Delegates at the International Whaling Commission (IWC) meeting in London this week will hear about the recovery of North Atlantic humpback whales. But the same research also reveals scars left by the overexploitation of humpbacks over the past two centuries, which suggest that current IWC whaling-quota assessments may be oversimplified.

Twenty years of research have shown that the North Atlantic humpback population has almost returned to levels before whaling began in the 17th century. Estimates suggest that there are now around 11,000 humpbacks cruising in the North Atlantic. Before whaling, the population is estimated to have been around 15,000 or more.

"Assuming continued population growth, they're indisputably doing well," says Phil Clapham of the Northeast Fisheries Science Center in Woods Hole, Massachussetts. Clapham's humpback research will be presented to the IWC this week.

Humpback whales - especially those in the North Atlantic - suffered the most at the hands of 19th- and 20th-century whaling fleets. "They are slow and inquisitive animals, so they really got nailed," says whale biologist Tony Martin of the British Antarctic Survey in Cambridge. The humpback's gratifying bounce-back is "more than anyone expected," he adds.

The character traits that made humpbacks so easy to exploit also make them easy to study. Between 1992 and 1993, Clapham and colleagues from seven other North Atlantic nations carried out a comprehensive census of humpbacks, the Years of the North Atlantic Humpback (YONAH). Taking tissue samples and tail photographs, they collected physical and genetic profiles of over 3,000 whales in the North Atlantic.

Although this and other work suggests that humpbacks are pulling through, it also indicates that exploitation may have permanently scarred the mammal's populations.

Genetic profiles reveal that certain humpbacks stick together, almost like tribes, preferring discrete feeding grounds. Humpback calves follow their mothers from breeding grounds back to feeding grounds. "That fidelity has been going on for long enough to show up in the genetic structure of the population," says Clapham.

North Atlantic humpbacks are known to feed in specific areas: the gulf of Maine, Newfoundland and Labrador, Western Greenland, Iceland and Norway. It was thought these whales mixed freely in one large breeding area in the Caribbean but the new genetic data suggest that there could be at least one other winter breeding ground.

"Once you start looking under the skin there's more fine-scale diversity in whale populations than we ever imagined," says Martin.

This may explain why few humpbacks are now found in traditional winter breeding grounds in the Cape Verdi islands off West Africa, or in the Southern and Eastern Caribbean - prime hunting grounds for 19th and early 20th century whalers. Certain populations may have suffered more at the hands of whalers than others, Clapham suspects.

If one feeding/breeding population is lost to whaling, says Clapham, "you would not get repopulation by migration."

Females from one breeding ground have occasionally been seen following males to other breeding grounds, so "there is a mechanism where a heavily depleted breeding stock could be replaced," says Martin. "But it would take a long time". This could have implications for managing humpbacks and other hunted or soon-to-be-hunted whale species.

The models used to calculate the IWC's comprehensive assessments - formal evaluations of whale populations required by the commission - which ultimately dictate catch quotas for certain species, do not account for discrete breeding groups of whales. "The models are very likely too simplistic," Clapham warns.