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

AUGUST 2005 FINAL REPORT
audiogram bandwidth was up to 128 kHz at a level of 100 dB above the minimum threshold. The
minimum thresholds were observed at frequencies of 60 to 70 kHz.
♦ Risso’s Dolphin (Grampus griseus)
Description—Risso’s dolphins are moderately large, robust dolphins reaching at least 3.8 m in length
(Jefferson et al. 1993). The head is blunt, without a distinct beak, and there is a vertical crease on the
front of the melon. The dorsal fin is tall and falcate and the flippers are sickle-shaped. Young Risso’s
dolphins range from light gray to dark brownish gray and are relatively unmarked (Jefferson et al.
1993). Adults range from dark gray to nearly white and are covered with white scratches and
splotches.
Status—This species is designated as data deficient on the IUCN Red List (Reeves et al. 2003).
Essentially nothing is known of stock structure of Risso’s dolphins in the western Pacific. Assuming
that several stocks may occur there, Miyashita (1993) used Japanese survey data to estimate that
about 7,000 Risso’s dolphins occur in the area to the north of the Mariana Islands.
Habitat Preferences—A number of studies have noted that Risso’s dolphins are most commonly
found along the continental slope (CETAP 1982; Baumgartner 1997; Davis et al. 1998; Mignucci-
Giannoni 1998; Kruse et al. 1999). Baumgartner (1997) hypothesized that the strong correlation
between Risso’s dolphin distribution and the steeper portions of the upper continental slope in the
Gulf of Mexico is most likely the result of cephalopod distribution in the same area.
Distribution—The Risso’s dolphin is distributed worldwide in tropical to warm-temperate waters,
roughly between 60ºN and 60ºS, where surface water temperature is usually greater than 10ºC
(Kruse et al. 1999). Water temperature appears to be a factor that affects the distribution of Risso’s
dolphins in the Pacific (Kruse et al. 1999). Changes in local distribution and abundance along the
California coast are probably in response to protracted or unseasonal warm-water events, such as El
Niño events (Shane 1994). Changes in both abundance and shoreward shifts of Risso’s dolphin
distribution have been reported during such periods.
Information Specific to the Marianas MRA Study Area—Risso’s dolphins are expected to occur in
the Marianas study area from the shelf break to seaward of the Marianas study area and vicinity
(Figure B-18). While there is a predominance of Risso’s dolphin sightings worldwide in areas with
steep bottom topography, this species is also found in deeper waters. The largest numbers for
this species will likely be in the vicinity of the shelf break and upper continental slope (Jefferson
personal communication). There is an area of low or unknown occurrence from the 50 m isobath
to the shelf break (Figure B-18). This takes into consideration also the possibility that this
species, with a preference for waters with steep bottom topography, might swim into areas where
deep water is close to shore. Leatherwood et al. (1979) and Shane (1994) reported on sightings
of Risso’s dolphins in shallow waters in the northeastern Pacific, including near oceanic islands.
These sites are in areas where the continental shelf is narrow and deep water is closer to the
shore (Leatherwood et al. 1979; Gannier 2000, 2002). Occurrence patterns are assumed to be
the same throughout the year.
Behavior and Life History—Little is known about the life history of this species. Risso’s dolphins are
quite social; groups usually average about 30 individuals, but can range up to over several hundred
(Kruse et al. 1999), or even several thousand (Jefferson personal communication). Risso’s dolphins
occur in stable, age- and sex-segregated groups, which interact fluidly with a larger population. This
species commonly associates with other cetacean species (Kruse et al. 1999). They may remain
submerged on dives for up to 30 min; typical swimming speed is 1.67 to 1.94 m/sec (Kruse et al.
1999). Cephalopods are the primary prey (Clarke 1996).
Acoustics and Hearing—Risso’s dolphin vocalizations include broadband clicks, barks, buzzes,
grunts, chirps, whistles, and simultaneous whistle and burst-pulse sounds (Corkeron and van Parijs
2001). The combined whistle and burst pulse sound appears to be unique to Risso’s dolphin
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