Species fact file

Common names: blue whale, sulphur-bottom, Sibbald's rorqual

Icelandic name: steypireyður

Scientific name: Balaenoptera musculus

Family: Balaenopteriidae (rorquals)

Max length: 27m (males), 32.6m (females) in Antarctica, Northern Hemisphere blue whales are smaller

Distribution: All oceans, predominantly offshore

Blue whales are the largest known animal to have lived on Earth. Although they were previously abundant worldwide, blue whale populations were greatly reduced during the peak of commercial whaling and they have still not fully recovered. The IUCN Red List considers blue whales to be Endangered.

Blue whales are divided into several subspecies. Antarctic blue whales are the largest of them all. It is thought that, in the past, there were more Antarctic blue whales than all the other subspecies combined – now, an estimated 3,000 remain. They are the only blue whale subspecies listed as Critically Endangered by the IUCN. From the beginning of the 20th Century, whaling was particularly intensive in the Southern Ocean which is why these populations were so severely depleted. Fortunately, Antarctic blue whales do now appear to be making a slow recovery.

The pygmy blue whale is the smallest subspecies. They are found in parts of the Southern Atlantic and Pacific Oceans, as well as the Indian Ocean, where its range overlaps with the Northern Indian Ocean blue whale. A more recently identified subspecies, the Chilean blue whale, has a relatively restricted distribution and is only found in the south-eastern Pacific.

Here in Iceland, we have the northern blue whale. This subspecies is found in the North Atlantic and North Pacific, with whales in the Atlantic typically reaching slightly larger body lengths than those in the Pacific.

Blue whales have a unique slate-grey colouring which makes them quite easy to distinguish from other baleen whales. The mottled patches on their back can be used for photo identification, and several catalogues have been compiled using this method in different regions around the world. They appear blue underwater, hence their common name, but sometimes look slightly yellow due to a layer of microscopic algae called diatoms that grow in cold water. This yellow tint gave the blue whale another name: sulphur-bottom. Although formerly a common moniker – in fact, Herman Melville used it in Moby Dick – this name has now fallen out of usage.

The usually grey skin of this blue whale is coloured yellow-brown by a layer of diatoms, microscopic algae. Photo by Allan Hopkins.

The blow of a blue whale is very tall – 10-12m high – and broad, which can be visible for several kilometres on clear days. They produce some of the loudest and lowest vocalisations in nature which, at around 10-20 Hz, are so low they are outside the range of human hearing. These low-frequency sounds have the potential to span whole ocean basins – blue whales can communicate with one another over vast distances!

Blue whales are migratory, but not all migration routes are well documented. The blue whales that frequent the waters around Iceland are part of a larger population that inhabits the whole north-eastern Atlantic (NEA). The photo ID catalogue for the NEA consists of nearly 600 whales. Blue whales in Iceland are known to migrate from places such as the Azores and the northwest coast of Africa, but there is still limited data on NEA migration routes. Photo ID catalogues and analysis of blue whale vocalisations suggest that the NEA and NWA blue whale populations are quite separate from one another.

Interbreeding with fin whales is not uncommon, and has been particularly well-documented in the NEA. The genetic distance between these two species is similar to that between humans and gorillas – their most recent common ancestor was about 3.5 million years ago – but despite this, hybrids of blue and fin whales are fertile and may be able to reproduce with either parent species.

Blue whales eat almost nothing but krill. Krill are tiny, shrimp-like crustaceans that aggregate in huge swarms – with thousands of individuals packed into a cubic metre, a single swarm can consist of hundreds of tonnes of krill. You may think it strange that the largest animal on earth survives entirely on a diet of such miniscule plankton, but they are able to do so because krill swarm in such densities. In fact, if all the krill on the planet were spaced out evenly throughout the oceans, whales would have to swim at 900 km/h to get enough food!

Here you can clearly see the ventral pleats on the underside of this blue whale’s mouth. Photo by Special Tours.

The ventral pleats, characteristic of all rorquals, expand when blue whales are feeding and allow them to engulf over 100 tonnes of water in a single mouthful. A recent study found that blue whales in the North Pacific probably eat between 10 and 22 tonnes of krill per day.

Like other rorquals, blue whales are lunge feeders. This is a very energetic method of feeding. Blue whales can lunge feed at the surface or sometimes at depths of 100m or more. Lunge feeding is so energetically demanding that it can seriously limit the amount of time blue whales stay underwater for; in theory, they should be able to dive for more than 30 minutes, but the average dive duration of a blue whale is between 8 and 15 minutes. Due to their energetic hunting techniques, blue whales have to carefully manage their oxygen consumption – during a dive their heart rate can drop to as low as 1 beat per minute!

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Hybridisation between blue and fin whales in the north-eastern Atlantic

How blue whales adjust their heart rate while diving and lunge feeding