In June or July several hundred humpback whales arrive in Tonga from their summer feeding areas in Antarctica. They spend the austral winter in Tonga, mating and bearing their young, before heading south again in October or November.
This annual migration is necessary because there's little food for whales in Tongan waters but the calves require warm seas to survive because they're poorly insulated at birth. During the first few months of their lives the baby whales grow at a rate of 45 kilograms a day. They're solely dependent on their mother's milk for sustenance, and by the end of the season a nursing female may have lost 25 percent of her body weight.
As soon as the offspring are ready in spring, the animals return to their summer home thousands of kilometers south to fatten up on a tiny plankton called krill. Pregnancy lasts 12 months, just long enough for the mother to put on adequate weight in Antarctica to have a child in Tonga.
While in Tonga the humpbacks engage in elaborate courtship displays and mating rituals that can last for hours. The males sing complex songs that have been studied and found to contain syllables and rhyming phrases.
During the displays the humpbacks often breach, and to see a 14.5-meter male rise from the sea, five-meter flippers flapping at his side, only to crash back on his side, is truly spectacular. A female chaperoning her calf is another favorite sight. Southern hemisphere humpbacks have a characteristic white belly, quite different from the black bellies of northern hemisphere humpbacks, and individuals are recognizable by their patterns.
Humpbacks prefer shallow waters close to shore, and are often curious about humans, characteristics that have worked to their disadvantage. From a population of around 100,000 in the 19th century, southern hemisphere humpback whales presently number only about 3,000, a drop of 97 percent. Subsistence shore whaling was only prohibited in Tonga in 1979 and the 10 whales previously taken each season did have an impact. However, it was the collapse of the Soviet Union in 1991 that contributed more to the whales' survival, since it put an end to illegal whaling from Soviet ships. Japanese whale boats continue the business even today in defiance of world opinion.