World's rarest whale is too buoyant, researchers find

Being able to float too easily may be helping to drive the world's rarest whale to extinction, research has found.

Being able to float too easily may be helping to drive the world's rarest whale to extinction, research has found.

The remarkable buoyancy of the North Atlantic right whale, whose numbers are now down to 300, means it has more difficulty in diving to avoid large ships. Fatal ship collisions are taking a big toll on the tiny population, with at least 16 recorded in the past 30 years.

Right whales, which migrate up and down the waters off the east coast of America, from their calving grounds off Florida to their feeding grounds off Canada, were the first of the great whales to be hunted to the point of extinction in the 19th century. Even then their buoyancy caused them trouble: they were popular targets for whalers, who identified them as the "right" whales to hunt, because it was known that their carcasses floated when they were killed.

But although right whales have hardly been hunted commercially since the First World War, and have been a protected species since 1935, their numbers have been much slower to recover than those of other species such as humpback whales.

Two years ago, researchers at the Woods Hole Oceanographic Institution in Massachusetts found that their population was actually declining and they were on the road to extinction. Human-induced mortality was known to be a primary factor. Now other researchers from Woods Hole have documented exactly why right whales seem to be colliding with large vessels with such fatal frequency.

By fixing sensors to the animals' tails, they have shown that right whales actually have positive buoyancy, a pronounced tendency to float, whereas most marine mammals have negative buoyancy and ­ other things being equal ­ will sink.

Negative buoyancy means whales and seals can dive quickly, and can actually glide downwards into the depths, thus saving energy.

Right whales, however, have more difficulty, needing to use strong strokes of their flippers to go down. They can glide upwards on the return ascent, but are likely to lose manoeuvrability in doing so. Both factors may increase the risk of a ship strike, say the scientists, whose conclusions are reported in the current edition of Proceedings of the Royal Society.

"If a right whale at the surface dives to avoid an oncoming vessel, its buoyancy may slow this vertical avoidance response," the researchers say. "Buoyancy may also pose risks during ascent from a dive. After whales begin the passive component of their ascent, their ability to manoeuvre is likely to be reduced relative to when they are under power."

The scientists hope that the new data and further research will lead to the development of methods to help ships to avoid right whale collisions.