The blue whale (Balaenoptera musculus) rules the ocean. It’s the largest animal ever to have lived on earth (yes, larger than any dinosaur).  

The species can grow up to 30 metres long and weigh in at a whopping 180 tonnes. A blue whale’s tongue alone weighs as much as an elephant and its heart as much as a small car.  

It belongs to the baleen whale group and can be recognised as such by the plates of baleen (rather than teeth) suspended from the upper jaw, and the two blowholes on the upper body. Its dorsal fin is very small and is set far back on the body.

The blue whale has a long, slender body and it gets its name from its blue colouration. However, it only looks true blue underwater. When it surfaces, the colour is more of a mixed grey-blue. Its underbelly has a yellowish hue from the millions of microorganisms that take up residence on its skin. The blue whale is sometimes called the sulphur bottom whale because of this yellow tinge to its skin.

On average, blue whales live around 80-90 years. The oldest blue whale recorded was estimated (using ear plug analysis) to be 110 years old. Blue whales swim at an average of 8 kilometres an hour but can accelerate to more than 32 kilometres an hour when they’re agitated.  

Blue whales are the loudest animals on earth, reaching around 188 decibels. That’s far more than a jet engine’s sound (140 decibels). Scientists believe that in good conditions, blue whales can hear each other up to 1,600 kilometres away. They vocalise to communicate with other blue whales and along with their awesome hearing, to sonar-navigate the ocean depths.  

Blue whales prefer living in deep temperate cold waters. There are two reasons why – they have a significant layer of blubber which helps to keep them insulated and the majority of their food also likes to live in cold waters.  

Blue whales can be found in all of the world’s oceans. They’re generally more common in the Southern Hemisphere (Antarctica, Australian and New Zealand waters). There’s also a resident population in the northern Indian Ocean.  

In the North Atlantic Arctic, the blue whale can be spotted around Norway, Iceland, the Faroe Islands, in southern Greenland, and in southern Svalbard. They’ve also been occasionally spotted in North Atlantic waters west of Ireland and Scotland. In 2021, there was great excitement when a blue whale was spotted just off the coast of Co Galway, the first time in six years that the species had been sighted in Irish waters. You can read more about the sighting in this Irish Post article.

2021 also saw an unusual blue whale sighting on Spain’s Atlantic Coast, following on from previous identifications in 2017, 2018 and 2020. Blue whales were spotted off Galicia, where they hadn’t been seen in 40 years. The species was almost extinct in that region as a result of historic whale hunting. You can read more about the Spanish sighting in this Guardian article.  

Generally speaking, blue whales spend their summers feeding in cold waters and then migrate long distances to warmer waters (nearer the equator) for mating season.   

The Eastern North Pacific population of blue whales mostly feed off California from summer to autumn and then move north to colder waters off Oregon, Alaska and Washington State to continue feeding. During winter and spring, they migrate south to the waters of Mexico (mostly the Gulf of California) and the Costa Rica Thermal Dome.  

Blue whales are occasionally seen swimming in small groups but are more often found migrating alone or in pairs (particularly with offspring). The gestation period for a blue whale is 10-12 months and offspring are always born in warmer waters.  

Like other baleen whales, blue whales have expandable pleats (or baleen) which allow them to take in huge amounts of water and food. They can often be seen rapidly performing twists and turns with their entire body to locate tiny shrimplike animals (krill) and grab as much of it as possible.  

They sieve the water out through their pleats and then ingest the krill remaining in their mouths. It had been thought that blue whales eat around four to eight tonnes a day but new research suggests they could be eating around three times as much a year as previously estimated. The study in Nature magazine shows evidence that a blue whale in the eastern North Pacific might eat between 10 and 20 tonnes of food a day! 

The blue whale was driven to almost extinction by commercial whaling in the 1900s. A single blue whale, being the size that it is, yielded a lot of whale oil so it sadly comes as no surprise that whale hunters were keen to track them down and slaughter them. 

It’s estimated that between 1900 and the mid 1960s, some 360,000 blue whales were killed. 

The species has been protected from hunting by the International Whaling Commission since 1966 and there is some evidence to suggest that populations are recovering. It’s difficult to assess blue whale numbers because many populations appear to still be small and because the species is widely distributed in offshore waters.  

In the Southern Hemisphere, the pre-hunting population size was estimated to have been as many as 200,000–300,000 whales. After intensive hunting in the Antarctic region took its toll, the numbers dropped dramatically. Blue whales were estimated to number around 2,300 in 1998 and to be increasing between 2.4 – 8.4% each year.  

Globally, the species is listed as Endangered on the International Union for Conservation of Nature Red List with the Antarctic subspecies listed as Critically Endangered.   

Vessel strikes present a risk to blue whales, especially in areas where their movement overlaps with busy shipping lanes e.g. off the coast of California and Sri Lanka. Larger vessels and ships travelling at high speeds particularly pose a threat of injury or death to whales.

Blue whales can become entangled in fishing gear and either swim off with the gear attached or become anchored. If they swim off with gear attached, it can ultimately cause fatigue and compromised feeding ability.  

Accidental entanglement poses a greater threat to other whales and dolphins though than it does to blue whales. Their bigger size and strength helps them to break freely more easily than other species. Death from entanglement is more rare in blue whales though it’s not unheard of.  

As with other whale species, increasing underwater noise from vessels can negatively affect blue whales, changing their normal behaviour and prompting them to move away from important breeding or feeding areas.  

Climate change is affecting our ocean and the creatures that live within it. Commercial exploitation of krill and climate change affecting the distribution of kill in our ocean both negatively impact blue whales. More research is needed, though, to quantify exactly what the effects have been.

The only known natural predator of blue whales is the orca but due to the blue whale’s massive size and its ability to outswim other whales, it’s usually calves who are targeted for predation. Interestingly though, in January 2022, a report was published with scientists documenting the first known case of a pod of orcas killing an adult blue whale.