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

AUGUST 2005 FINAL REPORT
range for this species. Beaked whales may be expected to occur in the area including around
seaward of the shelf break (Figure B-11). There is a low or unknown occurrence of beaked
whales on the shelf between the 50 m isobath and the shelf break, which takes into account that
deep waters come very close to the shore in this area. In some locales, beaked whales can be
found in waters over the shelf, so it is possible that beaked whales have similar habitat
preferences here. Occurrence patterns are expected to be the same throughout the year.
Longman’s beaked whale is not as rare as previously thought. However, the frequency with which
it has been sighted in the eastern and western tropical Pacific oceans (MacLeod et al. in press)
suggests that it is probably not as common as the Cuvier’s and Mesoplodon beaked whales
(Ferguson et al. 2001).
Behavior and Life History—Groups are relatively large and range in size from one to 100
individuals; mean group size for this species is 18.5 individuals (Pitman et al. 1999). There is no
available information on life history for Longman’s beaked whale (Dalebout et al. 2003).
All species of beaked whales probably feed at or close to the bottom in deep oceanic waters, taking
whatever suitable prey they encounter or feeding on whatever species are locally abundant (MacLeod
et al. 2003). There is no information available regarding feeding behavior and preferred prey of the
Longman’s beaked whale. Analysis of stomach contents from captured and stranded individuals
suggests that beaked whales are deep-diving animals, feeding by suction predominantly on
mesopelagic fish and squid or deepwater benthic invertebrates (Mead 1989; Heyning and Mead
1996; Santos et al. 2001; MacLeod et al. 2003). Individual dive times of 18 and 25 min have been
reported for this species (Gallo-Reynoso and Figueroa-Carranza 1995).
Acoustics and Hearing—MacLeod (1999) suggested that beaked whales use frequencies of
between 300 Hz and 129 kHz for echolocation, and between 2 and 10 kHz, and possibly up to 16
kHz, for social communication. There is no information available for Longman’s beaked whale
vocalizations.
There is no direct information available on the exact hearing abilities of beaked whales (MacLeod
1999). Beaked whale ears are predominantly adapted to hear ultrasonic frequencies (MacLeod
1999). Based on the anatomy of the ears of beaked whales, these species may be more sensitive
than other cetaceans to low frequency sounds; however, as noted earlier, there is no direct evidence
to support this idea (MacLeod 1999).
♦ Rough-toothed Dolphin (Steno bredanensis)
Description—This is a relatively robust dolphin with a cone-shaped head and the only one with no
demarcation between the melon and beak (Jefferson et al. 1993). The “fore-head” slopes smoothly
from the blowhole onto the long narrow neck (Reeves et al. 2002). The rough-toothed dolphin has
large flippers that are set far back on the sides and a prominent falcate dorsal fin (Jefferson et al.
1993). The body is dark gray, with a prominent narrow dorsal cape that dips slightly down onto the
side below the dorsal fin. The lips and much of the lower jaw are white, and many individuals have
white scars. The rough-toothed dolphin reaches 2.8 m in length (Jefferson et al. 1993).
Status—The rough-toothed dolphin is designated as data deficient on the IUCN Red List (Reeves et
al. 2003). There are no abundance estimates for this species in this area. Rough-toothed dolphins are
common in tropical areas, but not nearly as abundant as some other dolphin species (Reeves et al.
2002). Nothing is known about stock structure for the rough-toothed dolphin in the North Pacific
(Carretta et al. 2004).
Habitat Preferences—The rough-toothed dolphin is regarded as an offshore species that prefers
deep waters; however, it can occur in waters with variable bottom depths (e.g., Gannier and West
2005). It rarely occurs close to land, except around islands with steep drop-offs nearshore (Reeves et
al. 2002; Gannier and West 2005). In the Gulf of Mexico, the rough-toothed dolphin occurs primarily
over the deeper waters off the continental shelf (bottom depths of 950 to 1,100 m) (Davis et al. 1998),
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