Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
particularly along the cold eastern currents (Perry et al. 1999). Characteristics of preferred breeding
grounds are unknown.
Distribution—Sei whales have a worldwide distribution, but are found primarily in cold temperate to
subpolar latitudes, rather than in the tropics or near the poles (Horwood 1987). Sei whales are also
known for occasional irruptive occurrences in areas followed by disappearances for sometimes
decades (Horwood 1987; Schilling et al. 1992; Clapham et al. 1997).
Sei whales spend the summer months feeding in the subpolar higher latitudes and returns to the
lower latitudes to calve in winter. There is some evidence from whaling catch data of differential
migration patterns by reproductive class, with females arriving at and departing from feeding areas
earlier than males (Horwood 1987; Perry et al. 1999). For the most part, the location of winter
breeding areas remains a mystery (Rice 1998; Perry et al. 1999).
In the North Pacific, sei whales are thought to occur mainly south of the Aleutian Islands. They are
present all across the temperate North Pacific north of 40°N (NMFS 1998a) and are seen at least as
far south as 20°N (Horwood 1987). In the east, they range as far south as Baja California, Mexico,
and in the west, to Japan and Korea (Reeves et al. 1998). As noted by Reeves et al. (1999), reports
in the literature from any time before the mid-1970s are suspect, because of the frequent failure to
distinguish sei from Bryde’s whales, particularly in tropical to warm temperate waters where Bryde’s
whales are generally more common than sei whales.
Information Specific to the Marianas MRA Study Area—Sei whales are not expected to occur in
the Marianas study area. Two “sei” whales were tagged in the general vicinity of the Northern
Mariana Islands (1,515 km northwest of Riidian Point, Guam) during January 1972 (Masaki 1972;
Ohsumi and Masaki 1975). These two individuals were later killed a few hundred kilometers south
of the western Aleutian Islands during June 1972 (Ohsumi and Masaki 1975; Horwood 1987).
These were possibly misidentifications of Bryde’s whales (Jefferson personal communication).
Behavior and Life History—Sei whales are typically found in groups of one to five individuals
(Leatherwood et al. 1976). The sei whale is atypical of rorquals in that it primarily “skims” its food
(though it does some “gulping” as other rorquals do) (Pivorunas 1979). In the North Pacific, sei
whales take a diversity of prey, including calanoid copepods, krill, fish, and squid (Nemoto and
Kawamura 1977). Sei whales typically follow a reproductive cycle of two years: a gestation period of
about 10 to 12 months and a lactation period of 6 to 9 months (Gambell 1985).
Acoustics and Hearing—Sei whale vocalizations have been recorded only on a few occasions.
They consist of paired sequences (0.5 to 0.8 sec, separated by 0.4 to 1.0 sec) of 7 to 20 short (4
milliseconds [msec]) frequency-modulated (FM) sweeps between 1.5 and 3.5 kHz; source level is not
known (Thomson and Richardson 1995). While no data on hearing ability for this species are
available, Ketten (1997) hypothesized that mysticetes have acute infrasonic hearing.
♦ Fin Whale (Balaenoptera physalus)
Description—The fin whale is the second-largest whale species, with adults reaching 24 m in length
(Jefferson et al. 1993). Fin whales have a very sleek body, with a pale, V-shaped chevron on the
back, just behind the head. The dorsal fin is prominent, but with a shallow leading edge and is set
back two-thirds of the body length from the head (Jefferson et al. 1993). The head color is
asymmetrical, with a lower jaw that is white on the right and black or dark gray on the left. Fin and sei
whales are very similar in appearance, which has resulted in confusion about the distribution of both
species (NMFS 1998a).
Status—Fin whales are classified as endangered under the ESA and are designated as endangered
by the IUCN Red List (Reeves et al. 2003). There is no designated critical habitat for this species in
the North Pacific. The IWC recognizes two management stocks in the North Pacific: a single
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