Morphology: What Distinguishes the Sei Whale from Other Great Rorquals
The sei whale (Balaenoptera borealis) holds an intermediate size in the rorqual group: 12 to 17 meters for a weight of 20 to 30 tons. It is noticeably smaller than the fin whale (Balaenoptera physalus), but larger than the minke whale (Balaenoptera acutorostrata). This intermediate position is exactly what makes identification challenging.
Dark Pigmentation and Metallic Sheen
The coloration is uniformly slate-gray on the back, without the asymmetric white patch on the right lower jaw that marks the fin whale. Under grazing light, the back shows slight metallic or galvanized reflections, often described by observers as a "steel" tint. Irregular light marks, known as "cookie-cutter scars", are common on the flanks and result from bites by cookiecutter sharks (Isistius brasiliensis).
Falcate Dorsal Fin: Position and Shape
The dorsal fin is tall, strongly falcate and positioned at the posterior two-thirds of the body. It is proportionally larger than that of the fin whale. At sea, this fin often emerges with the back, creating a recognizable silhouette for the trained eye.
Baleen: Color and Structure
The baleen plates are gray-black at the front of the mouth, with fine, silky white fringes. This fine texture adapts to filtering small prey like copepods. At close range, the uniform baleen color without a distinct white band helps separate the sei whale from the fin whale.
Comparative Table
| Criterion | Sei Whale | Fin Whale | Bryde's Whale |
|---|---|---|---|
| Adult Size | 12-17 m | 18-24 m | 11-15 m |
| Pigmentation | Uniform slate gray | Asymmetric, white patch right | Uniform dark gray |
| Dorsal Fin | Tall, falciform, posterior 2/3 | Small, slightly falciform | Tall, falciform |
| Rostral Ridges | 1 | 1 | 3 |
| Baleen | Gray-black, silky white fringes | White right, striped left | Gray, coarse fringes |
The three rostral ridges of Bryde's whale (Balaenoptera edeni/brydei) are decisive but require a close view of the rostrum, rarely feasible at sea.
Recognizing the Sei Whale at Sea: Blow, Fin, and Surface Behavior
Bushy Blow
The sei whale's blow is conical to slightly bushy, reaching 2 to 3 meters high. It is much shorter and less visible than the fin whale's (4 to 6 m) or blue whale's (9 m). In strong wind or backlighting, it may be hard to distinguish from a large dolphin's (Tursiops truncatus) at distance. Shape is the best clue: vertical and narrow, never V-shaped.
Surface-Dive Cycle
The sei whale typically takes 4 to 6 blows at the surface over 1 to 2 minutes, then dives for 5 to 15 minutes. It never flukes on dives, unlike the humpback whale (Megaptera novaeangliae). The back arches gently, dorsal fin visible soon after the blow. This cycle is shorter than the fin whale's, complicating tracking.
Speed and Trajectory
Here the sei whale stands out most in practice. Cruising speed is 8 to 14 km/h, but it can hit 50 km/h over short distances (IUCN). It maintains a straight, steady path, without the unpredictable turns of the humpback. It vanishes from view before all traits can be noted.
Response to Boats
Behavior around vessels varies. Some individuals approach spontaneously and parallel the boat for hundreds of meters. Others flee at engine noise. This unpredictability bans interception maneuvers: waiting for the animal to approach is the only ethical and legal method.
Feeding and Ecology: A Selective Filter Feeder with Varied Strategies
Diet
The sei whale is an opportunistic filter feeder with a diet varying by area and season. It mainly eats copepods (especially Calanus finmarchicus in the North Atlantic), krill, and small pelagic fish like sprats or sand eels. This variety sets it apart from the almost exclusively krill-eating blue whale.
Surface Skimming Technique
The sei whale's most distinctive feeding is surface skimming. It swims slowly on its side, mouth agape, filtering dense zooplankton patches at the surface. Rare among rorquals, this is shared with the North Atlantic right whale (Eubalaena glacialis). It allows prolonged surface visibility during feeding, offering rare long observation windows.
