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

AUGUST 2005 FINAL REPORT
(Corkeron and van Parijs 2001). Corkeron and van Parijs (2001) recorded five different whistle types,
ranging in frequency from 4 to 22 kHz. Broadband clicks had a frequency range of 6 to greater than
22 kHz. Low-frequency narrowband grunt vocalizations had a frequency range of 0.4 to 0.8 kHz. A
recent study established empirically that Risso’s dolphins echolocate; estimated source levels were
up to 216 dB re 1 µPa-m (Philips et al. 2003).
Nachtigall et al. (1995) conducted baseline audiometric work. Because of the natural background
noise (the study was conducted in a natural setting), it was not possible to precisely determine peak
(or best) hearing sensitivity in the species. Maximum sensitivity occurred between 8 and 64 kHz.
Reported thresholds were 124 dB at 1.6 kHz, 71.7 dB at 4 kHz, 63.7 dB at 8 kHz, 63.3 dB at 16 kHz,
66.5 dB at 32 kHz, 67.3 dB at 64 kHz, 74.3 dB at 80 kHz, 124.2 dB at 100 kHz, and 122.9 dB at 110
kHz.
♦ Melon-headed Whale (Peponocephala electra)
Description—Melon-headed whales at sea closely resemble pygmy killer whales; both species have
a blunt head with little or no beak. Melon-headed whales have pointed (versus rounded) flippers and
a more triangular head shape than pygmy killer whales (Jefferson et al. 1993). The body is charcoal
gray to black, with unpigmented lips (which often appear light gray, pink or white) and a white
urogenital patch (Perryman et al. 1994). This species also has a triangular face “mask” and indistinct
cape (which dips much lower below the dorsal fin than that of pygmy killer whales). Melon-headed
whales reach a maximum length of 2.75 m (Jefferson et al. 1993).
Status—There are no abundance estimates available for the melon-headed whale in this area. This
species is designated as least concern on the IUCN Red List (Reeves et al. 2003).
Habitat Preferences—Melon-headed whales are most often found in offshore, deep waters. For
example, most melon-headed whale sightings in the Gulf of Mexico have been in deep waters, well
beyond the edge of the continental shelf (Mullin et al. 1994; Davis and Fargion 1996) and in waters
over the abyssal plain (Jefferson personal communication). Nearshore sightings are generally from
areas where deep, oceanic waters are found near the coast (Perryman 2002). Melon-headed whales
are found close to shore (within a few kilometers) around the Society and Marquesas Islands of
French Polynesia (Gannier 2000, 2002), and Lembata Island of the Indonesian archipelago (Rudolph
et al. 1997), as well as in some waters of the Philippines (Leatherwood et al. 1992). In the eastern
tropical Pacific, this species is primarily found in upwelling modified and equatorial waters (Au and
Perryman 1985; Perryman et al. 1994).
Distribution—Melon-headed whales are found worldwide in tropical and subtropical waters. They
have occasionally been reported from higher latitudes, but these sightings are often associated with
incursions of warm water currents (Perryman et al. 1994). Donaldson (1983) suggested that the
presence of melon-headed whales in the waters of Guam, Palau, and Japan suggests a possible link
between equatorial Pacific and northern Pacific populations of this species, via the Mariana Islands.
Information Specific to the Marianas MRA Study Area—The melon-headed whale is an oceanic
species. There are records of its occurrence for the Marianas study area and vicinity. There was
a live stranding on the beach at Inarajan Bay, Guam in April 1980 (Kami and Hosmer 1982;
Donaldson 1983), and there have been some sightings at Rota and Guam (Michael personal
communication; Odell personal communication). Melon-headed whales are expected to occur
from the shelf break (200 m isobath) to seaward of the Marianas study area and vicinity (Figure
B-19). There is also a low or unknown occurrence from the coastline to the shelf break which
would take into account any sightings that could occur closer to shore since deep water is very
close to shore at these islands. For example, during July 2004, there was a sighting of an
estimated 500 melon-headed whales and an undetermined smaller number of rough-toothed
dolphins at Rota in waters with a bottom depth of 76 m (Michael personal communication).
Occurrence patterns are assumed to be the same throughout the year.
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