The "complex" means the number and classification remains unclear because of a lack of definitive information and research.

The common Bryde's whale (Balaenoptera brydei, Olsen, 1913) is a larger form that occurs worldwide in warm temperate and tropical waters, and the Sittang or Eden's whale (Balaenoptera edeni, Anderson, 1879) is a smaller form that may be restricted to the Indo-Pacific.

The recently described Omura's whale (B. omurai, Wada et al. 2003), was formerly thought to be a pygmy form of Bryde's, but is now recognized as a distinct species.

B. brydei gets its specific and common name from Johan Bryde, Norwegian consul to South Africa, who helped establish the first modern whaling station in the country, while B. edeni gets its specific and common names from Sir Ashley Eden, former High Commissioner of Burma (Myanmar).

In Japan, early whalers called it "anchovy" (鰯, iwashi) or "skipjack whale" (鰹鯨, katsuo-kujira, カツオクジラ).

[8] In 1878, the Scottish zoologist John Anderson, first curator of the Indian Museum in Calcutta, described Balaenoptera edeni, naming it after the former British High Commissioner in Burma, Sir Ashley Eden, who helped obtain the type specimen.

Eden's Deputy Commissioner, Major A.G. Duff, sent a Mr Duke, one of his assistants, to Thaybyoo Creek, between the Sittang and Beeling Rivers, on the Gulf of Martaban, where he found a 11-metre (37 ft) whale, which had stranded there in June 1871 after swimming more than 32 kilometres (20 mi) up the creek—it was said to have "exhausted itself by its furious struggles" to get free and "roared like an elephant" before finally expiring.

[9] In 1913, the Norwegian scientist Ørjan Olsen, based on the examination of a dozen "sei whales" brought to the whaling stations at Durban and Saldanha, in South Africa, described Balaenoptera brydei, naming it after the Norwegian consul to South Africa Johan Bryde.

[9][10] In the 1950s it was discovered that there were two types of "sei whale" off Japan, a northern form with longer, finer baleen and shorter ventral grooves and a southern form with shorter, coarser baleen and longer ventral grooves.

[13] Several differences in anatomy are found between Bryde's and Eden's whales; morphological similarities have caused confusions regarding species identification.

The sei whale, like other rorquals, has a single median ridge, as well as a slightly arched rostrum, which is accentuated at the tip.

Both species are often covered with white or pink oval scars caused by bites from cookie-cutter sharks.

In the western North Pacific, Bryde's whales caught by Japanese scientific whaling vessels (2000–2007) mainly fed on Japanese anchovy (Engraulis japonicus, 52%) and various species of euphausiid (36%, including Euphausia similis, E. gibboides, Thysanoessa gregaria, and Nematoscelis difficilis), as well as oceanic lightfish (Vinciguerria nimbaria, nearly 3%), and mackerels (Scomber spp., less than 2%).

The latter, however, mainly feed on euphausiids (primarily Euphausia lucens, but also E. recurva, N. capensis, and Thysanoessa gregaria), as well as various deep-sea fish (including Mueller's pearlside, Maurolicus muelleri, and a species of Lestidium).

They have also been observed feeding on pelagic red crabs (Pleuroncodes planipes) off southern Baja California.

[22] Bryde's whales use several feeding methods, including skimming the surface, lunging, and bubble nets especially within Gulf of Tosa.

[25] In the Pacific and northern Indian Ocean, Bryde's whales have been observed employing "tread-water feeding" or "trap feeding,"[26] a behaviour more commonly known from the northern Gulf of Thailand which is also performed by humpback whales in the northeastern Pacific.

[29][30] B. brydei occurs in the Atlantic, Pacific, and Indian Oceans between the 40th parallels of latitude, preferring highly productive, tropical, subtropical, and warm, temperate waters of 16–22 °C (61–72 °F).

They occur throughout the eastern tropical Pacific, including Peru and Ecuador,[31] where they are absent from July to September.

[1][32][33] Mitochondrial DNA analysis showed that Bryde's whales caught in the pelagic western North Pacific and Bonin Islands (resident population), as well as biopsy samples taken from whales off Hawaii, the west coast of Baja California, and the southern Gulf of California, belonged to B.

[37] Bryde's whales do not occur within central to northern Sea of Japan on regular basis or at least in large numbers.

A population found off southern and southwestern Japan in the East China Sea has also been referred to B. edeni.

Bryde's whale (most had auxiliary ridges) of small size—estimated at 10.1 to 11.6 m (33 to 38 ft) in length—sighted off the northeastern side of the Solomon Islands during a survey in late November and early December 1993 may be referable to B. edeni.

[59] Prior to 2006, only two confirmed sightings of Bryde's whale had been reported in the eastern North Pacific north of Baja California—one in January 1963, only a kilometer off La Jolla (originally misidentified as a fin whale), and another in October 1991 west of Monterey Bay.

[60] Another sighting was made off Dana Point, California, on 19 September 2009, which was originally misidentified as a fin whale.

Bryde's whale is listed on Appendix II[61] of the Convention on the Conservation of Migratory Species of Wild Animals.

It is listed on Appendix II[61] as it has an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.

Around 2011, a videographer named Michael Fishbach filmed a Bryde's whale weighing 20 tonnes (20,000 kg) being hunted and killed by a pack of 20 orca in the Sea of Cortez.

[8] In March 2019, a diver off the coast of South Africa was accidentally caught in the mouth of a feeding Bryde's whale.

[68] Tourists on jet skis have harassed whales off the gulf coast of Phetchaburi Province, Thailand.