# Sei whale (*Balaenoptera borealis*)

Canonical page: https://creaturedata.com/animals/sei-whale  
Data as of: 2026-11-11

The sei whale (Balaenoptera borealis) is a marine mammal in the family Balaenopteridae, within the order Cetacea and suborder Mysticeti. It falls in the giant size class, with a representative adult body mass of 20 tonnes. An invertivore active by day, it feeds on copepod, euphausiacea and zooplankton. It is listed as Endangered on the IUCN Red List, a threatened category, and has a lifespan of 74 years.

## Quick facts

| Fact | Value |
|---|---|
| Weight | 20 t |
| Length | up to 19.5 m |
| Lifespan | 74 years |
| Diet | Invertivore |
| Main food | copepod, euphausiacea, zooplankton |
| Habitat | Not recorded |
| Gestation | 335 days – 346 days |
| Young per litter or clutch | 1.01 |
| Age at maturity | 8.97 years – 10 years |
| Litters or clutches a year | 0.45 |
| Active | By day (diurnal) |
| Lifestyle | Marine |
| Migration | Migratory |
| Conservation status | Endangered (IUCN) |
| Countries on record | Not recorded |

## Classification

- kingdom: Animalia
- phylum: Chordata
- class: Mammalia
- order: Cetacea
- family: Balaenopteridae
- genus: Balaenoptera
- species: *Balaenoptera borealis* Lesson, 1828

Synonyms: *Balaenoptera alba*, *Balaenoptera arctica*, *Balaenoptera iwasi*, *Balaenoptera laticeps*, *Balaenoptera schlegelii*, *Balaenoptera schlegellii*, *Belaenoptera schlegeli*, *Pterobalaena alba*, *Pterobalaena schlegeli*, *Pterobalaena schlegeli alba*, *Sibbaldius schlegelii*

## Common questions

### What kind of animal is the sei whale?

The sei whale is a mammal in the whales and dolphins group, belonging to the family Balaenopteridae and suborder Mysticeti.

### How heavy is the sei whale?

The sei whale has a representative adult body mass of 20 tonnes, placing it in the giant size class.

### What does the sei whale eat?

The sei whale eats copepod, euphausiacea and zooplankton. It is classed as an invertivore.

### Is the sei whale endangered?

Yes. The sei whale is listed as Endangered (EN) on the IUCN Red List, which is a threatened category.

### How long does the sei whale live?

The sei whale has a lifespan of 74 years.

### How long is the sei whale's gestation period?

The sei whale's gestation period is 346 days, with 1.01 young per litter and 0.45 litters per year.

### When does the sei whale reach sexual maturity?

The sei whale reaches sexual maturity at 9.5 years, with reported values from 9.0 to 10.0 years.

### Who named the sei whale?

The sei whale, Balaenoptera borealis, was named by Lesson, 1828.

## Sources

- [An amniote life-history database to perform comparative analyses with birds, mammals, and reptiles](https://doi.org/10.6084/m9.figshare.3563457.v1), CC0 1.0, E096-269. Myhrvold, N.P., Baldridge, E., Chan, B., Sivam, D., Freeman, D.L. & Ernest, S.K.M. (2015) An amniote life-history database to perform comparative analyses with birds, mammals, and reptiles. Ecology 96: 3109. https://doi.org/10.1890/15-0846R.1. Data: https://doi.org/10.6084/m9.figshare.3563457.v1
- [Catalogue of Life eXtended Release](https://www.checklistbank.org/dataset/316691), CC BY 4.0, 2026-10-16 XR. Catalogue of Life (2026). Catalogue of Life eXtended Release, version 2026-10-16 XR. Catalogue of Life Foundation, Amsterdam, Netherlands. https://doi.org/10.48580/dgzlf
- [Wikimedia Commons](https://commons.wikimedia.org), per file (CC0 / public domain / CC BY green, CC BY-SA amber, anything else not displayed), 2026-11-11. Wikimedia Commons contributors. Wikimedia Commons. https://commons.wikimedia.org (each file credited to its author under its own licence).
- [EltonTraits 1.0: species-level foraging attributes of the world's birds and mammals](https://doi.org/10.6084/m9.figshare.3559887.v1), CC0 1.0, 1.0. Wilman, H., Belmaker, J., Simpson, J., de la Rosa, C., Rivadeneira, M.M. & Jetz, W. (2014) EltonTraits 1.0: Species-level foraging attributes of the world's birds and mammals. Ecology 95: 2027. https://doi.org/10.1890/13-1917.1. Data: https://doi.org/10.6084/m9.figshare.3559887.v1
- [Wikidata](https://www.wikidata.org), CC0 1.0, 2026-11-11. Wikidata contributors. Wikidata. https://www.wikidata.org (data retrieved via the Wikidata Query Service).
- [Wikipedia (English)](https://en.wikipedia.org), CC BY-SA 4.0, 2026-11-11. Wikipedia contributors. Wikipedia, The Free Encyclopedia. https://en.wikipedia.org (text under CC BY-SA 4.0; each block names its article and revision).

Cite as: Sei whale (Balaenoptera borealis). Retrieved from https://creaturedata.com/animals/sei-whale