[7] On 21 February 1819, Swedish-born German naturalist Karl Rudolphi initially identified a 9.8 m (32 ft) whale stranded near Grömitz, in Schleswig-Holstein, as Balaena rostrata (Balaenoptera acutorostrata).

[10] In 1846, the English zoologist John Edward Gray, ignoring Lesson's designation, named Rudolphi's specimen Balaenoptera laticeps, which others followed.

Rorquals take their name from the Norwegian word røyrkval, meaning "furrow whale",[16] because family members have a series of longitudinal pleats or grooves on the anterior half of their ventral surface.

Balaenopterids diverged from the other families of suborder Mysticeti, also called the whalebone whales, as long ago as the middle Miocene.

Each plate is made of fingernail-like keratin, which is bordered by a fringe of very fine, short, curly, wool-like white bristles.

[39][24] During the southern Gulf of Maine influx in mid-1986, groups of at least three sei whales were observed "milling" on four occasions – i.e. moving in random directions, rolling, and remaining at the surface for over 10 minutes.

The American naturalist Roy Chapman Andrews compared the sei whale to the cheetah, because it can swim at great speeds "for a few hundred yards", but it "soon tires if the chase is long" and "does not have the strength and staying power of its larger relatives".

Between dives, the whale surfaces for a few minutes, remaining visible in clear, calm waters, with blows occurring at intervals of about 60 seconds (range: 45–90 sec.).

[41] The sei whale feeds near the surface of the ocean, swimming on its side through swarms of prey to obtain its average of about 900 kg (2,000 lb) of food each day.

The whale's diet preferences has been determined from stomach analyses, direct observation of feeding behavior,[42][43] and analyzing fecal matter collected near them, which appears as a dilute brown cloud.

[45] In the North Atlantic, it feeds primarily on calanoid copepods, specifically Calanus finmarchicus, with a secondary preference for euphausiids, in particular Meganyctiphanes norvegica and Thysanoessa inermis.

In addition, it eats larger organisms, such as the Japanese flying squid, Todarodes pacificus pacificus,[48] and small fish, including anchovies (Engraulis japonicus and E. mordax), sardines (Sardinops sagax), Pacific saury (Cololabis saira), mackerel (Scomber japonicus and S. australasicus), jack mackerel (Trachurus symmetricus) and juvenile rockfish (Sebastes jordani).

[46][49] Off central California, they mainly feed on anchovies between June and August, and on krill (Euphausia pacifica) during September and October.

These calls closely resembled and coincided with a peak in "20- to 35-Hz irregular repetition interval" downswept pulses described from seafloor recordings off Oahu, which had previously been attributed to fin whales.

[59] Between 2005 and 2007, low frequency downswept vocalizations were recorded in the Great South Channel, east of Cape Cod, Massachusetts, which were associated with the presence of sei whales.

This call has also been reported from recordings in the Gulf of Maine, New England shelf waters, the mid-Atlantic Bight, and in Davis Strait.

[66] As of February 2017[update], the U.S. National Marine Fisheries Service estimated that the eastern North Pacific population stood at 374 whales.

The only confirmed historical record is the capture of a 14 m (46 ft) female, which was brought to the Cap Lopez whaling station in Gabon in September 1950.

[74] Sei whales are commonly distributed along west to southern Latin America, including the entire Chilean coast down to the Beagle Channel.

Whales in the Labrador Sea as early as the first week of June may move farther northward and arrive at waters southwest of Greenland later in the summer.

[78] In the northeast Atlantic, the sei whale winters as far south as West Africa such as off Bay of Arguin, off coastal Western Sahara and follows the continental slope northward in spring.

Large females lead the northward migration and reach the Denmark Strait earlier and more reliably than other sexes and classes, arriving in mid-July and remaining through mid-September.

[79][34] A 1985 study suggested a correlation between appearances west of Greenland and the incursion of relatively warm waters from the Irminger Current into that area.

[81] An individual satellite-tagged off Faial, in the Azores, traveled more than 4,000 km (2,500 mi) to the Labrador Sea via the Charlie-Gibbs fracture zone (CGFZ) between April and June 2005.

Initially, the sei whale's speed and elusiveness partially protected them,[84] and later the comparatively small yield of oil and meat.

Heavy exploitation in the North Pacific began in the early 1960s, with catches averaging 3,643 per year from 1963 to 1974 (total 43,719; annual range 1,280–6,053).

[97] Conservation groups, such as the World Wildlife Fund, dispute the value of this research, claiming that sei whales feed primarily on squid and plankton which are not hunted by humans, and only rarely on fish.

[99] In 2010, a Los Angeles exclusive sushi restaurant confirmed to be serving sei whale meat was closed by its owners after a covert investigation and protests lead to prosecution by authorities for handling an endangered/protected species.

[98] In western Canadian waters, researchers with Fisheries and Oceans Canada observed five seis together in the summer of 2017, the first such sighting in over 50 years.

In June 2015, scientists flying over southern Chile counted 337 dead sei whales, in what is regarded as the largest mass beaching ever documented.