What is a blue whale? The blue whale (Balaenoptera musculus) is the largest living animal on Earth. Although the average length is 25 to 26 metres, females can reach more than 30 metres and weigh more than 160 tonnes at the end of the feeding season. Most blue whales seen off WA are probably a subspecies known as the pygmy blue (Balaenoptera musculus brevicauda) which occur only in the southern hemisphere and are most abundant in the Indian Ocean. However, the ‘pygmy’ blue is only marginally shorter than the true blue whale, at 21 to 24 metres long.

What do they look like? The huge size, mottled bluish-grey colour and small stubby dorsal fin well back on the body distinguish blue whales from other whales. When viewed from the air, their streamlined, slender shape is obvious. They also have a broad, flat U-shaped head, topped with a central ridge in front of the blowholes, and slender pointed flippers. Their blow is vertical and may be nine metres high. The large and notched tail flukes are sometimes raised when they dive.

Blue whale, Image by Kate Fitzgerald'
Pygmy blue whale image - courtesy of Kate Fitzgerald

Where do they live? Blue whales are animals that live mainly in the open ocean. Blue whales are seen near the WA coast in Ngari Capes Marine Park (between Busselton and Augusta), which is believed to be a breeding area for the species, and offshore from Rottnest Island.

What they eat and how: The species feeds principally on dense swarms of krill (tiny shrimp-like crustaceans) taken near the surface and probably do not dive deeply. Most feeding occurs in the Antarctic.

Threats: The population has been severely depleted by whaling, with nearly 30,000 caught in one season alone, The southern hemisphere population is estimated at less than 20,000, representing only a fraction of what it was in pre-whaling days. They are still threatened by illegal whaling by fleets primarily targeting other species, by defence operations that might interfere with their ability to echolocate, collision with ships, entanglement in fishing gear and pollution causing toxic chemicals to accumulate in body tissues.

Behaviour: Blue whales cover thousands of kilometres each year, migrating between subtropical or tropical waters, where they give birth in winter, and polar regions, which are the most productive areas for feeding, in summer. Blue whales may live an impressive 80 years or more. Sharks and killer whales may attack and kill young, wounded or diseased animals. They make low frequency moans, pulses and buzzes. Ultrasonic clicks are probably used for echolocation.

Breeding and caring for young: Calves are about seven metres long and 2.5 tonnes at birth. By the time they are weaned, seven or eight months later, they are almost 15 metres long. They are usually solitary or seen in pairs, but may be found in larger groups in feeding areas.

Conservation status: True blue whales are endangered. The subspecies has not recovered significantly since protection, with less than 1000 animals estimated to exist in the southern hemisphere. This compares with the pre-whaling population, which is estimated to have been greater than 150,000. There are an estimated 6000 whales of the pygmy blue subspecies.

How you can protect the blue whale: Follow the whale watchers code. Boats should not approach closer than 100 metres to a whale, a vessel should not separate a group or mother and calf and, if you are in the water and a whale approaches, you must stay at least 30 metres from the whale. If you should spot a whale entangled in rope or fishing equipment call the Wildcare Helpline so the department’s specially trained whale and dolphin disentanglement team can help the animal.