Rudolphi's rorqual, Pollack whale, Coalfish whale, Sardine whale, Japan finner
The Sei whale (Balaenoptera borealis) is a baleen whale, the third-largest rorqual after the Blue whale and the Fin whale. It inhabits most oceans and adjoining seas and prefers deep offshore waters. The whale's name comes from the Norwegian word for pollock, a fish that appears off the coast of Norway at the same time of the year as the Sei whale.
Crepuscular animals are those that are active primarily during twilight (that is, the periods of dawn and dusk). This is distinguished from diurnal...
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of a...
A planktivore is an aquatic organism that feeds on planktonic food, including zooplankton and phytoplankton. Phytoplankton are usually photosynthet...
A piscivore is a carnivorous animal that eats primarily fish. Piscivorous is equivalent to the Greek-derived word ichthyophagous. Fish were the die...
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract ...
Precocial species are those in which the young are relatively mature and mobile from the moment of birth or hatching. Precocial species are normall...
Natatorial animals are those adapted for swimming. Some fish use their pectoral fins as the primary means of locomotion, sometimes termed labriform...
Among animals, viviparity is the development of the embryo inside the body of the parent. The term 'viviparity' and its adjective form 'viviparous'...
Polygyny is a mating system in which one male lives and mates with multiple females but each female only mates with a single male.
Animal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migrati...
The Sei whale's body is typically a dark steel grey with irregular light grey to white markings on the ventral surface, or towards the front of the lower body. The whale has a relatively short series of 32-60 pleats or grooves along its ventral surface that extend halfway between the pectoral fins and umbilicus (in other species it usually extends to or past the umbilicus), restricting the expansion of the buccal cavity during feeding compared to other species. The rostrum is pointed and the pectoral fins are relatively short, only 9%-10% of body length, and pointed at the tips. It has a single ridge extending from the tip of the rostrum to the paired blowholes which are a distinctive characteristic of baleen whales. The whale's skin is often marked by pits or wounds, which after healing become white scars. These are now known to be caused by "cookie-cutter" sharks (Isistius brasiliensis). It has a tall, sickle-shaped dorsal fin that ranges in height from 38-90 cm (15-35 in) and averages 53-56 cm (21-22 in), about two-thirds of the way back from the tip of the rostrum. Dorsal fin shape, pigmentation pattern, and scarring have been used to a limited extent in photo-identification studies. The tail is thick and the fluke, or lobe, is relatively small in relation to the size of the whale's body. Adults have 300-380 ashy-black baleen plates on each side of the mouth, up to 80 cm (31 in) long. Each plate is made of fingernail-like keratin, which is bordered by a fringe of very fine, short, curly, wool-like white bristles. The Sei's very fine baleen bristles, about 0.1 mm (0.004 in) are the most reliable characteristic that distinguishes it from other rorquals.
Sei whales live in all oceans and seas, except tropical and polar regions. They occupy sub-polar and temperate regions during the summer, migrating to sub-tropical waters in the winter. These whales occur in the open ocean and usually avoid coastal waters. They are believed to migrate to warmer waters in lower latitudes during winter.
Sei whales are social and gather in groups that number 2 to 5 individuals, although larger groups can form where food is very plentiful. They prefer to feed at dawn and may demonstrate unpredictable behavior during foraging. Sei whales are filter feeders, using their baleen plates to obtain their food; they do this by opening their mouths, engulfing or skimming large amounts of the water containing the food, then straining the water out through the baleen, trapping any food items inside its mouth. Sei whales are one of the fastest cetaceans. However, they are not remarkable divers, reaching relatively shallow depths for 5 to 15 minutes. Between dives, the whale surfaces for a few minutes, remaining visible in clear, calm waters, with blows occurring at intervals of about 60 seconds. Unlike the Fin whale, the Sei whale tends not to rise high out of the water as it dives, usually just sinking below the surface. The blowholes and dorsal fin are often exposed above the water surface almost simultaneously. The whale almost never lifts its flukes above the surface, and are generally less active on water surfaces than closely related Bryde's whales; it rarely breaches. Sei whale makes long, loud, low-frequency sounds. Relatively little is known about specific calls, but in 2003, observers noted sei whale calls in addition to sounds that could be described as "growls" or "whooshes" off the coast of the Antarctic Peninsula. Many calls consisted of multiple parts at different frequencies. This combination distinguishes their calls from those of other whales. Most calls lasted about a half second, and occurred in the 240-625 hertz range, well within the range of human hearing.
Sei whales are carnivores (planktivores, piscivores) and eat whatever is abundant locally. In the North Atlantic, they feed primarily on calanoid copepods, with a secondary preference for euphausiids. In the North Pacific, they feed on similar zooplankton. In addition, Sei whales eat larger organisms, such as the Japanese flying squid and small fish, including anchovies, sardines, Pacific saury, mackerel, jack mackerel, and juvenile rockfish. Off central California, they mainly feed on anchovies between June and August, and on krill during September and October. In the Southern Hemisphere, prey species include the copepods, as well as the euphausiids and the pelagic amphipod.
Sei whales are polygynous, with one male having exclusive mating rights with many females. They breed in temperate waters during the winter. Females mate every 2-3 years and gestation lasts 11-13 months. Females typically give birth to one calf. Mothers nurse their calves and look after them until they are weaned at the age of 6 - 9 months. Sei whales become reproductively mature when they are about 8-10 years old.
Sei whales are under threat by climate change, pollution, entanglement in fishing gear, and ship strikes.
According to the U.S. National Marine Fisheries Service, the total size of the Sei whale population is around 80,000 individuals. Currently, this species is classified as Endangered (EN) on the IUCN Red List but its numbers today are increasing.