Foraging Dives and Diel Rhythm
Deep dives for bathypelagic prey are more common at night, when zooplankton rises. By day, it targets surface aggregates. This pattern explains why daytime sightings often involve active feeding, not just transits.
Prey Links and Seasonal Presence
Sei whale presence in temperate Northeast North Atlantic waters ties to spring and summer plankton blooms. When copepod concentrations crash in fall, it migrates to warmer waters, likely undocumented subtropical areas.
Distribution and Seasonality: Where and When to Encounter It in Northeast North Atlantic
Global Distribution and Atlantic Populations
The sei whale inhabits all oceans, from subpolar to subtropical waters. The North Atlantic hosts a distinct population, estimated at a few thousand after whaling collapses (IUCN, 2018). Distribution data remain patchy: acoustic tracking and SCANS surveys (Survey of Cetacean Abundance in the North Sea and adjacent waters) give spot estimates, but no precise migration routes are mapped.
Presence in Bay of Biscay and French Waters
The Bay of Biscay is a documented area, mainly offshore of the continental shelf. Ifremer's PELGAS campaigns and PELAGIS observatory data report regular sightings from March to September, especially in the central and southern gulf. Data are scarce near Breton coasts: most encounters occur over 50 nautical miles from ports.
March to September Window
Presence in French waters aligns with surface warming and zooplankton booms. June to August peak documented sightings. March-April arrivals match individuals ascending from winter grounds. Fall departures are poorly known: field observers note gradual decline from October.
Offshore Pelagic Nature and Coastal Rarity
The sei whale is fundamentally an offshore pelagic species. Coastal sightings are anecdotal, often linked to distressed individuals. Offshore trips from Breton or Basque ports toward Cap-Ferret Canyon or upwelling zones offer best odds. The 4 spots on the Whale Spotter map match recurrent report areas, ideal starting points for offshore outings.
Conservation Status: An Endangered Species Still Under Pressure
IUCN Listing and Decline History
IUCN lists the sei whale as Endangered (EN) since 2018 (IUCN, 2018). Global populations lost 60 to 80% of pre-whaling numbers. Recovery is slow: sexual maturity at 8 to 10 years, low reproduction (one calf every 2-3 years).
Commercial Whaling Impact
Intensively hunted from 1860 to 1970, first in the North Atlantic, then Southern Hemisphere. Over 100,000 individuals taken from the North Atlantic alone in the 20th century (IWC). Commercial whaling halted by the 1986 moratorium, but Iceland took some until 2008.
Current Threats and Collision Vulnerability
Ship strikes are the best-documented threat to great whales in Northeast North Atlantic. The sei whale's speed creates specific risk: at 50 km/h, it can cross a ferry or fast cargo path without reaction time for crew or animal. Noise pollution disrupts communication and feeding. Climate change shifts prey, with unclear migration impacts.
Monitoring Programs
No dedicated Northeast North Atlantic sei whale program exists. Data come from multi-species surveys (SCANS, PELGAS) and passive acoustics. Current North Atlantic numbers estimated at 2,000 to 5,000 individuals, with high uncertainty (IUCN, 2018).
Responsible Sei Whale Observation: Distances, Rules, and Code
Minimum Distances and French Regulations
France's July 1, 2011 decree mandates a 100-meter minimum distance for great cetaceans. This applies to all vessels, motorized or not. For the sei whale, it carries special weight: a fast-moving animal covers 100 meters in seconds. Intercepting its path is illegal and dangerous.
High Quality Whale Watching Code
The High Quality Whale Watching (HQWW) code sets stricter standards: never position on the animal's predictable path, slow to under 4 knots within 300 meters, cut engine if it approaches voluntarily. These fully apply to the sei whale—its speed never justifies speeding to "keep it in sight".
Behaviors to Avoid
Head-on approaches most disturb great rorquals: they block paths and trigger costly flight responses. Encircling by multiple boats, even at legal distance, creates acoustic stress zones. Sudden acceleration near feeding animals can halt rare skimming.
Citizen Science Contributions
Every documented sighting counts. I log all contacts on Obs-MAM, OFB's French marine mammal portal. If a clear dorsal fin photo exists, Happywhale enables photo-identification and movement tracking. These feed open scientific databases, filling Northeast North Atlantic distribution gaps.
