The Sei Whale (pronounce /ˈseɪ/ or /ˈsaɪ/), Balaenoptera borealis, is a baleen whale, the third largest rorqual after the Blue Whale and the Fin Whale. It can be found worldwide in all oceans and adjoining seas, and prefers deep off-shore waters. It tends to avoid polar and tropical waters and semi-enclosed bodies of water. The Sei Whale migrates annually from cool and subpolar waters in summer to temperate and subtropical waters for winter, although in most areas the exact migration routes are not well known.