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

AUGUST 2005 FINAL REPORT
♦ Spinner Dolphin (Stenella longirostris)
Description—This is a slender dolphin that has a very long, slender beak (Jefferson et al. 1993). The
dorsal fin ranges from slightly falcate to triangular or even canted forward. The spinner dolphin
generally has a dark eye-to-flipper stripe and dark lips and beak tip (Jefferson et al. 1993). This
species typically has a three-part color pattern (dark gray cape, light gray sides, and white belly).
Adults can reach 2.4 m in length (Jefferson et al. 1993). There are four known subspecies of spinner
dolphins, and probably other undescribed ones (Perrin 1998; Perrin et al. 1999). These include a
globally-distributed nominal subspecies, which is probably the one that occurs in the Marianas (S. l.
longirostris), as well as several forms with restricted distributions that almost certainly do not occur
there (S. l. orientalis, S. l. centroamerica, and S. l. roseiventris).
Status—There are no estimates of abundance for the spinner dolphin in this area. The spinner is the
most common species in the Marianas study area. This species is designated as lower risk on the
IUCN Red List (Reeves et al. 2003).
Habitat Preferences—Spinner dolphins occur in both oceanic and coastal environments. Most
sightings of this species have been associated with inshore waters, islands, or banks (Perrin and
Gilpatrick 1994). Oceanic populations, such as those in the eastern tropical Pacific, often are found in
waters with a shallow thermocline (Au and Perryman 1985; Reilly 1990). The thermocline
concentrates pelagic organisms in and above it, upon which the dolphins feed. Spinner dolphins are
associated with tropical surface water typified by extensive stable thermocline ridging and relatively
little annual variation in surface temperature (Reeves et al. 1999). Coastal populations usually are
found in island archipelagos, where they are tied to trophic and habitat resources associated with the
coast (Norris and Dohl 1980; Poole 1995). Norris et al. (1994) suggested that the availability of prey
and resting habitats are the primary limiting factors influencing the occurrence of spinner dolphins in
Hawai’i. As noted by Lammers (2004), presumably these are the same constraints faced by
populations at other islands.
Spinner dolphins at islands and atolls rest during daytime hours in shallow, wind-sheltered nearshore
waters and forage over deep waters at night (Norris et al. 1994; Östman 1994; Poole 1995; Gannier
2000, 2002; Lammers 2004; Östman-Lind et al. 2004). Suitable habitat for resting includes bay
complexes around islands (Poole 1995), or shallow waters near the coast (Lammers 2004); Lammers
(2004) noted a preference for the 10 fathom isobath. It should be noted, however, that while certain
bays or shoal areas are repeatedly occupied, others are only infrequently visited, or not occupied at
all (Norris et al. 1985; Poole 1995). Spinner dolphins often rest in lagoons (Gannier 2000; Trianni and
Kessler 2002). Preferred resting habitat is usually more sheltered from prevailing tradewinds than
adjacent areas and the bottom substrate is generally dominated by large stretches of white sand
bottom rather than the prevailing reef and rock bottom along most other parts of the coast (Norris et
al. 1994; Lammers 2004). These clear, calm waters and light bottom substrates provide a less cryptic
backdrop for predators like tiger sharks (Norris et al. 1994; Lammers 2004).
Distribution—The spinner dolphin is found in tropical and subtropical waters worldwide. Limits are
near 40ºN and 40ºS (Jefferson et al. 1993). These dolphins occur near islands such as Hawai’i, the
Mariana Islands, the South Pacific, the Caribbean, and Fernando de Noronha Island off Brazil.
Spinner dolphins have been documented to travel distances of 40 km between islands in the main
Hawaiian Islands (Maldini 2003). Long-term studies of island-associated spinner dolphins in the
Pacific have been conducted since the 1970s along the Kona coast of Hawai’i (Norris et al. 1994;
Östman 1994; Östman-Lind et al. 2004) and since the 1980s at Mo’orea, French Polynesia (Poole
1995). In Hawai’i, spinner dolphins occur along the leeward coasts of all the major islands and around
several of the atolls northwest of the main island chain. Long-term site fidelity has been noted for
spinner dolphins along the Kona coast of the island of Hawai’i, along O’ahu, and off the island of
Moorea in the Society Islands (Norris et al. 1994; Östman 1994; Poole 1995; Marten and Psarakos
1999), with some individuals being sighted for up to 12 years at Moorea (Poole 1995).
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