Animal Diversity Web

Ge­o­graphic Range

These whales are found in all oceans and ad­join­ing seas, ex­cept polar and trop­i­cal re­gions. These an­i­mals oc­cupy tem­per­ate and sub­po­lar re­gions in the sum­mer, but mi­grate to sub-trop­i­cal wa­ters dur­ing the win­ter.

• Biogeographic Regions
• indian ocean
  • native
• atlantic ocean
  • native
• pacific ocean
  • native

Habi­tat

These pelagic whales are found far from shore.

• Aquatic Biomes
• coastal

Phys­i­cal De­scrip­tion

The largest known Sei whale mea­sured 20 me­ters in length, al­though most whales are be­tween 12.2 and 15.2 me­ters long. Of this length, the head and body make up about 13 me­ters. Males are slightly smaller than fe­males. Sei whales have a rel­a­tively slen­der body with a com­pressed tail stock that abruptly joins the flukes. The snout is pointed, and the pec­toral fins are short. The dor­sal fin is sickle shaped and ranges in height from 25 to 61 cen­time­ters.

The body is typ­i­cally a dark steel gray with ir­reg­u­lar white mark­ings ven­trally. The ven­trum has 38-56 deeps grooves, which may have some feed­ing func­tion. Each side of the upper part of the mouth con­tains 300 - 380 ashy-black baleen plates. The fine inner bris­tles of these plates are whitish.

• Other Physical Features
• endothermic
• bilateral symmetry

• Average mass

  2e+07 g

  704845.81 oz

  AnAge

Re­pro­duc­tion

Dur­ing mat­ing sea­son, males and fe­males may form a so­cial unit, but strong data on this issue are lack­ing.

Mat­ing oc­curs dur­ing the win­ter months. Sei whales in the North­ern Hem­ish­pere mate be­tween No­vem­ber and Feb­ru­ary, whereas mat­ing in the south­ern hemi­sphere oc­curs be­tween May and July. Ges­ta­tion lasts from 10 1/2 to 12 months. Fe­males typ­i­cally give birth to a sin­gle calf mea­sur­ing 450 cm in length. There are re­ports of rare mul­ti­ple fe­tuses. The calf nurses for six or seven months. Young reach sex­ual ma­tu­rity at 10 years of age, but do not reach full adult size until they are about 25 years old. Sei whales may live as long as 74 years.

Fe­males typ­i­cally give birth every other year, but a re­cent in­crease in preg­nan­cies has been noted. Re­searchers think this may be a re­sponse to the pre­da­tion rate. Hu­mans kill a great many whales each year, and this might have ef­fects on their re­pro­duc­tive ac­tiv­ity.

• Key Reproductive Features
• iteroparous
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual
• viviparous

• Breeding interval

  Females typically give birth every other year

• Breeding season

  Mating occurs during the winter months

• Average number of offspring

  1

• Average number of offspring

  1

  AnAge

• Range gestation period

  10.5 to 12 months

• Range weaning age

  6 to 7 months

• Average age at sexual or reproductive maturity (female)

  10 years

• Average age at sexual or reproductive maturity (female)
  Sex: female

  3652 days

  AnAge

• Average age at sexual or reproductive maturity (male)

  10 years

• Average age at sexual or reproductive maturity (male)
  Sex: male

  3652 days

  AnAge

Lifes­pan/Longevity

• Average lifespan
  Status: wild

  70.0 years

  Max Planck Institute for Demographic Research

• Average lifespan
  Status: wild

  74.0 years

  Max Planck Institute for Demographic Research

Be­hav­ior

Lit­tle is known about the ac­tual so­cial sys­tem of these an­i­mals. Groups of two to five in­di­vid­u­als are typ­i­cally ob­served, but some­times thou­sands may gather if food is abun­dant. How­ever, these large ag­gre­ga­tions may not be de­pen­dent on food sup­ply alone, as they often occur dur­ing times of mi­gra­tion. Nor­we­gian work­ers call the times of great Sei whale abun­dance "in­va­sion years."

Sei whales are among the fastest cetaceans, swim­ming at speeds of up to 50 kilo­me­ters per hour. Al­though dis­tin­guished by their speed, Sei whales are not re­mark­able divers. These whales dive only to shal­low depths, and they re­main sub­merged only five to ten min­utes at a time.

• Key Behaviors
• natatorial
• motile
• migratory
• social

Com­mu­ni­ca­tion and Per­cep­tion

• Perception Channels
• tactile
• chemical

Food Habits

The Sei whale ob­tains food by skim­ming through the water and catch­ing prey in its baleen plates. These whales feed near the sur­face of the ocean, swim­ming on their sides through swarms of prey. An av­er­age Sei whale eats about 900 kilo­grams of cope­pods, am­phipods, eu­phausi­ids and small fish every day.

• Primary Diet
• planktivore

• Animal Foods
• fish
• zooplankton

Eco­nomic Im­por­tance for Hu­mans: Pos­i­tive

The cur­rent eco­nomic im­por­tance of this whale is ques­tion­able. How­ever, in the past, these large whales pro­vided a great deal of in­come to the whal­ing in­dus­try. It can­not be stressed enough, how­ever, that the pos­i­tive eco­nomic ef­fects of hunt­ing this an­i­mal have been acheived only by large scale dec­i­ma­tion of Sei whale pop­u­la­tions. By over­har­vest­ing the whales, the whal­ing in­dus­try ex­pe­ri­enced a short term eco­nomic gain at a long term cost-- the re­duc­tion in the num­ber of whales avail­able for har­vest.

Con­ser­va­tion Sta­tus

Sei whales are listed as CITES ap­pen­dix 1 from the equa­tor to Antarc­tica. All other pop­u­la­tions are listed as CITES ap­pen­dix 2. The global pop­u­la­tion of these whales is es­ti­mated at only 57,000. Hunt­ing of these whales by hu­mans has been high since the 1950s. The take of these an­i­mals peaked in the 1964-65 sea­son, when 25,454 of these whales were taken. The re­ported global catch of Sei whales in the 1978-79 sea­son was only 150, show­ing the dra­matic drop in whale pop­u­la­tions. Some re­searchers have con­cluded that Sei whale pop­u­la­tions are ris­ing as a re­sult of de­creases in Blue and Fin whale pou­la­tions. How­ever, this con­clu­sion must be taken with cau­tion, as ac­tual data are scarce, and the di­etary over­lap be­tween Sei whales and these other species is not com­plete.

• IUCN Red List

  Endangered
  More information

• CITES

  Appendix I

Con­trib­u­tors

Nancy Shef­ferly (au­thor), An­i­mal Di­ver­sity Web.

Ref­er­ences

Gaskin, D.E. 1982. The ecol­ogy of whales and dol­phins. Heine­mann, Lon­don, Ex­eter and New Hamp­shire.

Nowak, R.M. 1991. Walker's Mam­mals of the world, Fifth edi­tion. John Hop­kins Uni­ver­sity Press, Boston.