What is a fin whale?

The fin whale (Balaenoptera physalus) is the second largest animal on the planet, just after the blue whale. It is also the only large baleen whale that lives all year round in the Mediterranean, which makes it a major emblem of European marine wildlife. Nicknamed the “greyhound of the sea” for its exceptional streamlining, it combines enormous size with a remarkable elegance in the water.

A streamlined giant

Adult fin whales measure on average 18 to 20 metres long for a weight of 40 to 70 tonnes. Some exceptional specimens in the Antarctic have approached 27 metres. The body is extraordinarily slender, shaped for speed, hence the nickname “greyhound of the sea”. This streamlining allows it to reach bursts of 40 to 45 km/h in a sprint, despite its colossal mass.

Its most singular anatomical feature is the asymmetrical coloration of its lower jaw: the right side is white or very pale, the left side dark, almost black. This feature is unique among cetaceans. Hypotheses about its function vary: a hunting strategy (showing the pale side to drive prey in a given direction), camouflage at the surface, a social signal. No theory has been fully confirmed.

An impressive filtering system

Like all rorquals, it is a baleen whale that practises lunge feeding (engulfment feeding). It accelerates towards a shoal of prey with its mouth wide open, stretches its throat pleats (the longitudinal folds of its throat) like an accordion, and takes in up to 70,000 litres of water in a single mouthful. It then closes its jaws and forces the water out through its baleen, 260 to 480 pairs of keratin plates, retaining only the prey.

Its diet varies between regions. In the Mediterranean the fin whale feeds mainly on krill (Meganyctiphanes norvegica). In the North Atlantic it is more varied: krill, but also capelin, herring, mackerel and other small pelagic fish.

A Mediterranean resident

The fin whale is the emblem of the Pelagos Sanctuary, a large marine protected area of 87,500 km² created in 2002 by France, Italy and Monaco. A population estimated at 1,500 to 3,000 individuals lives all year round in the north-western Mediterranean, genetically distinct from the Atlantic population. It is one of the most remarkable features of the basin’s cetacean fauna.

The genetic isolation of this Mediterranean population makes it both a major case study and a particular vulnerability. Without regular genetic input from the Atlantic, the population depends on its own reproductive dynamics to maintain itself.

Behaviour and migrations

The fin whale is usually seen alone or in small groups of 2 to 7 individuals, though concentrations of several dozen can form on feeding grounds. It makes dives of 5 to 15 minutes, followed by series of breaths at the surface. Its blow, vertical and reaching 4 to 6 metres high, is visible from a distance and is often the first sign of its presence for observers.

Unlike other large whales (humpback, blue, grey), the fin whale does not make such a clear-cut migration between polar feeding grounds and tropical breeding grounds. Its movements are more diffuse, shaped by the availability of krill and fish shoals. This behavioural plasticity has allowed it to colonise many marine habitats.

Present-day threats

Decimated by industrial whaling in the 20^th century, the fin whale was one of the great victims of the whalers. The 1986 moratorium allowed a recovery to begin, but the species is still listed as “Vulnerable” by the IUCN, and the Mediterranean population as “Endangered”.

The main present-day threats include ship strikes: the Mediterranean being one of the most heavily crossed seas in the world, several dozen fin whales die there every year from collision; chemical pollution (PCBs, microplastics, which are particularly abundant in the Mediterranean); anthropogenic noise, which disturbs their communication; and bycatch in fishing gear. The shipping lanes laid out in the Pelagos Sanctuary are one of the concrete measures taken to reduce mortality from collisions.

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