Sei Whale – Balaenoptera borealis

Common Name: Sei Whale
Scientific Name: Balaenoptera borealis
Classification: Kingdom Animalia → Phylum Chordata → Class Mammalia → Order Cetacea → Family Balaenopteridae → Genus Balaenoptera → Species B. borealis
IUCN Conservation Status: Endangered (EN)

Subspecies:

1. Balaenoptera borealis borealis – North Atlantic and North Pacific populations
2. Balaenoptera borealis schlegelii – Southern Hemisphere population (the one most likely present in Sri Lankan waters)

Geographic Range (where the species is found):

The Sei Whale has a cosmopolitan distribution, occurring in all major oceans except the Arctic. It typically inhabits temperate and subtropical waters, migrating seasonally between high-latitude feeding grounds and low-latitude breeding areas. In the northern Indian Ocean, including Sri Lankan waters, it is considered rare, with occasional offshore sightings, especially along the southern and eastern coasts (Trincomalee and deep offshore regions).

Description:

A large, sleek baleen whale reaching lengths of 12–20 meters and weighing up to 40 tonnes. It has a dark bluish-gray back, lighter flanks, and a pale underside. The dorsal fin is tall, curved, and set far back on the body. The head is narrow and features a single central ridge, distinguishing it from the Blue and Fin whales. The Sei Whale has 300–400 short, dark baleen plates with fine bristles for filtering plankton and small fish.

Habitat:

Prefers deep offshore waters beyond the continental shelf, often in areas of high productivity such as upwelling zones and oceanic fronts. Around Sri Lanka, it inhabits deep pelagic waters near the continental slope and submarine canyons (e.g., Trincomalee Canyon).

Diet:

Carnivore (filter feeder).
Feeds mainly on zooplankton (copepods, krill) and small schooling fish. Uses baleen plates to filter prey from the water, often feeding in areas of dense plankton blooms.

Ecological Role of Balaenoptera borealis:

Functions as a top pelagic predator and plays a key role in regulating plankton and small fish populations. Through its feeding and excretion, it contributes to nutrient cycling, enhancing ocean productivity by distributing iron and nitrogen to surface waters.

Life Cycle:

Calves are born after a 10–12 month gestation period, measuring around 4.5 meters at birth. They are nursed for approximately 6–7 months, during which they double in size. Sexual maturity occurs at 5–12 years, and the species can live up to 70 years.

Reproduction (mating season, number of offspring):

Breeding takes place in winter in warm tropical or subtropical waters. Females give birth to one calf every 2–3 years. After weaning, both females and calves migrate to cooler, nutrient-rich feeding grounds.

Behaviour:

Generally solitary or found in pairs, though loose feeding aggregations can form in productive areas. Known for its speed, capable of bursts up to 50 km/h—one of the fastest baleen whales. The Sei Whale typically remains close to the surface while feeding and does not dive deeply. It surfaces quickly with minimal visible blow.

Key Adaptations:

• Streamlined body enabling high-speed swimming.
• Efficient baleen filtration system for small prey capture.
• Thick blubber layer for thermal insulation and energy storage.
• Migratory behavior allowing exploitation of seasonal food sources.
• Acute hearing and low-frequency vocalizations for long-distance communication.

Threats:

• Historical commercial whaling, which drastically reduced global populations.
• Ship strikes due to increasing ocean traffic.
• Bycatch and fishing gear entanglement.
• Noise pollution interfering with communication and migration.
• Pollutants (heavy metals, plastics, persistent organic compounds) accumulating in tissues.
• Climate change altering prey distribution and migration routes.

Interesting Fact:

The Sei Whale can consume up to 900 kg of plankton and small fish per day and is capable of swimming faster than most other large whales. During feeding, it sometimes leaves visible “clouds” of bubbles in the water—a technique thought to help concentrate prey.

REFERENCES

Here are the linked (anchor-text) references for the Sei Whale (Balaenoptera borealis) — ready to paste directly into your article, blog post, or web page:

• Wikipedia – Sei Whale – Overview of taxonomy, anatomy, distribution, and feeding ecology.
• NOAA Fisheries – Sei Whale – Comprehensive species profile including threats, conservation efforts, and migration data.
• IUCN Red List – Balaenoptera borealis – Global conservation status, population trends, and main threats.
• IWC Handbook – Sei Whale – International Whaling Commission report detailing management and recovery information.
• NAMMCO – Sei Whale – Facts on biology, identification, and regional distribution in the North Atlantic.
• Whale and Dolphin Conservation – Sei Whale – Informative summary with behavioral, ecological, and conservation notes.
• OBIS-SEAMAP – Balaenoptera borealis – Global occurrence and sighting data from research surveys and observations.

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