Sei Whale in Research: What We Still Don't Know Well
Acoustics and Passive Detection
The sei whale produces low-frequency vocalizations from 200 to 600 Hz, as repeated short sequences. Less studied than blue or fin whale songs. Passive acoustic detection (fixed or towed hydrophones) is a key method to estimate presence in visually sparse areas, especially winter and offshore.
Photo-Identification: Feasibility and Catalogs
Photo-ID relies on dorsal fin shape and cookie-cutter scars. Catalogs exist for some populations (e.g., Northwest Atlantic via Canada's MICS), but Northeast Atlantic data are fragmentary. Happywhale aggregates global contributions, the most accessible for non-pros.
Reproduction Gaps
North Atlantic sei whale calving grounds are unconfirmed. Hypothesized in warm subtropical winter waters, but no site documented with sufficient field data. Gestation estimated at 10.5 to 12 months, calves born 4 to 5 meters long, but direct birthing or nursing sightings are nearly nonexistent.
Amateur Observers' Role
These gaps reflect true spatiotemporal coverage deficits. Citizen data on Obs-MAM and Happywhale directly help, especially coastal and seasonal transitions. A documented encounter with a good fin photo can link a Bay of Biscay individual to one off the Azores or Canada via photo-ID.
Frequently asked
What is the difference between the sei whale and the fin whale?
The sei whale (Balaenoptera borealis) measures 12 to 17 meters, versus 18 to 24 meters for the fin whale (Balaenoptera physalus). Its dorsal fin is taller and more falcate, with uniform slate-gray pigmentation lacking the fin whale's asymmetric white patch on the right lower jaw. The sei whale's blow is also shorter (2 to 3 m vs 4 to 6 m) and less visible at distance.
Is the sei whale really the fastest cetacean?
It is the fastest among great baleen whales. Bursts to about 50 km/h documented over short distances (IUCN). This adapts to chasing fast surface prey like copepods, but makes it hard to follow at sea and more vulnerable to strikes by fast vessels like ferries or cargos.
Where to observe the sei whale in France?
The sei whale occurs in the Northeast North Atlantic, mainly offshore of the continental shelf. The Bay of Biscay is a documented area, especially March to September. Coastal sightings are rare; offshore trips from Breton or Basque ports offer best chances. The 4 spots on the Whale Spotter map provide concrete starting points.
How to distinguish the sei whale from Bryde's whale at sea?
Confusion is real, especially at distance. Bryde's whale (Balaenoptera edeni/brydei) has three longitudinal rostral ridges, versus one on the sei whale. This needs a close rostrum view. Geography helps: Bryde's is mainly tropical/subtropical, rare in cold temperate Northeast North Atlantic where sei whales are expected.
Is the sei whale endangered?
Yes. IUCN lists it as Endangered (EN) since 2018. Global populations lost 60 to 80% due to 20th-century commercial whaling. Current threats: ship strikes, noise pollution, climate-driven prey shifts (IUCN, 2018).
What minimum distance to maintain with a sei whale?
French rules require 100 meters minimum for great cetaceans. The High Quality Whale Watching code advises never intercepting the path and cutting the engine if it approaches voluntarily. Given its speed, interception maneuvers are illegal and potentially dangerous for the animal.
Can amateurs contribute to sei whale monitoring?
Yes. Log sightings on Obs-MAM (OFB portal) or Happywhale with dorsal fin photos for photo-identification. These feed scientific databases, partially filling major Northeast North Atlantic distribution gaps.
What does the sei whale eat?
Mainly copepods, krill, and small pelagic fish. It uses surface skimming, rare in rorquals, swimming slowly sideways to filter zooplankton patches. This ties it to high-plankton zones and allows prolonged observation during active feeding.
Does the sei whale make sounds?
Yes. It produces low-frequency vocalizations from 200 to 600 Hz as repeated short sequences. Less studied than blue or fin whale songs. Passive acoustic detection estimates presence in visually sparse areas, especially winter.