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

AUGUST 2005 FINAL REPORT
locale. Rock (1993) reported that killer whales have been reported in the tropical waters around
Guam, Yap, and Palau “for years.” There is, however; a paucity of sighting documentation to
substantiate this claim (Reeves et al. 1999; Visser and Bonoccorso 2003). There are a few
sightings (most are unconfirmed) of killer whales off Guam (Eldredge 1991). Gerry Davis
(personal communication) has observed killer whales off Cocos Island, Guam on more than one
occasion and off Galvez Bank, Guam. There was also a confirmed sighting of these animals by
the USCG in the late 1980s off Santa Rosa Bank, Guam (Davis personal communication). One
sighting of a killer whale was made 27 km west of Tinian during January 1997 and reported to the
NOAA Fisheries Platforms of Opportunity Program. There was also a badly decomposed killer
whale found stranded on Guam in August 1981 (Kami and Hosmer 1982). Killer whales are
infrequently sighted and found stranded around the Hawaiian Islands (Shallenberger 1981;
Tomich 1986; Mobley et al. 2001; Baird et al. 2003a), though with increasing numbers of boaters,
sightings each year could be expected (Baird personal communication). Since this species has a
sporadic occurrence in tropical waters and can be found in both coastal areas and the open
ocean, there is a low or unknown occurrence of the killer whale from the coastline (except in the
industrial Apra Harbor) to seaward of the Marianas study area and vicinity (Figure B-23).
Occurrence patterns are assumed to be the same throughout the year.
Behavior and Life History—Killer whales have the most stable social system known among all
cetaceans. In all areas where longitudinal studies have been carried out, there appear to be long-term
associations between individuals and limited dispersal from maternal groups called pods (Baird
2000).
Killer whales have a diverse diet, feeding on bony fishes, elasmobranchs, cephalopods, seabirds, sea
turtles, and other marine mammals (Jefferson et al. 1991; Fertl et al. 1996). Diet is specific to the type
of killer whale. Transients are primarily mammal-eaters, residents are mostly fish-eaters, and
offshores appear to eat mostly fish as well. Killer whales in the tropics have been observed feeding
on fishes, elasmobranchs, and sea turtles (e.g., Fertl et al. 1996; Gannier 2002; Visser and
Bonoccorso 2003; Pitman and Dutton 2004). Killer whale interference with fisheries in the tropics is
well known; for example, they remove fish from longlines (Iwashita et al. 1963; Visser and
Bonoccorso 2003). Killer whales use passive listening as a primary means of locating prey and use
different echolocation patterns for different hunting strategies (Barrett-Lennard et al. 1996). For
example, they mask their clicks and encode their signals in background noise when hunting other
cetaceans, prey that can hear their high-frequency clicks. In contrast, killer whales do not mask their
high-frequency signals when hunting fish that are not capable of hearing in this frequency range.
The maximum depth recorded for free-ranging killer whales diving off British Columbia is 264 m
(Baird et al. 2005b). On average, however, for seven tagged individuals, less than 1% of all dives
examined were to depths greater than 30 m (Baird et al. 2003a). A trained killer whale dove to a
maximum of 260 m (Dahlheim and Heyning 1999). The longest duration of a recorded dive from a
radio-tagged killer whale was 17 min (Dahlheim and Heyning 1999).
Acoustics and Hearing—The killer whale produces a wide variety of clicks and whistles, but most of
its sounds are pulsed and at 1 to 6 kHz (Thomson and Richardson 1995). Source levels of
echolocation signals range between 195 and 224 dB re 1 µPa-m (Au et al. 2004). Acoustic studies of
resident killer whales in British Columbia have found that there are dialects, in their highly
stereotyped, repetitive discrete calls, which are group-specific and shared by all group members
(Ford 2002). These dialects likely are used to maintain group identity and cohesion, and may serve
as indicators of relatedness that help in the avoidance of inbreeding between closely related whales
(Ford 2002). Dialects also have been documented in killer whales occurring in northern Norway, and
likely occur in other locales as well (Ford 2002).
The killer whale has the lowest frequency of maximum sensitivity and one of the lowest highfrequency
hearing limits known among toothed whales (Szymanski et al. 1999). The upper limit of
hearing is 100 kHz for this species. The most sensitive frequency, in both behavioral and in auditory
brainstem response audiograms, has been determined to be 20 kHz (Szymanski et al. 1999).
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