There are approximately 40,000 to 60,000 sei whales in the world population. Many of the populations are severely depleted or have disappeared, especially during the 1960's and early 1970's. According to C.I.T.E.S the sei whale is an endangered species as of 2000.
Sei whales are distributed worldwide, but in some parts their presence is sporadic and unpredictable. Sei whales do not venture as far pole ward as other whales, but they do have a greater tendency to enter tropical waters. Sei whales are typically found in the western North Pacific, western North Atlantic, East Atlantic, and in the Southern Hemisphere they favor sub-Antartic waters.
Like most other baleen whales, sei whales spend the winters in warm, sub-tropical waters, migrating to temperate or polar seas to feed in summer. Sei whale abundance in the Antarctic peaks later than blue and fin whale abundance.The movements and distributions of sei whales are often unpredictable and are not well documented. Off Korea, Norway and Japan, and in the western North Atlantic researchers have reported great fluctuations in the abundance of sei whales in irregularly spaced years. This may result from changes in environmental conditions which render an area more or less productive from year to year. There is a pronounced segregation of the sexes during migration - generally the pregnant females arrive earlier and mature whales stay longer.