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

AUGUST 2005 FINAL REPORT
♦ Short-finned Pilot Whale (Globicephala macrorhynchus)
Description—There are two species of pilot whales worldwide; only the short-finned pilot whale is
expected to occur in the Marianas study area. Pilot whales are among the largest members of the
family Delphinidae. In general, the short-finned pilot whale is smaller than the long-finned pilot whale,
reaching lengths of 5.5 m (females) and 6.1 m (males) (Jefferson et al. 1993).
Pilot whales have bulbous heads, with a forehead that sometimes overhangs the rostrum; there is
little or no beak (Jefferson et al. 1993). The dorsal fin is distinctive, being generally broader-based
than it is tall. It is falcate and usually rounded at the tip, and is set well forward of the middle of the
back. The flippers of the short-finned pilot whale are long and sickle-shaped and range from 16% to
22% of the total body length (Jefferson et al. 1993). Both pilot whale species are black on the back
and sides; in many individuals, there is a light gray saddle patch located behind the dorsal fin. Pilot
whales also have a white to light gray anchor-shaped patch on the chest (Jefferson et al. 1993).
Status—This species is designated as lower risk on the IUCN Red List (Reeves et al. 2003). There
are no abundance estimates for the short-finned pilot whale in this area. Stock structure of shortfinned
pilot whales has not been adequately studied in the North Pacific, except in Japanese waters,
where two stocks have been identified based on pigmentation patterns and head shape differences of
adult males (Kasuya et al. 1988). The southern stock of short-finned pilot whales (Kasuya et al.
1988), which is probably the one associated with the Marianas Islands area, has been estimated to
number about 18,700 whales in the area south of 30°N latitude (Miyashita 1993). This is the closest
population estimate for the Marianas area.
Habitat Preferences—Pilot whales are found over the continental shelf break, in slope waters, and in
areas of high topographic relief (Olson and Reilly 2002). While pilot whales are typically distributed
along the continental shelf break, movements over the continental shelf are commonly observed in
the northeastern U.S. (Payne and Heinemann 1993) and close to shore at oceanic islands, where the
shelf is narrow and deeper waters are nearby (Mignucci-Giannoni 1998; Gannier 2000). A number of
studies in different regions suggest that the distribution and seasonal inshore/offshore movements of
pilot whales coincide closely with the abundance of squid, their preferred prey (Hui 1985; Waring et
al. 1990; Payne and Heinemann 1993; Waring and Finn 1995; Bernard and Reilly 1999). Short-finned
pilot whale occurrence in the Caribbean seems to coincide with the inshore movement of spawning
octopus (Mignucci-Giannoni 1998). Short-finned pilot whale distribution off southern California
changed dramatically after the El Niño event in 1982 through 1983, when squid did not spawn as
usual in the area, and pilot whales virtually disappeared from the area for nine years (Shane 1994).
Distribution—The short-finned pilot whale is found worldwide in tropical to warm-temperate seas,
generally in deep offshore areas. The short-finned pilot whale usually does not range north of 50ºN or
south of 40ºS (Jefferson et al. 1993). The long-finned pilot whale (Globicephala melas) is not known
to presently occur in the North Pacific (Kasuya 1975); the range of the short-finned pilot whale
appears to be expanding to fill the former range of the long-finned pilot whale (Bernard and Reilly
1999). Pilot whales are sighted throughout the Micronesia region (Reeves et al. 1999).
Information Specific to the Marianas MRA Study Area—There are a very small number of
occurrence records for the short-finned pilot whale in the Marianas study area and vicinity.
Miyashita et al. (1996) reported sightings in the vicinity of the Northern Mariana Islands during
February through March 1994, but did not provide the actual sighting coordinates. A group of
more than 30 individuals was sighted in late April 1977 near Uruno Point, off the northwest coast
of Guam (Birkeland 1977). A stranding occurred on Guam in July 1980 (Kami and Hosmer 1982;
Donaldson 1983; Schulz 1980).
Expected occurrence of the short-finned pilot whale in the Marianas study area and vicinity is
seaward of the 100 m isobath (Figure B-24). The known preference of this species globally for
steep bottom topography, which is most probably related to distribution of squid, was considered.
With a narrow shelf and deep waters in close proximity to the shore, there is also a low or
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