Blue Whale

Physical Description

The Blue Whale is the largest animal, possibly the largest animal, to ever inhabit the earth. Its body is long, somewhat tapered and streamlined, with the head making up less than one-fourth of its total body length. The upper part of the head is very broad and flat and almost U-shaped, with a single ridge that extends just forward of the blowholes to the tip of the snout. The body is smooth and relatively free of parasites, but a few barnacles attach themselves to the edge of the fluke and occasionally to the tips of the flippers and to the dorsal fin. There are 55-68 ventral grooves or pleats extending from the lower jaw to near the navel.

Length & Weight

The longest blue whale ever recorded was a 33 metre adult female caught by whalers in Antartica. Today, blue whales in the Southern Hemisphere reach lengths of 27-30 metres, but those in the Northern Hemisphere are smaller (23 to 24.5 metres). Blue whales can weigh over 90 tonne and females are larger than males of the same age, the largest perhaps weighing as much as 135 tonne.

Newborn calves weigh 2,700-3,600kg.

Mating and Breeding

Blue whales reach sexual maturity between the ages of 6-10 years, or when males average about 23 metres and females are about 24 metres. Calves are born at intervals of 2 to 3 years and gestation is about 12 months.

Calves usually nurse for 7 to 8 months and are weaned when they reach 16 metres in length. During the nursing period, calves consume almost 380 litres of the fat-rich mother’s milk each day, gain 90 kilograms a day (about 3.5 kg per hour) and grow 2.5 to 3.5cm in length a day.

Feeding

The blue whale is thought to feed almost exclusively on small, shrimp-like creatures called euphausiids or krill. During the summer feeding season the blue whale gorges itself, consuming an astonishing 3.6 tonne or more per day. This means it may eat up to 40 million krill in a day.

The blue whale is a baleen whale and during feeding large volumes of water and food can be taken into the mouth because the pleated grooves in the throat expand. As the mouth closes water is expelled through the baleen plates, which trap the food on the inside near the tongue to be swallowed.

Distribution and Migration

Blue whales can be found in all oceans of the world. they migrate to tropical-to-temperate waters during winter months to mate and give birth to calves. They can feed throughout their range, in polar, temperate, or even tropical waters. Blue whales typically travel either singly or in pairs, although sometimes more whales may be found within close range in areas high in krill concentrations.

Natural History

Though they are sometimes found singly or in small groups, it is more common to see blue whales in pairs. They are sometimes seen in larger groups and loosely defined concentrations of 50-60 have been observed. They are fast, strong swimmers, capable of reaching almost 50 kph when alarmed, but they usually cruise at about 20 kph.

Status

Because of their size and speed, blue whales were safe from early whalers, who could not pursue them in open boats with hand harpoons. The invention of the exploding harpoon gun and steam and diesel powered factory and catcher boats allowed the whaling industry to focus on blue whales after 1900. A single blue whale could yield up to 120 barrels of oil, and the blues were killed by the thousands. In 1931 over 23,000 were killed in one season. The IWC banned the taking of blue whales in 1966 but recovery has been extremely slow and only in the last few years have there been signs that the population is recovering. Pre-whaling estimates of the population were over 350,000, but up to 99% of blue whales were killed during commercial whaling. Presently, there are an estimated 5 – 10,000 blue whales in the Southern Hemisphere, and only around 3 – 4,000 in the Northern Hemisphere.

Acknowledgements
Australian Museum; American Cetacean Society; National Geographic Society; Department of the Environment, Water, Heritage and the Arts