A right whale may weigh some 70 tons, but unlike other marine mammals studied so far, it tends to float rather than sink at great depths.
Electronic monitoring of 95 right whale dives shows that the animals swim the whole downward leg of 70-meters plus plunges, says Douglas P. Nowacek of Woods Hole Oceanographic Institution in Woods Hole, Mass. On the way back up, however, the whales stop swimming and just float to the surface, Douglas and his colleagues report in the Sept. 7 Proceedings of the Royal Society of London B.
Seafarers have long noted the animal’s unusual buoyancy at the surface, an attribute that supposedly made this whale the “right” one to harpoon. Since the heyday of whaling, however, scientists have grown curious about buoyancy changes underwater.
The handful of species tested so far, including blue whales, elephant seals, and bottlenose dolphins, stop swimming as they work their way downward. As the increasing pressure squeezes air out of their lungs and other tissues, their buoyancy drops. Then, without additional energy-sapping strokes, the animals let gravity take them down to feeding depths.
“There’s a video of a bottlenose dolphin at 40 to 60 meters, and its chest is caved in,” marvels Nowacek. “The diving mechanisms of these animals are just incredible.” Until the new work, he explains, the prevailing hypothesis predicted that all diving marine mammals “glide down and power up.”
In a study examining the risk of right whales colliding with a ship in their coastal habitats, Nowacek and his colleagues tagged eight whales with an electronic data-gathering device known as a DTAG. To do this, the researchers stealthily inched their boat toward right whales that they spotted lolling on the surface. With a 40-foot pole, the scientists then stuck a DTAG with suction cups on an animal’s back.
Once in place, each of the 18-inch-long, torpedo-shaped tags records such details as how deep the whale goes, water temperature, underwater sounds, and how much the animal pitches forward or back or rolls side to side. Hours later, the suction cups slip off and the radio-emitting tag bobs to the surface. Scientists then home in on the tag and retrieve its trove of data.
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From data collected during dives, Nowacek noticed regular up-and-down wavers in the pitch of a whale’s body as the dive deepened. These data indicate that the whale is swimming as it dives, he says. The right whales were still swimming at depths beyond 40 m, where seals, dolphins, and blue whales can simply sink, Nowacek says. However, on the way back up, the right whales save energy by gliding much of the way.
During that slow rise, the whales don’t have much maneuverability and could easily get smacked by big ships, worries Frank E. Fish of West Chester (Pa.) University, who has studied aquatic locomotion.
Of the identified causes of death among the highly imperiled North Atlantic right whales, ship collisions cause the largest share–about 30 percent, says whale biologist Philip Hamilton of the New England Aquarium in Boston. He helps keep a database of all known North Atlantic right whales, and his best guess is that the species now consists of only 325 individuals. “It’s a sad species to study,” he says.