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 distinguishing sei whales from Bryde's whales.
They may be distinguished from fin whales and all other rorquals by the
following characteristics: 1) The color of the baleen plates--uniformly
ash black with a blue tinge and fine white bristles (Table 2). 2) The
density of bristles on the plates--sei whales have from 35 to 60 baleen
fringes per centimeter; all other rorquals have far fewer (less than
35). 3) The relative lengths of the ventral grooves--the grooves of sei
whales end well before the navel; those of blue, fin, and Bryde's whales
extend at least to the navel. 4) The relatively small numbers of ventral
grooves (38-56)--both blue and fin whales have more; Bryde's whales have
approximately the same number.

If the animal is not in an advanced state of decomposition, the region
of white coloration of the belly may also be visible.

[Illustration: Figure 24.--The head of a sei whale is intermediate in
shape between that of the blue whale and that of the fin whale. When
viewed from the side it is slightly arched. Note the single central head
ridge, from just in front of the blowholes to near the tip of the snout.
Bryde's whales, with which sei whales are most likely to be confused in
the tropical and subtropical portions of their range, have two auxiliary
ridges, one on each side of the top of the head, in addition to this
main central ridge. (_Photo from the North Pacific by Japanese Whales
Research Institute, courtesy of H. Omura._)]

[Illustration: Figure 25.--Sei whales are dark gray on the right lower
lip. They can be distinguished from fin whales, which have a white right
lower lip, by approaching them from the right side. (_Photo from the
North Pacific by Japanese Whales Research Institute, courtesy of H.
Omura._)]

[Illustration: Figure 26.--Three views of swimming sei whales.