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

AUGUST 2005 FINAL REPORT
Spinner dolphins feed primarily on small mesopelagic fishes, squids, and sergestid shrimps, diving to
at least 200 to 300 m (Perrin and Gilpatrick 1994). Foraging takes place primarily at night when the
mesopelagic community migrates vertically towards the surface and also horizontally towards the
shore at night (Benoit-Bird et al. 2001; Benoit-Bird and Au 2004). Rather than foraging offshore for
the entire night, spinner dolphins track the horizontal migration of their prey (Benoit-Bird and Au
2003). This tracking of the prey allows spinner dolphins to maximize their foraging time while foraging
on the prey at its highest densities (Benoit-Bird and Au 2003; Benoit-Bird 2004). Spinner dolphins
begin foraging at a protruding bank, most likely because the prey layer enters shallow waters here
before anywhere else along the coast (Lammers 2004). Spinner dolphins dive to meet the rising layer
of prey organisms and forage cooperatively (Benoit-Bird and Au 2003). Life history of the spinner
dolphin has been well-described for the eastern tropical Pacific Ocean, where the species is killed in
large numbers in tuna purse seine nets (reviewed in Perrin 1998). Gestation lasts about 10 months,
length of lactation is about 1 to 2 years, and sexual maturity occurs at lengths and ages of 1.65 to
1.70 m and 4 to 7 years (females) and 1.60 to 1.80 m and 7 to 10 years (males). There is some
geographic variation, but other spinner dolphin populations probably have life history characteristics
similar to those listed.
Sazima et al. (2003) and Silva Jr. et al. (2004) reported that spinner dolphins off northeast Brazil
vomit after a meal rich in squid, and that reef fishes were observed feeding on this vomit. Würsig et
al. (1994) did not report similar vomiting behavior for Hawaiian spinner dolphins, possibly due to their
feeding primarily on fishes. Trianni and Kessler (2002) reported on a stranded individual in Saipan
whose stomach was engorged with seagrass.
Spinner dolphins are well known for their propensity to leap high into the air and spin before landing
in the water; the purpose of this behavior is unknown. Calving peaks in different spinner dolphin
populations range from late spring to fall (Jefferson et al. 1993).
Acoustics and Hearing—Pulses, whistles, and clicks have been recorded from this species. Pulses
and whistles have dominant frequency ranges of 5 to 60 kHz and 8 to 12 kHz, respectively (Ketten
1998). Spinner dolphins consistently produce whistles with frequencies as high as 16.9 to 17.9 kHz,
with a maximum frequency for the fundamental component at 24.9 kHz (Bazúa-Durán and Au 2002;
Lammers et al. 2003). Clicks have a dominant frequency of 60 kHz (Ketten 1998). The burst pulses
are predominantly ultrasonic, often with little or no energy below 20 kHz (Lammers et al. 2003).
Source levels between 195 and 222 dB have been recorded for spinner dolphin clicks (Schotten et al.
2004).
♦ Striped Dolphin (Stenella coeruleoalba)
Description—The striped dolphin is uniquely marked with black lateral stripes from eye to flipper and
eye to anus. There is also a white V-shaped “spinal blaze” originating above and behind the eye and
narrowing to a point below and behind the dorsal fin (Leatherwood and Reeves 1983). There is a dark
cape and white belly. This is a relatively robust dolphin with a long, slender beak and prominent
dorsal fin reaching 2.6 m in length.
Status—This species is designated as lower risk on the IUCN Red List (Reeves et al. 2003). The
stock structure of striped dolphins in the western Pacific is poorly known, although there is evidence
for more than one stock (Miyashita 1993). A putative population south of 30°N in the western Pacific
was estimated to number about 52,600 dolphins, and this is probably the group from which any
striped dolphins around the Marianas would come.
Habitat Preferences—Striped dolphins are usually found beyond the continental shelf, typically over
the continental slope out to oceanic waters, often associated with convergence zones and waters
influenced by upwelling (Au and Perryman 1985). In the eastern Pacific, striped dolphins inhabit
areas with large seasonal changes in surface temperature and thermocline depth, as well as
seasonal upwelling (Au and Perryman 1985; Reilly 1990). This species appears to avoid waters with
sea temperatures of less than 20ºC (Van Waerebeek et al. 1998). Off the coast of Japan, striped
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