Sei whale (Balaenoptera borealis) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

The sei (pronounced “say”) whale (Balaenoptera borealis) (Lesson 1828)is the third largest member of the Balaenopteridae family, after the blue (B. musculus) and fin (B. physalus) whales. The name is derived from “sejhval”, the Norwegian name for the species. The species was so named by Norwegian whalers because its arrival in Scandinavian waters coincided with the “seje”, or pollock (Pollachius virens) (Andrews 1916). Other historic English names include coalfish whale, pollock whale, Rudolph’s rorqual, sardine whale and Japan finner. French names include rorqual du nord, rorqual de Rudolph, rorqual boréal, and baleine noire (Gambell 1985), although baleine noire is now used to refer to the Right Whale (Véronique Lesage, personal communication).

Gambell (1985) lists purported “Eskimo” and Aleut names as Komovokhgak and Agalagitakg. However, the cited source (Klinowska 1980) is obscure, and the names are not familiar to Inuit groups in Atlantic Canada, although Komovokhgak may have its origins in the western Arctic (Allan McNeill, personal communication).

As detailed in Horwood (1987), the classification of B. borealis is Lesson’s Latin translation of Cuvier’s (1823) “rorqual du Nord”. A southern hemisphere form, Sibbaldius schlegelii, was first proposed by Flower in 1865. However, this racial distinction between northern (B. b. borealis) and southern (B. b. schlegelii) hemisphere populations, based on size difference and little or no mixing (Tomilin 1967, Zemsky 1980 cited inHorwood 1987) has generally been ignored.

Description

Sei whales are generally dark steel to bluish-grey in colour, with a tendency towards lighter pigmentation down the sides and on the posterior of the ventral surface (Horwood 1987). The ventral grooves almost always exhibit a white or light-coloured area that extends back from the chin sometimes as far as the umbilicus. However, Andrews (1916) warns that the colour variation for this species is enormous. The lateral and ventral sides may appear mottled with grey or white circular scars caused by various parasites and predators including ectoparasitic copepods, Penella spp. (Andrews 1916, Ivashin and Golubovsky 1978), lampreys(Pike 1951, Rice 1977), and cookie cutter sharks (Schevchenko 1977). The dorsal fin is tall and slender and further forward on the body when compared to blue or fin whales (Andrews 1916). The baleen is much finer than that of the other balaenopterids, making it a reliable feature for species identification (Mead 1977).

Sei whales are easily confused with Bryde’s whales (B. brydei), especially in subtropical waters where the species historically overlapped. The morphological differences between sei and Bryde’s whale are minor. While the sei whale is larger, the only reliable way of distinguishing between the two at sea is to examine the head. The Bryde’s whale has three distinct ridges along the length of the rostrum while the sei whale has only one (Horwood 1987). However, since Bryde’s whales tend to be restricted to warmer latitudes, generally below 40ºN (Omura 1959), this is not likely to be a source of confusion in Canadian waters.

Sei whales can also be confused with fin and minke whales, especially by inexperienced observers (Horwood 1987). Distinguishing sei whales from fin whales requires observation of the right jaw or the ventral side of the animal to confirm that it is grey in colour, not yellowish-white, as in fin whales. Kate Wynne (personal communication) reports that there is considerable overlap in body size, colouration, and dorsal fin shape between sei and fin whales in the northeastern Pacific, and confirms that observation of the right jaw or the ventral fluke surface is necessary to confirm species identification.

The possibility of underestimating population sizes for this species due to uncertainties in distinguishing between the two species has been noted in Atlantic Canada (Whitehead et al. 1998 cited in Breeze et al. 2002). This effect is likely more pronounced on the Pacific coast, where observers generally have less familiarity with sei and fin whales.

Page details

Date modified: