Humpback whales one of the largest mammals in the world. Adult humpback whales are about 15 meters long and can weigh 40 tonnes. That's about as much as 30 cars in total!
Humpback whales undertake the longest migration of any mammal on Earth and therefore need a lot of energy for the long journey. During migration, a whale generally only feeds at high latitudes during summer and into early winter, but must survive the rest of the year on their fat stores. Budgeting their time and energy is therefore crucial.
Researchers from UiT The Arctic University of Norway followed the journey of a whale named ‘Theresia’ for 18.000 km. And they actually discovered that Theresia was pregnant. They tagged the female humpback whale with a satellite transmitter, and followed her for a whole year.
A satellite transmitter was attached to the whale's skin and sent signals up to satellites in space every time the whale reached the surface. This allowed the researchers to pin point where in the world the whale was at any given time.
– This is the first time researchers have been able to document how a whale varies its swim speed throughout the year to adhere to their annual schedules, and how a mother whale may adjust her behavior when traveling with a calf, says PhD-student Lisa Elena Kettemer from UiT.
She is currently doing a PhD on humpback whales and their migration patterns.
Theresia started the year with a stop-over herring feeding frenzy off the coast of Northern Norway, where she was tagged.
A pregnant whale probably eats 2-3% of its own body weight per day. If Theresia weighed 30 tonnes, she would then have to eat between 600 and 900 kilos of herring a day. That's as much as 4,500 herring per day!
– Pregnant whales need extra energy both for the long journey and to nurse the newborn baby whale they give birth to in tropical waters. Therefore, they eat extra food and are the last to leave the eating areas outside northern Norway in the winter, says Professor Audun Rikardsen.
He leads the research project on these whales.
Because of the unusual herring feeding in northern Norway and the long journey, the researchers wanted to know how fast Theresia swam and when she would arrive at the mating and birthing grounds.
When Theresia started her journey southwards, the researchers noticed that she was swimming very fast. They think she did this to make up for the extra time she spent grazing on herring in northern Norway.
– Because swimming faster costs more energy, we wondered if she could make this long journey and still be back in time for the feeding season in the Arctic summer, says Kettemer.
Suddenly, the whale slowed down in the middle of the Atlantic.
– Because Theresia suddenly slowed down, we think that she gave birth before reaching the warm and sheltered waters of the breeding grounds - in the middle of the North Atlantic, Kettemer says.
After the birth, she swam slower before reaching the warm and sheltered waters of the mating area in the Caribbean.
– She then had a small baby whale in tow, the researcher says.
Kettemer explains that Theresia and her calf spent fewer days resting than they really needed. Mother whales elsewhere use coastal areas, close to land, to rest and nurse their calves.
– This is the first time scientists have found out that a whale can vary its swimming speed throughout the year to follow its schedule, and how a mother whale can adapt when traveling with a calf, says Kettemer.
Whales like Theresia traverse the Northeast Atlantic, far from any coastline.
This means that most of the trip crosses the open ocean, an area called the ‘high seas’, an area with limited scientific knowledge. The high seas area represents two-thirds of the ocean surface and is currently not protected or managed, because it does not fall under the jurisdiction of any government.
– A treaty to protect high seas biodiversity in the future would ensure protection and sustainable management of both whales and other animals that spend much of their time in these remote areas, Kettemer says.