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

AUGUST 2005 FINAL REPORT
Behavior and Life History—Melon-headed whales are typically found in large groups, ranging
between 150 and 1,500 individuals (Perryman et al. 1994; Gannier 2002), though Watkins et al.
(1997) described smaller groupings of 10 to 14 individuals. These animals often log at the water’s
surface in large schools composed of noticeable subgroups. Melon-headed whales are often found in
mixed-species aggregations, commonly with Fraser's dolphins (Miyazaki and Wada 1978; Perryman
et al. 1994; Jefferson and Barros 1997; Reeves et al. 1999; Gannier 2000), as well as on occasion,
spinner, bottlenose, and rough-toothed dolphins (Reeves et al. 1999; Gannier 2000; Perryman 2002).
Very few data are available on life history. It is unclear whether there is significant seasonality in
calving (Jefferson and Barros 1997). Females reach sexual maturity at about 11.5 years and males at
16.5 years (Jefferson and Barros 1997). These life history parameters are estimated from work on a
single school of melon-headed whales that mass-stranded at Aoshima, southern Japan (Miyazaki et
al. 1998), and therefore must be taken as highly preliminary. Melon-headed whales prey on squid,
pelagic fishes, and occasionally crustaceans. Most of the fish and squid families eaten by this species
consist of mesopelagic forms found in waters up to 1,500 m deep, suggesting that feeding takes
place deep in the water column (Jefferson and Barros 1997). There is no information on specific
diving depths for melon-headed whales.
Acoustics and Hearing—The only published acoustic information for melon-headed whales is from
the southeastern Caribbean (Watkins et al. 1997). Sounds recorded included whistles and click
sequences. Whistles had dominant frequencies around 8 to 12 kHz; higher-level whistles were
estimated at no more than 155 dB re 1 µPa-m (Watkins et al. 1997). Clicks had dominant frequencies
of 20 to 40 kHz; higher-level click bursts were judged to be about 165 dB re 1 µPa-m (Watkins et al.
1997). No data on hearing ability for this species are available.
♦ Fraser's Dolphin (Lagenodelphis hosei)
Description—The Fraser's dolphin reaches a maximum length of 2.7 m and is generally more robust
than other small delphinids (Jefferson et al. 1993). This species has a short, stubby beak, small
flippers, and a subtriangular dorsal fin. The most conspicuous feature of the Fraser's dolphin
coloration is the dark band running from the face to the anus, although it is not present in younger
animals (Jefferson et al. 1997). The stripe is set off from the surrounding areas by thin, pale, creamcolored
borders. There is also a dark chin-to-flipper stripe.
Status—This species is designated as data deficient on the IUCN Red List (Reeves et al. 2003).
There are no abundance estimates available for the Fraser’s dolphin in this area.
Habitat Preferences—This is an oceanic species, except in places where deep water approaches
the coast (Dolar 2002). Fraser’s dolphins are found close to shore in some regions, such as around
the Society Islands of French Polynesia (Gannier 2000), around several islands of the Indo-Malay
archipelago in the Indo-Pacific area (Rudolph et al. 1997), and in some waters of the Philippines
(Leatherwood et al. 1992). In the offshore eastern tropical Pacific, this species is distributed mainly in
upwelling-modified waters (Au and Perryman 1985).
Distribution—The Fraser's dolphin is found in tropical and subtropical waters around the world,
typically between 30ºN and 30ºS (Jefferson et al. 1993). Strandings in temperate areas are
considered extralimital and usually are associated with anomalously warm-water temperatures (Perrin
et al. 1994b). As noted by Reeves et al. (1999), the documented distribution of this species is skewed
towards the eastern Pacific, which may reflect the intensity of research associated with the tuna
fishery rather than an actual higher density of occurrence there than in other tropical regions.
Information Specific to the Marianas MRA Study Area—There are no occurrence records for this
species in the Marianas study area and vicinity, but this area is within the known distribution
range for this species. The Fraser’s dolphin is an oceanic species. In the Gulf of Mexico, this
species has been seen in waters over the abyssal plain (Leatherwood et al. 1993). Therefore, the
Fraser’s dolphin is expected to occur from the shelf break to seaward of the Marianas study area
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