|Range of the Sei whale|
Sei Whales (Balaenoptera borealis) are a type of Baleen whale (filtering food from water), and one of the largest animals on Earth. The Sei whale lives in many oceans, adjoining seas, and deep off-shore waters. they live in both polar and tropical water. In the summer, they live in cool and sub-polar waters, but when it becomes winter they move to temperate and sub-tropical waters. However, scientists don't know the exact routes of migration.
The Sei Whale is a member Balaenopteridae and they are third biggest member, the first is Blue Whale, which is up to 180 tonnes, (200 tons) and the second member is Fin Whale which is over 70 tonnes (77 tons), the Sei Whale is between 12 and 15 meters (40~50ft) and weighs 22~33tons. There are two types of Sei Whale; the Northern Sei Whale and, Southern Sei Whale. The Southern Sei Whale is larger than the Northern Sei Whale. Also, the female is larger than the male. At birth, the baby Sei Whale is around 4~5 meters, which is 13~16ft. Most the Sei Whale’s body is dark grey and light grey, with white markings on their ventral surface. 
The Sei Whales have baleen (32~60 pleats or grooves) in their mouth that help with feeding by allowing them to filter small organisms and debris from the water.
They have short pectoral fins and a single ridge. Also, they have a dorsal fin which is sickle shaped and 25~61 centimeters in heights. The adult Sei Whales have ashy-black baleen plates on each side of mouth that are 48 centimeters, which equals 19in.
When it comes to mating season, male and females become a social unit, in winter months is when the Sei Whale mate; between November and February. This is when the Sei Whales which live in the Northern Hemisphere are typically mating. However, in the Southern hemisphere, they mate in between May and July. The Sei Whale's pregnancy lasts from 10 1/2 to 12 months. They birth only one baby whale, and most baby whale is 450cm in length. Rarely does the Sei Whale have multiple fetuses. The baby Whale is taken care of by parents for six or seventh months. When the baby whale becomes 10 years old, they become a sexually mature, but they grow until 25 years old. Some Sei Whale lives as long as 74 years. Today human's hunting was effects on their reproductive activity, so most reproduction is decrease. 
The Sei Whales live in lots of different regions, in polar and tropical waters. Those Sei Whales which live n the North Atlantic, spread from Southern Europe to Norway in the eastern North Atlantic, also they spread from Southern United States to Greenland during the Western in the winter, the Sei Whales stay in the 20°N~23°N latitude in the North pacific, and when it becomes summer, they move to 35 °N~ 50 °N latitude. Most Sei Whales live in the North Pacific, east of the International Date Line however, today scientists don’t have enough information to find them in the North Pacific, however, we could see data of catches in between 40°S and 50°S latitude, but we don’t know the exact area of distribution. In the summer the Sei Whale live in cool and sub polar waters however when it become winter. They move to temperate and subtropical waters with abundant food. 
The Sei Whale did not need protect their population until 1970, however when humans started hunting them, they need protect from be threatened. So the International Whaling Commission (IWC) is limited about hunting so after 1976, the Sei Whale can be protecting from commercial whaling. Also in 1986, the IWA voted to protect from threatened with extinction so the commercial whaling stopped by voting also Sei Whaling could protect from hunting. However in the late 1970’s some pirate hunted whale which was illegal in North Pacific, also Soviet Union misreported about whaling data. In 2000, many whales remained on the IUCN Red List which mean, we need more protect whales from hunting. 
Balaenoptera borealis schleglii
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