Sei whale

 

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Classification:

Population Mondiale

Description Et Histoire Naturelle

Physical Description
Sei whales (pronounced, "say") are the least known of the Family, Balaenopteridae, which includes the blue, Bryde’s, fin and minke whales.

Smaller than the fin whale and larger than the Bryde’s whale, the sei whale shares the same streamlined body shape and broad, flat, u-shaped rostrum as the other species. There is a noticeable longitudinal ridge from the blowhole to the tip of the rostrum.

Adult sei whales measure 12 - 16 meters (39.4 - 52.5 feet) in length. Females are typically larger than males. Calves are about 4.5 meters (15 feet) long at birth. The body is slate-gray above and lighter below, although there is individual variation. Some have light-colored oblong spots on their sides that are probably scars resulting from parasites.

The head is uniformly dark on both sides, unlike the asymmetrical coloring of the fin whale, and there is an area of grayish-white that is almost always confined to the ventral grooves on the throat. There may be a white or light-colored area on the flipper and a white streak may extend down the leading edge of the flipper from behind the eye.

The tall, sickle-shaped dorsal fin, located midway along the back, appears simultaneously with the blow when the animal surfaces.

Natural History
Little is known about the biology, ecology, and behavior of the sei whale. They are cosmopolitan in distribution and seem to favor deep, pelagic (open ocean), temperate to sub-polar waters.

Sei whale movements and distribution are not well documented and are often unpredictable during the year, and from year to year. Sei whales appear to be strongly migratory, spending the winters in warm, subtropical waters and migrating to temperate or polar seas to feed in the summer. They do not, however, penetrate as far into polar seas as other rorqual whales, usually not entering the ice zone.

Some segregation may occur during migration with pregnant females leading the way to and from the summer feeding grounds. The sei whale tends to travel alone or in groups of two to five, although greater numbers can be found on the feeding grounds. They often skim-feed on planktonic crustaceans such as copepods and krill at, or just below, the surface. Because of this feeding method, they are quite unobtrusive.

Sei whales do not tend to breach and do not raise their tail flukes before a deep dive. Instead of arching its back before a dive, a sei whale sinks quietly, leaving a series of swirls or "tracks" at the surface. Sei whales may show their rostrum as they surface but usually reveal little of their body above the water. They are the fastest swimming of the great whales, reaching bursts of over 20 knots.

Sei whales are thought to become sexually mature between six and eight years of age. Intervals between calving are believed to be about two to three years, with calves born during the winter after a gestation of 10.5 to 13 months. Calves are weaned between six and nine months. While these figures are generally accepted, further scientific studies may show that they are not accurate.

There are no current population estimates available for this species and their status is poorly known.

                                                            

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Situation De L'Espèce

Status
Today, the sei whale is listed as Endangered by the International Union for the Conservation of Nature and Natural Resources (IUCN) and is included on Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).

Threats to the Species

As with other fast swimming baleen whales, the sei whale was spared from early hunting because of its size, speed and deep-water habits. However, by the early 20th century, whaling technology had changed to include steam-powered vessels and the exploding harpoon. In 1925, the floating factory ship was introduced and could operate independently around the clock and stay at sea for months. After the blue whale populations were depleted in the 1930s, whalers directed their energies to catching the next largest species, the fin whale. When the fin whale populations declined, whalers moved on to the sei whale. It is difficult to isolate the history of whaling for this species because sei and Bryde’s whales were combined in the catch statistics up until about 1975. It is known, however, that the sei whale was heavily exploited in all areas where it was previously abundant. Because sei whale numbers were so depleted, it was provided worldwide protection by the International Whaling Commission (IWC) in 1978. It gained further protection under the IWC moratorium on commercial whaling in 1986, although Iceland continued "scientific" whaling for sei whales until 1988. In March 2002, Japan announced its intention to expand its "scientific" whaling in the North Pacific to include 50 sei whales, despite the fact that there is no available population estimate for the North Pacific population. Norway has also been whaling commercially since 1993 under objection to the IWC moratorium but has not taken any sei whales. Other threats to the sei whale include disturbance from seismic operations, collision with large vessels, entanglement in fishing gear, and pollution (including noise pollution and increasing amounts of plastic debris at sea, oil spills and dumping of industrial wastes). International Trade Listed on CITES Appendix I which prohibits international trade.

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Auteur Et Sources

Sources
CITES. 2001. Balaenoptera borealis. http://www.cites.org.

IUCN. 2001. The 2000 IUCN Red List of Threatened Species. www.redlist.org.

Jefferson, T.A., S. Leatherwood and M.A. Webber. 1993. Marine Mammals of the World. FAO Species Identification Guide. UNEP, Rome. 320pp.

Leatherwood, S. and R.R. Reeves. 1983. The Sierra Club Handbook of Whales and Dolphins. Sierra Club Books, San Francisco. 302 pp.

                                               

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