Most humpback whales make exceptionally long journeys every year between their feeding and breeding sites. Humpbacks can travel up to 8 km/h but during their long migration journey they average only 1.6km/h, resting and socialising along the way.
Because seasons are reversed on either side of the equator, northern and southern hemisphere populations of humpbacks probably never meet. Those in the north travel towards their breeding grounds in tropical waters as those in the south are traveling towards the pole to feed, and vice versa.
When a whale surfaces, the simple act of exhaling results in a ‘blow’ of spray and air that shoots up to four metres in the air as it empties 90 per cent of the contents of its lungs in less than a second. The whale’s lung capacity is about the same volume as a small car.
Then, when it breathes in and dives, it can stay submerged for up to 30 minute
Sometime between July and August and somewhere in the protected waters of Queensland’s Great Barrier Reef, a baby humpback whale is born (it’s a girl!). She has been carried by her mother for the last 11 or 12 months. After a one hour labour, the young whale emerges tail first with its long pectoral fins folded forward—five metres long and weighing between one and two tonnes. The mother supports the new calf and lifts it to the surface for its first breath. At birth, the young whale consumes 40 kilograms of milk a day and gains up to one kilogram in weight every hour. In the first week, it starts to develop the pattern of markings that will distinguish it from other whales.
By the end of August, mother and calf turn south and begin the 4,500 kilometre migration to the Antarctic. Along the way the calf will stay close to its mother, suckling up to 40 times a day. The calf also receives her first lessons in navigating the route that she will retrace the following March.
On the way south the mother and her calf are likely to enter Hervey Bay for a few days. The sheltered waters of the Bay are visited by many of the whales on their southern migration between August and October. Here they remain for up to a week before continuing south. The adults arrive here first in August, with some still actively courting and mating. They are followed by the juveniles, and then the mothers and calves arrive between mid-September and October, sometimes escorted by another whale. By this time the calf is between four and six weeks old and taking around 500 litres of milk a day. When not feeding she is learning: copying her mother’s behaviour, breaching, and fin and tail slapping.
By November, the humpback whales have reached the Southern Ocean. First to arrive are the pregnant females, followed by the immature whales, and the mature males and ‘resting females’–mature females that aren’t pregnant or suckling. The final group to return is the mothers and calves.
Here the whales spend the spring and summer feeding on the immense populations of krill that build up in response to blooms of phytoplankton. An adult whale can eat an estimated 2,000 kilograms of krill a day. There may be up to 35 kilograms of krill in a cubic metre of water.
The calf is now five to six months old and is starting to socialise with other whales and feed on krill. It will still stay near its mother and will continue to suckle from her for a few more months.
By the end of February the whales have laid down the blubber they will live off for the next nine months—if they take part in the northerly migration (some whales don’t migrate each year). Of the whales that do migrate, all of the mothers and their six to seven metre long calves leave first. A couple of weeks later the juveniles leave with the mature males. Resting females and males follow them after another two or three weeks. The pregnant females stay and feed for another two weeks before leaving in April.
The whales move north in loose groups, covering about 850 nautical miles a month. The mature males start singing to attract a mate. The immature whales along with last year’s mothers and calves reach the tropical waters first around June or July. By now the calf is 8 metres long and has completed its first round trip to where it was born the previous year. At almost a year old, she is fully weaned. She will soon leave her mother, spending more time with other immature young until she becomes a mature adult. Whales become mature at between four and six years of age.
The mature males and females without young then arrive and courting and mating takes place between July and September.
The pregnant females are the last to arrive and give birth sometime between July and August. Occasionally humpback whales give birth anywhere along their migratory route, with an orphaned calf being found near Sydney in August 2008. As the whale population increases, so will the number of orphaned calves that are found.
The calf born the year before may now live for 50 years—that means she would swim up to 500,000 kilometres during her lifetime.