Slightly more than 60 percent of the females had high progesterone levels in their blubber, indicating that they were pregnant, according to the new study, published today in the journal Royal Society Open Science. There were more pregnant whales in recent years than earlier, according to the research. Genetic analysis indicated gender and enabled the scientists to avoid counting the same whale twice.
Although some other whale species also appear to be rebounding in the southern oceans, humpbacks seem to be faring the best, Dr. Friedlaender said. It may have been easier for the humpbacks to recover than the bigger fin and blue whales because the humpbacks mature faster,have a short period between pregnancies, and have centralized breeding grounds. Fin and blue whales mate in the open ocean, making it harder for them to find a match, he said.
A moratorium on commercial whaling took effect in the early 1980s, though some whaling regulations had been imposed as early as the 1940s. Twelve countries signed a treaty in 1959 to protect the Antarctic Peninsula. Humpback whales south of the Equator are no longer considered endangered, though. A few northern populations remain on the National Oceanic and Atmospheric Administration’s list of endangered animals.
It is likely that a large portion of the animals alive today were born after the age of whale hunting ended, Dr. Friedlaender said.
So far, climate change around the Antarctic Peninsula has been beneficial for humpbacks, he said, providing about 80 more ice-free days per year when these whales that prefer open water can feed on abundant stocks of tiny shrimplike crustaceans called krill.
Whale researchers are concerned that this moment of health and easy access to food will be short-lived. Krill stock around Antarctica is being fished by some countries, and threatened by climate change.
Additionally, reduction in sea ice endangers krill, which feed on the underside of sea ice, said Sean Todd, the chair in marine sciences at the College of the Atlantic in Bar Harbor, Me., who was not involved in the new research.
Dr. Todd said his repeated trips to Antarctica have proven to him that krill are essential to life in the southern oceans. “You either eat krill, or you eat something that eats krill,” he said. “If krill stocks collapse, then there’ll be profound changes to that region.”
Dr. Todd said the new study confirmed the growing abundance of humpbacks that he had noticed on annual trips to Antarctica. When he first started going in the early 2000s, he would see a few humpbacks every trip in February and March. Now, he sees them as early as December, he said, “to the point that you can lose count of how many are around you.”
He said he had also seen a smaller but obvious rebound among fin and blue whales – the planet’s largest animals – as well as the Southern right whale, a cousin to the highly endangered Northern Right Whale.
“It’s a really stark contrast,” Dr. Todd said.