Growing up to 27 metres long and weighing as much as 120 tonnes, the slender body of the fin whale is capable of speeds of up to 37km/h, with brief bursts of up to about 47km/h which led to its nickname, ‘greyhound of the sea'. The upper half of the body is dark grey or brownish, while the belly is white. The head has a single medial ridge and has distinctive asymmetrical markings: on the right side the lower lip, mouth cavity and baleen plates are white, whereas on the left these features are dark. The fin whale has a small, falcate dorsal fin and slightly concave trailing edges to its flukes, which are rarely raised out of the water.
The fin whale is the world's second largest animal after the blue whale. They can dive to depths of 230metres and make low-frequency noises (infrasound) which can be as loud as 188 decibels. These sounds cannot be heard by humans, but can be detected by other fin whales up to 850km away. An individual fin whale is identified by the pattern of light-coloured chevrons on its back as well as by the size and shape of its dorsal fin. Records of females have been found with multiple foetuses, but it is unlikely that more than one would survive. The oldest specimen captured in Antarctica was 111 years old. Fin whales have also been known to mate with blue whales and to produce first generation offspring.
The fin whale feeds on animal plankton (including krill) and schooling fishes, lunging to take in great quantities of water and prey, then filtering the water out through its baleen plates leaving the prey trapped inside its mouth. When a fin whale eats it often turns on its side with the right side facing downward; in this position the lighter head colouration makes it less visible to prey. The fin whale generally travels alone or in pairs, and rarely breaches or spyhops. Its exceptionally large size, asymmetrical head colouration and small dorsal fin are probably the best identifying features. Overall range and distribution is not well studied but they are known to inhabit primarily pelagic habitats of both northern and southern hemispheres. Most populations are thought to be migratory whilst several resident populations are known to exist in the Gulf of California, East China Sea and the Mediterranean.
Fin whales, being much faster than many other whale species, were not hunted to a great extent until fast catcher boats and the explosive harpoon were developed as part of commercial whaling. Prior to this time, fin whales were one of the most abundant cetaceans found in the oceans. Hunting practices severely depleted natural populations. Currently, small 'aboriginal subsistence' hunts are conducted off Greenland. Japan and Iceland have resumed hunting the fin whale, and intend to increase takes in the near future. In the summer of 2009 Iceland took 125 fin whales. Threats to fin whales include entanglement in fishing gear, vessel strikes (they are the most commonly struck whale worldwide), commercial whaling, hunting by indigenous people and environmental change including noise and chemical pollution. The fin whale is categorised as Endangered by the IUCN (2008).