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

AUGUST 2005 FINAL REPORT
dolphins are born spotless and develop spots as they age, although the degree of spotting varies
geographically (Perrin and Hohn 1994). Adults may reach 2.6 m in length (Jefferson et al. 1993).
Status—Pantropical spotted dolphins may have several stocks in the western Pacific (Miyashita
1993), although this is not confirmed at present. There are estimated to be about 127,800 spotted
dolphins in the waters surrounding the Mariana Islands (Miyashita 1993). This species is designated
as lower risk on the IUCN Red List (Reeves et al. 2003). Three subspecies are recognized in the
Pacific Ocean. One inhabits nearshore waters around the Hawaiian Islands, another occurs in
offshore waters of the eastern tropical Pacific, and a third occurs in coastal waters between Baja
California and the northwestern coast of South America (Reeves et al. 2002).
Habitat Preferences—In the eastern Pacific, the pantropical spotted dolphin is an inhabitant of the
tropical, equatorial and southern subtropical water masses, characterized by a sharp thermocline at
less than 50 m depth, surface temperatures greater than 25ºC and salinities less than 34 ppt (Au and
Perryman 1985). Most sightings of this species in the Gulf of Mexico and Caribbean occur over the
lower continental slope (Davis et al. 1998; Mignucci-Giannoni et al. 2003). Pantropical spotted
dolphins may also be sighted in shallow waters near the edge of the continental shelf (Peddemors
1999; Gannier 2002).
Distribution—The pantropical spotted dolphin is distributed in tropical and subtropical waters
worldwide (Perrin and Hohn 1994). Range in the central Pacific is from the Hawaiian Islands in the
north to at least the Marquesas in the south (Perrin and Hohn 1994).
Information Specific to the Marianas MRA Study Area—The pantropical spotted dolphin is
primarily an oceanic species (Jefferson et al. 1993). Based on the known habitat preferences of
the pantropical spotted dolphin, this species is expected to occur seaward of the shelf break (200
m isobath) (Figure B-14). Low or unknown occurrence of the pantropical spotted dolphin from the
coastline (except in harbors and lagoons) to the shelf break (Figure B-14) is based on sightings
of pantropical spotted dolphins being reported in coastal waters of Guam by Trianni and Kessler
(2002) and coastal populations of pantropical spotted dolphins in tropical locations, such as off
Central America and Hawai’i. Occurrence patterns are assumed to be the same throughout the
year.
Behavior and Life History—Group size for the pantropical spotted dolphin may range from just a
few dolphins to several thousand (Jefferson et al. 1993). Observations of dolphin groups caught in
tuna purse seines in the eastern tropical Pacific show that there are subgroups containing mother/calf
pairs, adult males, or juveniles (Pryor and Shallenberger 1991). Pantropical spotted dolphins in the
eastern tropical Pacific Ocean and Indian Ocean are often found in mixed species aggregations that
include spinner dolphins, tunas, and various oceanic bird species. In the eastern tropical Pacific,
where this species has been best studied, there are two calving peaks–one in spring and one in fall
(Perrin and Hohn 1994). Pantropical spotted dolphins prey on epipelagic fish, squid, and crustaceans,
with mesopelagic species dominating stomach contents (Perrin and Hohn 1994; Robertson and
Chivers 1997; Wang et al. 2003). Results from various tracking and food habit studies suggest that
pantropical spotted dolphins in the eastern tropical Pacific and off Hawai’i feed primarily at night on
epipelagic species and on mesopelagic species which rise towards the water’s surface after dark
(Robertson and Chivers 1997; Scott and Cattanach 1998; Baird et al. 2001). Dives during the day are
generally shorter and more shallow than dives at night; rates of descent and ascent are higher at
night than during the day (Baird et al. 2001). Similar mean dive durations and depths have been
obtained for tagged pantropical spotted dolphins in the eastern tropical Pacific and off Hawai’i (Baird
et al. 2001).
Acoustics and Hearing—Pantropical spotted dolphin whistles have a dominant frequency range of
6.7 to 17.8 kHz (Ketten 1998). Click source levels between 197 and 220 dB have been recorded for
pantropical spotted dolphins (Schotten et al. 2004). There are no published hearing data for
pantropical spotted dolphins (Ketten 1998). Anatomy of the ear of the pantropical spotted dolphin has
been studied (Ketten 1992, 1997).
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