Blue whale
Balaenoptera musculus

The measurements of the blue whale are known to all: up to 33 m for the largest Antarctic specimens, 100 to 150 tonnes for adults. But these raw figures mask remarkable biological mechanisms.

Grey-blue coloration and light spots

The skin is slate-grey, dotted with irregular light spots. Underwater, light diffusion in the superficial layers gives the animal an intense blue hue, the origin of the common name. This spot pattern is individual: it forms the basis of photo-identification used by researchers and citizen platforms such as Happywhale.

Baleen and krill feeding

The blue whale is a mysticete: it has no teeth but 300 to 400 pairs of black baleen plates, 50 to 100 cm long, that filter the water. The feeding technique is lunge feeding: the animal accelerates, opens its mouth, engulfs a colossal volume of water, then expels the water while retaining the krill (Euphausiacea spp.) against the baleen. An adult can ingest up to 4 tonnes of krill per day during intense feeding periods (GREMM, compiled data).

Heart, lungs and diving

The blue whale heart weighs about 180 kg and beats at 4 to 8 pulses per minute during a dive, versus 25 to 37 at the surface. This diving bradycardia reduces oxygen consumption. Dives rarely exceed 200 m depth because krill concentrates in superficial layers, but their duration can reach 20 minutes.

Infrasonic vocalizations

The vocalizations of the blue whale are among the most powerful in the animal kingdom: they reach 188 decibels and lie between 10 and 40 Hz, below the human hearing threshold. These infrasounds travel hundreds or even thousands of kilometres through the ocean. Their exact role remains debated: long-distance communication between individuals, locating reproductive partners, orientation, probably a combination of the three (Croll et al., 2002).

The taxonomy of the blue whale is often simplified in popular sources. Three subspecies are recognized, plus a population with distinct behavioural characteristics. This distinction has direct implications for actual numbers and conservation priorities.

True blue whale (B. m. musculus): North Atlantic and North Pacific

This is the nominal subspecies, the largest. It frequents the North Atlantic, notably the St. Lawrence, and the North Pacific, with concentrations off California and Mexico. Its global population is estimated at only a few thousand individuals (IUCN, 2018).

Pygmy blue whale (B. m. brevicauda): Indian Ocean and South Pacific

Slightly shorter (generally 20 to 24 m), it is distinguished by a proportionally longer tail and a more massive caudal region. It mainly frequents the Indian Ocean and the South Pacific. Its population is considered numerically larger than that of the nominal subspecies, but data remain fragmentary.

Antarctic blue whale (B. m. intermedia): austral waters

This is the largest subspecies and the one most affected by 20th-century industrial whaling. Austral waters once harboured hundreds of thousands of individuals. Current estimates indicate fewer than 2 000 individuals, making it the most threatened population (IUCN, 2018). Its recovery is extremely slow.

Sri Lanka population: partial sedentariness

The population frequenting the waters of Sri Lanka, notably off Mirissa, shows unusual behaviour: some individuals appear resident year-round, without the long polar migrations typical of the species. Local operators and researchers from the Sri Lanka Blue Whale Project document this partial sedentariness, which remains poorly explained and potentially linked to local krill availability.

On the water, identifying large rorquals requires method. Here are the criteria I use systematically, in the order they become visible on approach.

The blow: height, shape and verticality

This is often the first signal. The blue whale blow is very vertical, narrow and can exceed 9 m in height. It is visible from afar in calm weather. A short, diffuse or inclined blow points to another species. Verticality is the most reliable criterion at long range.

The dorsal fin: small, very rearward

The dorsal fin of the blue whale is small (about 30 cm high) and positioned in the last quarter of the body, very close to the tail. It appears only after the blow has dissipated, during descent for a dive. This is an essential discriminating criterion against the fin whale (Balaenoptera physalus), whose fin is larger and more forward.

The tail during diving

Unlike the sperm whale (Physeter macrocephalus), the blue whale rarely shows the fluke (tail) during diving. When it does, it is generally during deep dives, which remains rare in coastal feeding areas. The absence of a visible fluke is therefore an additional clue, but not absolute.

Comparative table

I have not observed the blue whale from Camaret, and for good reason: the species is absent from our Breton waters. The data below come from operator reports, GREMM publications and IUCN site sheets.

Which month to see a blue whale?

No site covers the entire year, but the calendars overlap. Here, month by month, are the 16 places from our profiles where the species is documented. The links lead to our detailed guides.

Why the Indian Ocean extends the season

Studies suggest that blue whales in the northern Indian Ocean (Oman, Maldives, Sri Lanka) could form a distinct resident population that does not follow the great migration cycle of the others. This is where we find our longest windows: eight months in Muscat, ten months in Sri Lanka. In Sri Lanka, the south coast (Mirissa, November to April) and the east coast (Trincomalee, March to August) take turns: by changing coasts, one covers November to August.

