Description & Characteristics:
A smaller relative of the Fin whale, Sei whales occupy temperate and subpolar regions in the summer, migrating to sub-tropical waters during the winter months. Only the largest individuals are found south of the Antarctic Convergence. Despite their small size and lower profitability, Seis–like Humpbacks, Blues and Fin whales before them–were overexploited and by the 1960’s were considered ‘commercially extinct.’ Since 1979 the International Whaling Commission (IWC) has banned hunting thus offering them some protection.
Sei whales have arched steel-gray backs with irregular white markings on their bellies. They have relatively slender bodies with pointed snouts, short pectoral fins and a compressed tail section that abruptly joins the flukes. The dorsal fin is sickle shaped and measures 10 to 20 inches in height. Seis have 38-56 deep grooves in their ventral (front) regions which may aid in feeding. Each side of their upper jaw contains 300 – 380 grayish-black baleen plates with fine whitish inner bristles.
Sei whales feed by skimming through the water on their sides through swarms of prey and straining food through their comb-like baleen plates. An average Sei whale eats about one ton of copepods, amphipods, krill, and small fish every day. Although distinguished by their speed, Sei whales are not remarkable divers, preferring to stay near the surface or diving only to shallow depths where they remain submerged for only five to ten minutes at a time.
Sei whales reach sexual maturity at 10 years of age, but do not reach full adult size until they are about 25 years old. They mate in the Northern Hemisphere between November and February, whereas in the Southern Hemisphere mating occurs between May and July. Females usually give birth every other year after a gestation period of 10 to 12 months. Typically a single calf is born which measures 13 feet in length and may weigh 1 1/2 tons. The calf is weaned at six or seven months during which time it has grown to 25 feet in length. Little is known about the actual social system of these animals though pods of two to five individuals are typically observed during mating season.