominant feeding mode in this region,” says University of New Hampshire professor of data visualization Colin Ware. “You’ve got this prominent species, and until now nobody knew how they were doing most of their feeding.”

The research team included Ware, of UNH's Center for Coastal and Ocean Mapping, David Wiley of the Stellwagen Bank National Marine Sanctuary of the National Oceanic and Atmospheric Administration (NOAA) and Ari Friedlaender of Duke University Marine Laboratory and Pratt School of Engineering. This team gathered data from 53 humpback whales in the Stellwagen Bank National Marine Sanctuary and the Great South Channel near Cape Cod, Massachusetts.

The researchers affixed DTAGs -- synchronous motion and acoustic recording tags -- to the whale's backs with four suction cups. This allowed them to track the movements of the whales below the ocean's surface for the first time. Ware developed a custom software tool, called TrackPlot, to translate the tags' data into a 3D ribbon that illustrated the whales’ paths as they repeatedly dove to the bottom of the ocean, rolled onto their sides, tilted their heads down, and feasted on sand lance, a favorite food that is abundant there.

The data, collected between 2004 and 2009, allowed the researchers to identify distinct types of behavior during what they called bottom side-roll feeding. This type of feeding consists of simple side-rolls, side-roll inversions, and repetitive scooping. The data confirms the bottom-feeding behavior that scientists had suspected from visible scarring along the jaws of some whales.

Ware calls these humpbacks "by far the most acrobatic of all baleen whales." The data showed that they were not only performing bottom side-rolls and seafloor scooping, but also indicated that this bottom feeding does not include lunging. Previously, scientists assumed lunging to be the humpbacks' primary feeding behavior.

Whales accelerate to propel water full of prey into their enlarged mouths during lunge feeding. They then filter the water out through the hair-like filaments of their baleen, retaining the prey. In this study, the tagged whales were moving at too low rate of speed to characterize their behavior as lunge feeding.

National Geographic's Crittercam -- a video camera attached to the whale -- provides additional insight into the whales' time at the seafloor, however Ware cautions that there's still plenty to learn about what the whales are doing in the deep.

“The big mystery is we still don’t know exactly how they’re feeding. We don’t know the mechanism,” he says.