St. Lawrence (Quebec)

The St. Lawrence is one of the best-documented sites in the world for the species. According to the GREMM (Group for Research and Education on Marine Mammals), blue whales frequent the estuary from June to October, with a peak in August. The Tadoussac and Pointe-Noire area concentrates the observations. Canadian regulations require a minimum distance of 400 m for endangered species in this zone.

Azores

The geographical position of the Azores, at the crossroads of several Atlantic migration routes, makes it the most reliable European site. Local certified operators report regular sightings from March to June. Several companies adhere to the High Quality Whale Watching (HQWW) charter, which facilitates choosing a responsible provider.

Sri Lanka (Mirissa)

The waters off Mirissa concentrate blue whales from November to April. Field observers report high densities, but also concerning over-frequentation: several dozen boats can be around the same animal simultaneously. Local NGOs and Whale and Dolphin Conservation regularly alert on non-compliance with minimum distances (WDC, annual reports).

Baja California (Mexico)

The Sea of Cortez and the coastal waters of Baja California host blue whales from January to March, coming to feed on krill. Operators in La Paz and Loreto offer guided outings. The coastal concentration facilitates observations, but also exposes the animals to collision risks with maritime traffic.

Iceland and Antarctica

In Iceland, sightings are reported sporadically in the north and west of the island, especially in summer, but they remain rare and not guaranteed. In Antarctica, polar cruises sometimes allow observation of the subspecies B. m. intermedia, but weather conditions and Antarctic Treaty regulations impose significant logistical constraints.

The blue whale has benefited from international protection since 1966, when the International Whaling Commission (IWC) banned its commercial hunting. Fifty years later, the global population has not recovered its historical levels: the IUCN estimates it between 5 000 and 15 000 individuals (2018 assessment). It is no longer harpoons that concern researchers, but ship collisions, fishing gear, underwater noise and climate effects on krill.

Historical numbers and impact of industrial whaling (1900-1966)

Before industrial whaling, the global blue whale population is estimated at 200 000 to 300 000 individuals. Between 1900 and 1966, more than 360 000 animals were killed, mainly in the southern hemisphere (IWC data, compiled by IUCN). The Antarctic subspecies was the most affected, with annual takes exceeding 30 000 individuals in some years.

IUCN "Endangered" status: current figures

The IUCN has classified the blue whale as "Endangered" since 2018. The total global population is estimated between 10 000 and 25 000 individuals, all subspecies combined (IUCN, 2018). This figure represents less than 10 % of pre-industrial numbers. Recovery is slow: a female produces only one calf every 2 to 3 years, and sexual maturity is reached around 10 years.

Ship collisions: leading cause of anthropogenic mortality

Today, ship collisions are identified as the leading cause of anthropogenic mortality for the species (Redfern et al., 2013). Commercial shipping routes cross directly through feeding and migration areas in the North-East Pacific, Sri Lanka and the Mediterranean. Route displacement or speed reduction measures have been tested in California and the St. Lawrence, with encouraging but insufficient results.

Noise pollution, entanglement and climate change

Noise pollution generated by maritime traffic disrupts infrasonic vocalizations, potentially reducing long-distance communication capacity. Entanglement in fishing gear remains rare but documented. Climate change alters krill distribution, forcing animals to modify migration routes and expend more energy to find food (Hazen et al., 2013).

Observing a blue whale from a boat is a privilege that entails concrete responsibilities. The rules are not abstract recommendations: they respond to documented stress and flight behaviours in animals approached too closely.

Recommended minimum distances

The High Quality Whale Watching (HQWW) charter recommends a minimum distance of 100 m for large cetaceans. In Canada, in the St. Lawrence, regulations require 400 m for species listed as "endangered", including the blue whale (Species at Risk Act, Canada). In Sri Lanka, no binding national regulation existed yet in 2023, which explains the over-frequentation situations documented at Mirissa.

Behaviours to avoid

Frontal approach is particularly stressful: it cuts across the animal's trajectory and can provoke a sudden change of direction. Engine running in immediate proximity generates vibrations and underwater noise. Unregulated drones flying over the animal at low altitude disrupt breathing and surface behaviour. These three behaviours are explicitly discouraged by the HQWW charter and by Whale and Dolphin Conservation.

Choosing a responsible operator

Before boarding, I advise checking adherence to the HQWW charter or an equivalent label, asking whether a trained naturalist is present on board, and ensuring the operator communicates clearly on distances respected. A serious operator refuses to guarantee a sighting and does not pursue an animal that moves away.

Contributing to citizen science

Every clear photo of the dorsal fin or the spot pattern of a blue whale can be submitted to Happywhale or Obs-MAM for photo-identification. These platforms allow tracking of identified individuals over years and thousands of kilometres. This is a concrete contribution to research, accessible to any observer equipped with a camera with a minimum focal length equivalent of 300 mm.