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

AUGUST 2005 FINAL REPORT
where sperm whales are found in shelf waters, this is likely due to prey occurrence. There are two
stranding records for this area (Kami and Lujan 1976; Eldredge 1991, 2003). In June 2001, a
group of sperm whales that included a newborn calf was sighted off the west coast of Guam
(Eldredge 2003). Sperm whale occurrence patterns are assumed to be similar throughout the
year. North of the map extent in Figure B-7 is one sighting of 3 sperm whales made 8.3 km north
of Anatahan during March 2003 (Vogt 2004).
Behavior and Life History—Female sperm whales live a highly social life, while large male sperm
whales typically occur alone or in pairs, at times joining groups of adult females for breeding
(Whitehead 2003). Female and immature sperm whales form groups that move together in a
coordinated fashion over periods of days (Whitehead 2003). Mean group size is approximately 20 to
30 individuals, although there is much variation (Whitehead 2003). For a review of sperm whale social
organization, see Whitehead and Weilgart (2000) and Whitehead (2003). Mating behavior is observed
from winter through summer and calving during spring through fall. Gestation is 14 to 15 months,
lactation is approximately 2 years, and the typical inter-birth interval is 4 to 7 years. Sperm whales
prey on large mesopelagic squid and other cephalopods as well as demersal fishes and occasionally
benthic invertebrates (Rice 1989; Clarke 1996).
Sperm whales forage during deep dives that routinely exceed a depth of 400 m and 30 min duration
(Watkins et al. 2002). Sperm whales are capable of diving to depths of over 2,000 m with durations of
over 60 min (Watkins et al. 1993). Sperm whales spend up to 83% of daylight hours underwater
(Jaquet et al. 2000; Amano and Yoshioka 2003). Males do not spend extensive periods of time at the
surface (Jaquet et al. 2000). In contrast, females spend prolonged periods of time at the surface (1 to
5 hours [hrs] daily) without foraging (Whitehead and Weilgart 1991; Amano and Yoshioka 2003). The
average swimming speed is estimated to be 0.7 m/sec (Watkins et al. 2002). Dive descents averaged
11 min at a rate of 1.52 m/sec, and ascents averaged 11.8 min at a rate of 1.4 m/sec (Watkins et al.
2002).
Acoustics and Hearing—Sperm whales produce short-duration (generally less than 3 sec),
broadband clicks. These clicks range in frequency from 100 Hz to 30 kHz, with dominant energy in
two bands (2 to 4 kHz and 10 to 16 kHz). Generally, most of the acoustic energy is present at
frequencies below 4 kHz, although diffuse energy up to past 20 kHz has been reported (Thode et al.
2002). The source levels can be up to 236 dB re 1 µPa-m (Møhl et al. 2003). Thode et al. (2002)
suggested that the acoustic directivity (angular beam pattern) from sperm whales must range
between 10 and 30 dB in the 5 to 20 kHz region. The clicks of neonate sperm whales are very
different from usual clicks of adults in that they are of low directionality, long duration, and lowfrequency
(centroid frequency between 300 and 1,700 Hz) with estimated source levels between 140
and 162 dB re 1 µPa-m (Madsen et al. 2003). Clicks are heard most frequently when sperm whales
are engaged in diving/foraging behavior (Whitehead and Weilgart 1991; Miller et al. 2004; Zimmer et
al. 2005). These may be echolocation clicks used in feeding, contact calls (for communication), and
orientation during dives. When sperm whales are socializing, they tend to repeat series of clicks
(codas), which follow a precise rhythm and may last for hours (Watkins and Schevill 1977). Codas are
shared between individuals of a social unit and are considered to be primarily for intragroup
communication (Weilgart and Whitehead 1997; Rendell and Whitehead 2004).
The anatomy of the sperm whale’s ear indicates that it hears high-frequency sounds (Ketten 1992).
Anatomical studies also suggest that the sperm whale has some ultrasonic hearing, but at a lower
maximum frequency than many other odontocetes (Ketten 1992). The sperm whale may also
possess better low-frequency hearing than some other odontocetes, although not as extraordinarily
low as many baleen whales (Ketten 1992). Auditory brainstem response in a neonatal sperm whale
indicated highest sensitivity to frequencies between 5 and 20 kHz (Ridgway and Carder 2001).
♦ Hawaiian Monk Seal (Monachus schauinslandi)
Description—Hawaiian monk seals are similar in body shape to female and young elephant seals,
with a moderately robust, spindle-shaped body and short muzzle. Adults are 2.1 to 2.4 m in length
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