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

AUGUST 2005 FINAL REPORT
dolphins congregate at the periphery of the Kuroshio Current where warm water meets up with cold
water (Miyazaki et al. 1974). Gannier (1999) noted diel variations in distribution in the northwestern
Mediterranean Sea consistent with nocturnal feeding by dolphins close to the shelf break with a
diurnal offshore-inshore movement.
Distribution—The striped dolphin has a worldwide distribution in cool-temperate to tropical waters.
This species is well documented in both the western and eastern Pacific off the coasts of Japan and
North America (Perrin et al. 1994a); the northern limits are the Sea of Japan, Hokkaido, Washington
state, and along roughly 40ºN across the western and central Pacific (Reeves et al. 2002). Scattered
records exist from the South Pacific as well (Perrin et al. 1994a).
Information Specific to the Marianas MRA Study Area—Striped dolphins are not common in most
truly tropical areas (Jefferson personal communication). There is only one record of striped
dolphin occurrence in the Marianas study area and vicinity—a stranding that occurred in July
1985 (Wilson et al. 1987; Eldredge 1991, 2003). Eldredge (1991) erroneously reported that
additional records, aside from the stranding he documented, appeared in Wilson et al. (1987); it is
actually the same record. There is a low or unknown occurrence of the striped dolphin in this area
seaward of the 100 m isobath (Figure B-16), since this is an oceanic species. Occurrence
patterns are assumed to be the same throughout the year.
Behavior and Life History—Striped dolphins are found in groups numbering between 100 and 500
individuals, though sometimes they gather in the thousands. Striped dolphins have been found in
association with seabirds and other species of marine mammals (Baird et al. 1993).
Life history information is based mostly on western North Pacific specimens (Archer and Perrin 1999).
Males reach sexual maturity between 7 and 15 years of age, at an average body length of 2.2 m.
Females become sexually mature between 5 and 13 years of age (Archer and Perrin 1999). Off
Japan, where their biology has been best studied, there are two calving peaks: one in summer,
another in winter (Perrin et al. 1994a).
Striped dolphins often feed in pelagic or benthopelagic zones along the continental slope or just
beyond oceanic waters. A majority of the prey possess luminescent organs, suggesting that striped
dolphins may be feeding at great depths, possibly diving to 200 to 700 m to reach potential prey
(Archer and Perrin 1999). Striped dolphins may feed at night, in order to take advantage of the deepscattering
layer's diurnal vertical movements. Small, mid-water fishes (in particular, myctophids or
lanternfish) and squids are the dominant prey (Perrin et al. 1994a).
Acoustics and Hearing—Striped dolphin whistles range from 6 to 24+ kHz, with dominant
frequencies ranging from 8 to 12.5 kHz (Thomson and Richardson 1995). The striped dolphin’s range
of most sensitive hearing (defined as the frequency range with sensitivities within 10 dB of maximum
sensitivity) was determined to be 29 to 123 kHz using standard psycho-acoustic techniques;
maximum sensitivity occurred at 64 kHz (Kastelein et al. 2003). Hearing ability became less sensitive
below 32 kHz and above 120 kHz (Kastelein et al. 2003).
♦ Short-beaked Common Dolphin (Delphinus delphis)
Description—There are two species in the genus Delphinus: the long-beaked common dolphin
(Delphinus capensis) and the short-beaked common dolphin (Delphinus delphis) (Heyning and Perrin
1994). The short-beaked common dolphin is the species most likely to occur in the Marianas MRA
study area (Jefferson personal communication).
Short-beaked common dolphins are moderately slender animals with a relatively long beak and a tall,
slightly falcate dorsal fin. Common dolphins are distinctively marked; they have a V-shaped dark
saddle that produces an hourglass pattern on the side of the body (Jefferson et al. 1993). The back is
a dark, brownish gray; the belly is white; and the anterior flank patch is tan to cream in color. The lips
are dark and there is a stripe running from the apex of the melon to encircle the eye. There is also a
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