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Basking sharks may be among the largest fish in the world at more than 30 ft long, but they are hardly monsters of the deep.

July 15, 2006 12:00 AM IST

4 min read

Basking sharks may be among the largest fish in the world at more than 30 ft long, but they are hardly monsters of the deep. They spend much of their time slowly swimming in coastal areas around the world, feeding on plankton. Their slow movements make them an easy target. They have been hunted for their liver oil, fins, cartilage and meat, although hunting them is now banned in some countries. They are also very slow reproducers: it takes one to three years of gestation to produce a few pups. Because of all that, they are listed as vulnerable by the World Conservation Union. A study by A. Rus Hoelzel of Durham University in England and colleagues, published online last month by the journal Biology Letters, shows the sharks have low genetic diversity. The researchers analysed mitochondrial DNA from 62 tissue samples and found little variation among sharks from all parts of the world. This was unusual; often populations of the same species from different oceans will vary greatly. As to what might have caused the diversity bottleneck, the researchers suggest that a warming period might have disrupted the sharks8217; food source enough to cause a major die-off.

When the world wobbles

Late last November, as a big low-pressure system built over Europe and Asia and high pressure settled in over the Pacific and Atlantic Oceans, the shifts in the atmosphere caused the earth to jiggle ever so slightly. As a result, the North Pole and its southern counterpart moved about four inches by one measure. There are several ways to define the poles. Scientists have long known that as the atmosphere shifts, it influences the earth8217;s rotation. There were well-known, regularly occurring wobbles in the earth8217;s rotation that could shift the poles 30 ft over a year or more. These shifts blocked the detection of subtler, quicker movements caused by day-to-day changes in the atmosphere and the oceans. Now, these small shifts are being measured by institutions devoted to tracking the planet8217;s behavior, including the earth orientation department of the United States Naval Observatory and the 8220;time, earth rotation and space geodesy section8221; of the Royal Observatory of Belgium.Experts at the Belgian observatory and the Paris Observatory found the November polar shift and a series of other little loops by looking particularly closely at a period from last November through February, when two of the larger regular wobbles in the axis canceled each other out. They reported their analysis in the July 1 issue of Geophysical Research Letters.

Warm times down south

Scientists have discovered the remnants of elephant seal colonies in Antarctica near the Ross Ice Shelf, implying that the region8217;s climate was warmer then than it is today. The finding may also have some implications for the future of the ice shelf itself. Breeding grounds of southern elephant seals currently are found well north of Antarctica. But Brenda L. Hall of the University of Maine and colleagues found seal skin, hair and other remains at several sites along the coast of Victoria Land on the Ross Sea.

The finding, published last week in The Proceedings of the National Academy of Sciences, demonstrates that seals used to breed and molt there. Radiocarbon analysis dated the remains from about 250 years ago to more than 6,000 years ago. In particular, the seals flourished from 1,100 to 2,300 years ago, indicating a previously unrecognized period of unusual warmth in the region. Many scientists are concerned that as the climate continues to warm, huge expanses of ice like the Ross Ice Shelf will thin and eventually disintegrate, even within this century. But if the new study is correct, the Ross shelf apparently survived very warm conditions more than 1,000 years ago. 8212;NYT

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