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

AUGUST 2005 FINAL REPORT
Acoustics and Hearing—Humpback whales are known to produce three classes of vocalizations: (1)
“songs” in the late fall, winter, and spring by solitary males; (2) social sounds made within groups on
the wintering (calving) grounds; and (3) sounds made on the feeding grounds (Thomson and
Richardson 1995).
The best-known types of sounds produced by humpback whales are songs, which are thought to be
breeding displays used only by adult males (Helweg et al. 1992). Singing is most common on
breeding grounds during the winter and spring months, but is occasionally heard outside breeding
areas and out of season (Matilla et al. 1987; Clark and Clapham 2004). Humpback song is an
incredibly elaborate series of patterned vocalizations, which are hierarchical in nature (Payne and
McVay 1971). There is geographical variation in humpback whale song, with different populations
singing different songs, and all members of a population using the same basic song. However, the
song evolves over the course of a breeding season, but remains nearly unchanged from the end of
one season to the start of the next (Payne et al. 1983).
Social calls are from 50 Hz to over 10 kHz with the highest energy below 3 kHz (Silber 1986). Female
vocalizations appear to be simple; Simão and Moreira (2005) noted little complexity. The male song,
however, is complex and changes between seasons. Components of the song range from under 20
Hz to 4 kHz and occasionally 8 kHz, with source levels of 144 to 174 dB re 1 µPa-m, with a mean of
155 dB re 1 µPa-m. Au et al. (2001) recorded high-frequency harmonics (out to 13.5 kHz) and source
level (between 171 and 189 dB re 1 µPa-m) of humpback whale songs. Songs have also been
recorded on feeding grounds (Mattila et al. 1987; Clark and Clapham 2004). The main energy lies
between 0.2 and 3.0 kHz, with frequency peaks at 4.7 kHz. “Feeding” calls, unlike song and social
sounds, are highly stereotyped series of narrow-band trumpeting calls. They are 20 Hz to 2 kHz, less
than 1 sec in duration, and have source levels of 175 to 192 dB re 1 µPa-m. The fundamental
frequency of feeding calls is approximately 500 Hz (D’Vincent et al. 1985).
No tests on humpback whale hearing have been made. Houser et al. (2001) produced the humpback
audiogram (using a mathematical model). The predicted audiogram indicates sensitivity to
frequencies from 700 Hz to 10 kHz, with maximum relative sensitivity between 2 and 6 kHz.
♦ Sei Whale (Balaenoptera borealis)
Description—Adult sei whales are up to 18 m in length and are mostly dark gray in color with a
lighter belly (Jefferson et al. 1993). There is a single prominent ridge on the rostrum and a slightly
arched rostrum with a downturned tip (Jefferson et al. 1993). The dorsal fin is prominent and very
falcate. Sei whales are extremely similar in appearance to Bryde’s whales, and it is difficult to
differentiate them at sea and even in some cases, on the beach (Mead 1977).
Status—Sei whales are listed as endangered under the ESA; they are also designated as
endangered by the IUCN Red List (Reeves et al. 2003). The IWC designates the entire North Pacific
Ocean as one sei whale stock unit (Donovan 1991), although some evidence exists for multiple
stocks (NMFS 1998a; Carretta et al. 2004). There are no abundance estimates available for this
species in this area.
The taxonomy of the baleen whale group formerly known as sei and Bryde’s whales is currently
confused and highly controversial (see Reeves et al. 2004 for a recent review, also see the Bryde’s
whale species account below for further explanation).
Habitat Preferences—Sei whales are most often found in deep, oceanic waters of the cool
temperate zone. They appear to prefer regions of steep bathymetric relief, such as the continental
shelf break, canyons, or basins situated between banks and ledges (Kenney and Winn 1987;
Schilling et al. 1992; Gregr and Trites 2001; Best and Lockyer 2002). These areas are often the
location of persistent hydrographic features, which may be important factors in concentrating
zooplankton, especially copepods. On the feeding grounds, the distribution is largely associated with
oceanic frontal systems (Horwood 1987). In the North Pacific, sei whales are found feeding
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