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This week, by popular choice, we are talking about the sei whale. This enigmatic baleen whale is both mysterious and unique, showing up in large numbers when food is plentiful only to disappear for months on end! Keep reading to find out more about this intriguing marine mammal.


Of the rorqual (streamlined, throat-pleated, baleened) whales, the sei whale (Balaenoptera borealis) is the third largest behind the fin and blue whales, reaching lengths of 64 feet and a maximum weight of 28 tons.

Found in all the world’s oceans, these whales are a migratory species, travelling from cooler subpolar waters in the summer to temperate and subtropical waters in the winter due to prey availability and mating, though exact routes and mating locations are still up for debate. The sei whale is remarkably similar to both the Bryde’s (pronounced “broodus”) and fin whale in appearance, creating confusion around positive identification and therefore understanding population counts and movement. The most recent estimate has the sei whale population at around to 80,000 individuals.

Sei whale mother and calf. Photo: Christin Khan, NOAA/NEFSC

The sei whale is darker grey in color, with white and lighter grey markings scattered across their ventral surfaces. They also commonly have scars scattered on their botties from bites from cookie cutter sharks (which are also useful for photo-ID of individuals!). Compared to its massive elongated body, this species has fairly small and pointed pectoral fins and small, slightly curved dorsal fin. If you have ever seen a minke whale here in the Salish Sea, think that, but much larger!

Diet & Behavior

The name sei whale is related to their behavior, and who they hang out with: “sei” comes from the Norwegian Seje which means Pollock or coalfish, which the whales are often seen with. Other common names have to do with their tendency to be found with a certain species, and include pollack whale, coalfish whale, sardine whale, Rudolphi’s rorqual and Japan finner.

Being a rorqual (baleen whale), sei whales have grooves on their underside from the chin/lower jaw to the belly. These allow their throat to expand (like a balloon) so they can gulp-feed, taking in a lot of water and fish, then pushing the water out straining their prey in their baleen. What is interesting about the sei whale is that although they do gulp-feed, they are more likely to be seen skim feeding. This is where they open their mouths and swim along, skimming the surface of the water, so what you see is the top part of their mouth up in the air as they swim along the surface.

Sei whales skim-feeding. Photo: Peter Duley/NEFSC/NOAA Fisheries

This makes sense with what they eat: up to 2000lbs/day of plankton (including copepods and krill), small schooling fish and sometimes cephalopods (including squid). They basically eat what is abundant in the region, as long as it is a shoaling (schooling) species. Because their food is close to the surface, they make relatively shallow dives (under 300m) and will stay submerged only 5-20 minutes at a time. When they dive they do so a little differently. You won’t see them arch their back, or fluke up (where the fluke comes up out of the water) like other whales. They simply sink below the surface. What you will see are ‘fluke prints’ – smooth circles on the surface of the water created by the flukes as they dive.

Generally, this species is solitary or found in small groups of 2-5 individuals but there have been up to 100 seen in a limited area when food is abundant. They are known for their ‘invasions’ of particular areas to exploit prey, Norwegian’s call them invasion years. This leads them to have more unpredictable distribution compared with other large whales. They may be found in one area for a period of time, then not return for years or decades! Certainly makes them a bit more challenging to study!

When sei whales surface you will see both the blow and the dorsal fin at the same time. Their blow can reach as high as 10-13ft! You may also see them breach, or leap out of the water, though they will do this at a lower angle than other whales.

There is much we still don’t know about this species. We know they mate and breed in warmer areas, but no one knows exactly where. They are sexually mature between 6-12 years (but won’t be fully grown until around 25 years), are pregnant for 11-13 months and nurse for 6-9 months. Beyond that we don’t know much about their reproduction, social structure or behaviors.

Status, Threats & Fun Facts

As mentioned previously, the worldwide population estimate for sei whales is estimated at around 80,000, however due to our lack of knowledge about this species that number is likely a “best guess”. Sei whales are listed as “Endangered” under the Endangered Species Act (ESA) and “Depleted” under the Marine Mammal Protection Act (MMPA).

Their biggest threats are ship strikes and entanglement in fishing gear, particularly gill nets, pots, and traps. Ship strikes are becoming more of an issue for many large whale species as global transportation becomes more widespread and Arctic routes become more accessible due to sea-ice melt. This is especially true for the sei whales due to their wide-ranging distribution and the fact that they likely travel the same routes as many large ships. Whaling was traditionally a huge threat to sei whales, however since the moratorium on whaling by the International Whaling Commission (IWC), nowadays only a small amount of whaling still occurs in Japan.

So here are some fun facts about the sei whale:

  • They are the FASTEST cetacean (whale, dolphin, or porpoise) of them all! Sei whales can reach speeds of up to 35 miles per hour (around 50kilometers per hour), however they are more of a sprinter: they cannot sustain such high speeds.
  • Sei whales can live up to 65 years.
  • Their only natural predator is mammal-eating killer whales (orcas), and of course humans.
  • Female sei whales are bigger than the males.
  • In 2019 a whole new type of vocalization called a “triplet call”, made by sei whales, was discovered and published by Tremblay et al. (2019)! This type of vocalization is unique among other baleen whale call types, and may be an indicator that sei whales produce song, similar to their cousins the humpback whale!

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