Species fact file
Common names: Sei whale
Icelandic name: sandreyður
Scientific name: Balaenoptera borealis
Family: Balaenopteridae (rorquals)
Max length: 17m (males), 18.6m (females) in the Northern Hemisphere, larger in the Southern
Distribution: Globally, but most common in offshore waters and rarer in tropical and polar regions
Sei whales are the third largest baleen whale, after the blue and fin whales, but much less is known about them compared to their larger relatives. They prefer offshore waters and typically travel alone or in small groups. Their range doesn’t often overlap with other baleen whales, and their migration patterns are more irregular, so their distribution can fluctuate significantly over time and is currently not well understood. Surveys suggest that there are more than 10,000 sei whales in the North Atlantic, and that they are most abundant in the waters between Iceland and Greenland.
Although sei whales prefer deep, offshore waters, they aren’t deep divers. They feed mostly nearly the surface of the water and their dives typically last less than 15 minutes. Sei whales have a much wider and varied diet compared to other rorquals. They have very fine baleen which allows them to switch between different feeding strategies and prey – they can lunge feed, which is how most rorquals prefer to feed, but they are also able to skim feed like bowhead and right whales. You can watch a video of a sei whale lunge feeding on a school of small fish here. Sei whales are thought to be the fastest of all baleen whales, capable of reaching speeds up to 50 km/h in a sprint, although they cannot maintain this for very long.
Sei whales favour different prey in different regions and at different times of year. They are known to feed on small crustaceans like copepods and krill, multiple species of fish, crabs, and lobsters. In the eastern North Atlantic, copepods and krill appear to make up the majority of their diet, but sei whales in Iceland also feed on some kinds of fish.
‘Sei’ is the Norwegian word for pollock, a fish similar to cod. The whales came to be known by the same name because they arrive along the Norwegian coast at the same time of year as the pollock.
The largest mass mortality of baleen whales known to science occurred in 2015 when over 360 whales, of which at least 343 were sei whales, stranded off southern coast of Chile. The deaths of these whales were attributed to a toxic algal bloom. Blooms like this are thought to be related to the El Niño event, which is an oceanographic occurrence associated with warm periods in the Southern Hemisphere. As the effects of climate change continue to increase, El Niño events are predicted to become more frequent and stronger and mass mortality events like this one may become a more common sight.
Modern commercial whaling impacted sei whale populations in the Southern Hemisphere particularly strongly. Sei whale hunting in Antarctica increased dramatically in the late 1950s and early 1960s, following the depletion of humpback, blue, and fin whale stocks. By the time commercial whaling in the Southern Hemisphere ended in 1979, over 200,000 sei whales had been killed by whalers and their population had been reduced from an estimated 100,000 to 24,000 whales.
However, since the end of commercial whaling in the late 1970s, the global population of sei whales is slowly recovering. The IUCN estimates that there are now around 50,000 sei whales across the globe and that their numbers are increasing, however this is still less than a third of their pre-whaling population size. Exploitation in the North Atlantic occurred over a longer period than in the Southern and Pacific Oceans, ending in 1989, and was less intensive, but here in the eastern North Atlantic the population has not yet recovered from early commercial whaling.
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