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

AUGUST 2005 FINAL REPORT
black to dark gray chin-to-flipper stripe. Adults can reach lengths of up to 2.6 m (Jefferson et al.
1993).
Status—There are no abundance estimates for the short-beaked common dolphin in this area. This
species is designated as least concern on the IUCN Red List (Reeves et al. 2003).
Habitat Preferences—Common dolphins occupy a wide range of habitats, including waters over the
continental shelf, along the continental shelf break, and over prominent underwater topography (e.g.,
seamounts) (Hui 1979; Evans 1994). The long-beaked common dolphin appears to be restricted to
waters relatively close to shore (Jefferson and Van Waerebeek 2002); there is no known occurrence
for this species around archipelagos great distance from a continent (Jefferson personal
communication). Common dolphins in some populations appear to preferentially travel along bottom
topographic features such as escarpments and seamounts (Evans 1994). In tropical regions, where
common dolphins are routinely sighted, they are found in upwelling-modified waters (Au and
Perryman 1985; Ballance and Pitman 1998; Reilly 1990). Common dolphins prefer areas with large
seasonal changes in surface temperature and thermocline depth (Au and Perryman 1985).
Distribution—Delphinus is a widely distributed genus of cetacean. It is found worldwide in
temperate, tropical, and subtropical seas. The range of the short-beaked common dolphin may
extend entirely across the tropical and temperate North Pacific (Heyning and Perrin 1994). All animals
observed at sea and collected as specimens from the offshore eastern tropical Pacific, ranging as far
south as northern Peru, have been of the short-beaked form (Heyning and Perrin 1994). In the
eastern North Pacific, all sightings of long-beaked common dolphins have been within about 100 NM
of shore (Heyning and Perrin 1994).
Information Specific to the Marianas MRA Study Area—There are no occurrence records for this
species in the Marianas study area and vicinity, but this area is within the known distribution
range for this species. There is a low or unknown occurrence of the short-beaked common
dolphin from the shelf break to seaward of the Marianas study area and vicinity (Figure B-17).
Short-beaked common dolphins are thought to be more common in cool temperate waters of the
North Pacific, although there are populations in cooler, upwelling modified waters of the eastern
tropical Pacific (Au and Perryman 1985). The absence of known areas of major upwelling in the
western tropical Pacific suggests that common dolphins will not be found there, although there
have been some reports of sightings of this species (Masaki and Kato 1979). However, the
species identification of these records is not confirmed, and therefore is in doubt. Occurrence
patterns are assumed to be the same throughout the year.
Behavior and Life History—Group size ranges from several dozen to over 10,000 (Jefferson et al.
1993). Common dolphins are fast-moving swimmers, active bowriders, and often jump in the air.
Calving peaks differ from stock to stock. Calving peaks in spring and autumn, or spring and summer,
have been reported for various populations (Jefferson et al. 1993).
Delphinus feed on a wide variety of epipelagic and mesopelagic schooling fishes and squids
associated with the deep scattering layer (Ohizumi et al. 1998). Common dolphins feed
opportunistically on those species most abundant locally and change their diet according to
fluctuations in the abundance and availability of prey (Young and Cockcroft 1994). Based on a small
sample size from the eastern North Pacific, the short-beaked common dolphin may feed more
extensively on squid than the long-beaked form (Heyning and Perrin 1994). Diel fluctuations in vocal
activity of this species (more vocal activity during late evening and early morning) appear to be linked
to feeding on the deep scattering layer as it rises during the same time (Goold 2000).
Acoustics and Hearing—Recorded Delphinus vocalizations include whistles, chirps, barks, and
clicks (Ketten 1998). Clicks and whistles have dominant frequency ranges of 23 to 67 kHz and 0.5 to
18 kHz, respectively (Ketten 1998). Popov and Klishin (1998) recorded auditory brainstem responses
from a common dolphin. The audiogram was U-shaped with a steeper high-frequency branch. The
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