Boning up on bowhead habitats.

Boning up on bowhead habitats

For the bowhead whales, "life has notalways been zooplankton and cream," says Donald M. Schell. During the late 1800s, whalers almost drove the animals to extinction, and now some people are concerned that the bowheads may be threatened by the offshore oil industry's expansion into Beaufort Sea, north of Alaska and the Yukon Territory, where these whales summer.

In order to determine whether a whalespecies is threatened, Schell says, scientists must understand its life cycles and favorite habitats. Most of the information on bowhead feeding habits is based on their migration route: It has been assumed that the bowheads do most of their feeding in the Beaufort Sea and then live on their fat reserves as they migrate west and south through the Chukchi Sea and into the Bering Sea, where they spend the winter.

But now Schell and his colleagues saythey have found a more precise method for learning about bowhead feeding patterns and for age-dating the animals. And their preliminary results, presented at the recent San Francisco meeting of the American Geophysical Union and the American Society of Limnology and Oceanography, are challenging conventional thinking about the bowhead.

Schell and Norma Haubenstock, bothat the University of Alaska inFairbanks, have studied the levels of carbon and nitrogen isotopes contained in whale baleen, or the plates that grow from the roof of the animal's mouth. These plates -- which, like fingernails, are made of keratin -- fray on their inner edge into a fringe of coarse filaments. The filaments enable the whale to filter out food particles from the seawater.

The researchers have found that, alongthe length of a plate, there are cyclic changes in isotopic content that can be related to the whale's geographic movement. In particular, Schell and Ken Dunton at the University of Texas in Port Aransas reported at the San Francisco meeting on their measurements of the carbon-13 to carbon-12 ratio in zooplankton, which whales consume. This ratio increased as the researchers moved westward from the Beaufort Sea to the Bering Sea. As the whales migrate back and forth between the Beaufort and Bering seas each year, the isotopic content of their growing baleen records their diet and movement.

Schell also looked at how the carbon-14levels in baleen had changed with time. Scientists know the rate at which global carbon-14 levels have been falling in the environment after large amounts were released by nuclear weapons testing in the 1960s. The carbon-14 decrease along the baleen gave Schell and Haubenstock an independent way of determining the length of the carbon-13/carbon-12 cycles. With this method they confirmed that the cyclic isotopic changes were annual.

The isotopic shifts in the baleen, alongwith isotopic studies of whale muscle tissue, enabled Schell and Haubenstock to estimate where the bowheads were doing most of their feeding. Contrary to previous thinking, says Schell, the whales are getting about 60 percent of their food in the Bering Sea, and not from the Beaufort. Schell hopes that with further work comparing carbon and nitrogen isotope ratios, his group will be able to pinpoint where the whales spend their time.

"One of the most important aspects ofthis whole study is to find out where the animals' energy comes from at different times of the year," he says.

The annual isotopic shifts also enableSchell and Haubenstock to determine the age of the animals. Until now, scientists have estimated bowhead ages by looking at the size distribution of bowheads harvested by Eskimos and assuming that the largest numbers were 1 and 2 years old. With the baleen, however, the researchers have found that the whales are considerably older than they were reported to be in the literature. This technique, says Schell, provides marine biologists with a powerful tool for describing life histories of the bowheads, including their age of sexual maturation and their growth rates. It could also play a role in decisions about their management.

Schell plans to focus next on the isotopicvariations of the zooplankton and to determine how oceanographic conditions, including El Ninos, affect these variations. "This study has led us in so many directions so fast that it's been a full-time effort to keep up with it," he says.

Beyond its importance to the bowhead,Schell adds, the study demonstrates the use of stable isotope chemistry in ecological studies in general. "This use," he says," ... is the field of the future," as evidenced by the "rapid growth in the audience size of these stable isotope lectures" at meetings like the one in San Francisco.