Balaenoptera acutorostrata

Balaenoptera acutorostrata

The minke whale or lesser rorqual, (Balaenoptera acutorostrata Lacépède, 1804) is a marine mammal belonging to the Balaenopteridae family.

Systematics –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Phylum Chordata,
Mammalia class,
Order Cetacea,
Suborder Mysticeti,
Family Balaenopteridae,
Genus Balaenoptera,
Species B. acute rostrata.
The terms are synonyms:
– Balaena gibbosa Cope, 1868;
– Balaena microcephala Tomilin, 1957;
– Balaena minima Rapp, 1837;
– Balaena minima subsp. borealis Knox, 1838;
– Balaena rostrata Fabricius, 1780;
– Balaenoptera acutorostrata subsp. thalmaha Deraniyagala, 1963;
– Balaenoptera acuturostrata Lacépède, 1804;
– Balaenoptera eschrichtius Rasch, 1845;
– Balaenoptera microcephala Gray, 1850;
– Balaenoptera rostrata (Fabricius, 1780);
– Balaenoptera thalmaha;
– Balaenoptera thalmaha subsp. thalmaha;
– Pterobalaena acutorostrata (Lacépède, 1804);
– Pterobalaena minor Eschricht, 1849;
– Pterobalaena minor subsp. bergensis Eschricht, 1849;
– Pterobalaena minor subsp. groenlandica Eschricht, 1849;
– Pterobalaena nana Barkow, 1862;
– Pterobalaena nana subsp. pentadactyla Barkow, 1862;
– Pterobalaena nana subsp. tetradactyla Barkow, 1862;
– Pterobalaena pentadactyla Flower, 1865;
– Rorqualus minor Hamilton, 1837;
– Rorqualus rostratus (Fabricius, 1780);
– Sibbaldius mondinii Capellini, 1877.
Within this species the following subspecies are recognised:
– Balaenoptera acutorostrata subsp. acutorostrata Lacépède, 1804;
– Balaenoptera acutorostrata subsp. scammoni Deméré, 1986.

Geographic Distribution and Habitat –
The Balaenoptera acutorostrata is one of the two smallest and most common species of the family of the Balaenopteridae present throughout the northern hemisphere, but probably with discontinuous distribution, being rarer in tropical waters than in colder waters.
This cetacean has a disjointed distribution. In the North Atlantic, they are found as far north as Baffin Bay, Svalbard and Nova Zembla and as far south as New Jersey (USA) and the North Sea during summer. There are some records in Hudson Bay. They have been sighted on Madeira and are seen all year round in the Canary Islands. There are occasional sightings and strandings in Spain and Portugal, Western Sahara, Mauritania and Senegal. It is rare in the Azores and vagrant in the Gulf of Mexico and in the Mediterranean Sea, with some reports also in the Black Sea.
It therefore frequents the following ecozones: Antarctic ecozone, Neotropical ecozone, Palearctic ecozone, Nearctic ecozone, Australasian ecozone, Indo-Malay ecozone and Afrotropical ecozone.
Its distribution habitat sees it more concentrated at high latitudes in summer and at lower latitudes in winter, but migrations vary from year to year. It seems that some populations are sedentary and according to some indications it seems that in certain areas some individuals have their own territory. It often enters estuaries, bays and inlets and, in summer, it sometimes feeds around islets and capes. Sometimes it becomes trapped in small areas of open water.

Description –
The Balaenoptera acutorostrata has a rather variable aspect for which some scholars recognize more than 2 subspecies (3 or even 4). It measures between 7 and 9.8 meters in length and weighs between 5 and 10 tons. On average, females are about 0.5 meters larger than males.
The body is compact, with a pointed, narrow and triangular head, with a keel or crest characteristic of minke whales.
Coloration is dark on the head, back and dorsal region of the caudal fin, white ventrally and on the underside of the pectoral fins, and gray on the sides, caudal peduncle and upper part of the pectoral fins.
The pectoral fins are thin and pointed. The dorsal fin is tall and falcate, located in the rear third of the back. The caudal fin has long thin distal lobes.
It has a white patch on the pectoral fins and pale bands in the shape of an inverted “V” behind the head and breathing holes.
It also has 30 to 70 throat folds extending to the tip of the pectoral fins. It has 231 to 360 wattles on each upper jaw, yellowish-white to gray, up to 27 cm long.

Biology –
The Balaenoptera acutorostrata mates and gives birth to the newborns in the warm waters of both hemispheres, where these cetaceans migrate from the poles.
The gestation period lasts about 10 months.
At birth, the young whales measure about 2.5 meters and at less than a year of age, when they migrate with their mother towards the food-rich polar waters, they are weaned.

Ecological Role –
The Balaenoptera acutorostrata is a difficult cetacean to approach but it may happen that some curious individuals go into areas where there are boats, approaching without warning.
It rarely swims bow, but it does happen to swim close to a boat for a considerable distance.
Movements under water are unpredictable, it can vanish without a trace.
It is quite fast swimming (it is the most common and fastest of all Mysticeti) with the typical dive sequence consisting of 5-8 breaths at intervals of less than a minute, followed by a long dive lasting from 3 at 8 minutes. When moving, he usually breathes 1 or 2 times between dives.
As for its feeding habits, it is sometimes seen feeding on the surface under a group of seabirds.
It mainly feeds on fish belonging to the Salmonidae, Clupeidae and Gadidae families.
This cetacean is the only Mysticeti hunted commercially.
In regards to its ecological status it was recently assessed for the IUCN Red List of Threatened Species in 2008, where it is listed as Least Concern.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Facilidad Global de Información sobre Biodiversidad.
– Gordon Corbet, Denys Ovenden, 2012. Guide to mammals of Europe. Franco Muzzio Publisher.
– John Woodward, Kim Dennis-Bryan, 2018. The Great Encyclopedia of Animals. Gribaudo Publisher.

Photo source:
– https://ala-images.s3.ap-southeast-2.amazonaws.com/store/4/a/3/b/34b848c5-84ec-4035-9451-36331ecfb3a4/thumbnail_large