SEI WHALE (B) 



Balaenoptera horealis Lesson 1828 



Other Common Names 



Pollack whale, sardine whale, Rudolphi's rorqual. 



Description 



Sei (pronounced "say") whales have been reported to 

 reach 62 feet (19 m). 



The snout is less acutely pointed than that of the fin whale 

 but when viewed from the side appears slightly arched. In 

 general, the head is intermediate in shape between that of the 

 blue whale and that of the fin whale. The dorsal fin, which is 

 from 10 to 24 inches (25.4 to 61 cm) tall and strongly falcate in 

 adult animals, is located about two-thirds of the way back on 

 the back, farther forward than that of the blue or fin whales. 

 Sei whales are dark steel gray on the back and sides, and on 

 the posterior portion of the ventral surface. The body often 

 has a galvanized appearance due to scars possibly resulting 

 from lamprey bites inflicted during migrations into warmer 

 waters. These scars may be dark gray to almost white in 

 color. On the belly there is a region of grayish white that is 

 confined to the area of the ventral grooves. Neither the 

 flippers nor the tail flukes are white underneath. The right 

 lower lip and the mouth cavity, unlike those of the fin whale, 

 are uniformly gray. The baleen plates are uniformly grayish 

 black with fine grayish-white fringes. (A small number of sei 

 whales have been noted to have a few half- white plates near 

 the front of the mouth, a feature which might result in their 

 confusion with fin whales.) 



Natural History Notes 



The blow of sei whales is an inverted cone rarely taller 

 than 15 feet (4.6 m). 



Sei whales are generally skimmer feeders and do not 

 usually dive very deeply. For that reason they usually surface 

 at a shallower angle than fin whales. The head rarely 

 emerges at a steep angle (except when the whales are 

 chased). Instead, the blowholes and a major portion of the 

 back, including the dorsal fin, become visible almost 

 simultaneously and remain visible for relatively long periods 

 of time. In this species it can be generally stated that the 

 amount of the back in the area of the dorsal fin which is 

 exposed above the surface as the animal sounds is 

 approximately the same height as the dorsal fin. When they 

 begin another dive, sei whales do not arch the tail stock or 

 flukes high. Instead, they normally submerge by slipping 

 quietly below the surface, often remaining in view only a few 

 feet down and leaving a series of tracks or swirls on the 

 surface as they move their tail flukes. When they are feeding 

 in this manner, sei whales may exhibit a highly regular 

 blowing and diving pattern over long periods of time. 



Sei whales usually travel in groups of from two to five 

 individuals, though they may concentrate in large numbers 

 on the feeding grounds. 



In the northern portion of their range, sei whales feed on 

 copepods. Throughout the remainder of the range, however, 

 their food is more varied and also includes krill and a variety 



of small schooling fishes. The species derived its common 

 name, in fact, from its frequent occurrence with or near sei 

 fish. 



May Be Confused With 



The sei whale's smaller size and decidedly taller, more 

 falcate dorsal fin located well more than one-third forward 

 from the tail should prevent confusion with the blue whale. At 

 a distance, however, sei whales are difficult to distinguish 

 from either fin whales or Bryde's whales. The primary clues 

 for distinguishing them from fins are the differences in 

 swimming, blowing, and diving characteristics tabularized in 

 detail on p. 26 and the asymmetrical coloration of the fin 

 whale. 



Sei whales may be distinguished from Bryde's whales only 

 upon close examination. The dorsal fin of Bryde's whales is 

 small, to 18 inches (45.7 cm), sharply pointed, and often worn 

 on the rear margin. If close examination is possible, the sei 

 whale will be seen to have only a single head ridge. Bryde's 

 whales have two additional ridges— one on each side of the 

 main ridge. Bryde's whales are primarily fish feeders and 

 their diving behavior more closely resembles that of a fin 

 whale than that of a sei whale. (See p. 37). 



Distribution 



The distribution and migrations of the sei whale during 

 most of the year are rather poorly known. The species is 

 known to avoid the colder regions near the pack ice and to 

 range from Iceland south to the northeast Venezuelan coast 

 and the northeast and southwest Gulf of Mexico. There are 

 also records from Cuba and the Virgin Islands. Along the 

 northeast United States and eastern Canadian coasts, where 

 most research on sei whales has been conducted, the species 

 migrates from New England through the Blanford, Nova 

 Scotia area in June and July, is found in small numbers off 

 eastern Newfoundland in August and September (abundant 

 in Placentia Bay, Newfoundland in August), and continues 

 northward to the Davis Straits in September and October. 

 An offshore stock may be found year-round in the Labrador 

 Sea. The summer range (May to September or October) 

 extends from New England to southern Arctic waters. 

 Though some individuals remain behind through November, 

 the southward movement of the bulk of the population 

 presumably begins in October. In general, sei whales do not 

 venture as far north as fin whales but may have a greater 

 tendency to enter tropical waters. 



Stranded Specimens 



Stranded sei whales are most likely to be confused with fin 

 whales or Bryde's whales. The three head ridges of the 

 Bryde's whale (sei whales have only one) assist in distinguish- 

 ing sei whales from Bryde's whales. They may be distin- 

 guished from fin whales and all other rorquals by the follow- 

 ing characteristics: 1) The color of the baleen plates— uni- 

 formly ash black with a blue tinge and fine white bristles 



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