The Sei Whale (Balaenoptera borealis) is the third largest species of rorqual, a group of baleen whales known for their speed and streamlined bodies. These whales are remarkably fast, capable of short bursts that can reach up to 30 miles per hour, earning them the nickname “the greyhounds of the sea.” They inhabit temperate and subpolar waters across the globe, migrating between warmer breeding grounds in winter and cooler feeding areas in the summer. Determining the exact number of Sei Whales remaining is difficult due to their offshore habitat and elusive nature.
The Current Global Population Estimate
The task of counting Sei Whales across the world’s oceans is complicated by their massive, deep-water range and the expense of conducting comprehensive marine surveys. The most widely accepted global population estimate suggests there are approximately 50,000 mature individuals alive today, a number that reflects a species still recovering from historical lows. This figure is an estimate, however, and is often subject to high uncertainty because the species is frequently confused with Bryde’s Whales in the field.
Population estimates vary significantly across the major ocean basins, indicating uneven recovery rates for different regional stocks. The North Pacific stock is considered the largest, with estimates suggesting around 35,000 individuals as of 2011. Conversely, the North Atlantic population is considerably smaller, estimated to be around 12,000 whales in total, with one regional stock off the Canadian Atlantic coast likely numbering fewer than 1,000 mature individuals.
The Southern Hemisphere population was reduced most severely by commercial whaling, falling from an estimated 64,000 whales in 1960 to only about 11,000 by 1979 in some regions. Current estimates for the Southern Hemisphere remain low, hovering around 10,000 individuals based on older survey data from the 1980s.
Historical Drivers of Population Decline
The dramatic decline in Sei Whale numbers was a direct consequence of industrial-scale commercial whaling, which intensified during the mid-20th century. Whalers initially targeted the largest species, the Blue and Fin Whales, until their populations were severely depleted. This depletion forced the whaling industry to shift its attention to the next available large species, the Sei Whale, beginning in the late 1950s.
This change in focus led to a rapid and catastrophic exploitation of the global Sei Whale stock. It is estimated that approximately 300,000 Sei Whales were killed worldwide for their meat and oil during the commercial whaling era. The culling peaked in the 1964–65 season, when more than 20,000 Sei Whales were taken in a single year, primarily in the Southern Hemisphere. For example, between 1910 and 1979, over 150,000 Sei Whales were harvested in the Southern Hemisphere alone, decimating what was once a robust population.
Modern Conservation Status and Threats
The Sei Whale is now protected globally and is officially classified as Endangered by the International Union for Conservation of Nature (IUCN) based on the severe population reduction caused by 20th-century whaling. The species is also protected by the International Whaling Commission (IWC) moratorium on commercial whaling, which took effect in 1986, and is listed on Appendix I of CITES.
Despite these protective measures, contemporary threats continue to complicate the species’ recovery. One of the most significant dangers is entanglement in commercial fishing gear, which can lead to injury, exhaustion, or drowning. Ship strikes also pose a major risk, as Sei Whales often feed near the surface and along the edges of the continental shelf, intersecting with busy shipping lanes.
Ocean noise pollution, generated by activities like seismic surveys and commercial shipping, disrupts the whales’ ability to communicate and locate food. Climate change represents a long-term threat by altering oceanographic conditions that impact the Sei Whale’s primary food source, such as copepods and krill. Some Sei Whales are still taken annually in the North Pacific under a controversial scientific research program.