Sei whales have a long,sleek body that is dark bluish-gray to black in colour and pale underneath.  They have 30 to 65 ventral grooves or pleats that extend from below the mouth to the navel area.   Sei whales have a 2 foot high dorsal fin, relatively short, pointed flippers and a single ridge along the rostrum ( top of the head ).  Sei whales can be confused with Bryde's whale which possess 3 ridges along the rostrum.   Sei whales can attain speeds of 35 mph or 56 kilometers per hour.  Being up to 60 feet in length, the Sei whale can weigh 100 thousand pounds or 45 thousand kilograms.  Sei whales can live for seventy years.

Usually Sei whales travel in groups of 2 to 5 whales and they generally stay far offshore.  Sei whales possess 219 to 410 baleens plates that are dark in colour with greyish fringes.  Usually Sei whales use the baleen plates as a sieve to strain out copepods and krill from the water that they swallow using force from their tongue and gular pouch to expel the water from between the baleen plates. Sei whales can dive for up to twenty minutes to feed upon copepods, krill, small schooling fish and cephalopods by both gulping and skimming.  When diving the Sei whale does not arch its back or raise its flukes.

Sei whales become sexually mature at 6 to 12 years of age when they reach 45 feet in length.  Female Sei whales breed every 2 to 3 years and carry baby Sei whales in their womb for 11 to 13 months ( gestation period ). Female whales give birth to a single calf that is about 15 feet long and weighs 1500 pounds. Calves are nursed  for 6 to 9 months before being weaned on the feeding grounds.  Often the body of Sei whales is covered in oval-shaped scars from cookie cutter sharks and lamprey bites.

Ship strikes and fishing gear entanglements are probably the two biggest threats to Sei whales.  Between 1959 and 1971 an estimated 106,886 Sei whales were killed by whaler ships and this reduced the global population to a third of their original numbers.  There are at least ten thousand Sei whales in the North Pacific ocean alone and they are totally protected from being hunted by whaler ships by the International Whaling Commission.  Sei whales vocalize at low frequencies below the 120 Hz range and their repetoire is dominated by short, pulsing sounds from 23 Hz to 18 Hz.

photo courtesy of DFO