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

AUGUST 2005 FINAL REPORT
Information Specific to the Marianas MRA Study Area—The dugong is not expected to occur in
the Marianas study area during any time of the year. The endangered status of this species
necessitates mentioning these extralimital occurrences in the Marianas study area. Nishiwaki et
al. (1979) reviewed dugong distribution worldwide and concluded that this species likely never
occurred in the Mariana Islands. There have been extralimital sightings recorded in this area,
including a sighting in Cocos Lagoon at the southern end of Guam in 1974 (Anonymous 1974;
Randall et al. 1975); Eldredge (1991, 2003) erroneously noted the year to be 1975. Several
sightings were made in 1985 (date unspecified) of a dugong along the southeastern coast of
Guam (Eldredge 1991, 2003). As noted earlier, Palau is the closest location (1,076 km) of regular
occurrence of the dugong to the Mariana Islands. Dugongs are known to travel over deep waters
for long distances; they can occur in both shallow and deep waters.
Behavior and Life History—Most dugongs are sighted alone or in groups of two animals (Marsh
2002; Davis 2004). Large aggregations of up to several hundred animals are regularly seen at some
locations, feeding on seagrass meadows (Marsh 2002; Lanyon 2003). Dugongs are slow-growing
mammals, with females only reaching sexual maturity after 6 to 17 years and producing a single calf
every 2.5 to 5 years (Marsh et al. 2002). The estimated gestation period is about 13 months; calves
are nursed for at least 13 months (Marsh 1995). Dugongs start eating seagrasses soon after birth
(Marsh et al. 2002). Dugongs are benthic feeding specialists; depending on the species of seagrass
being consumed, dugongs target either the leaves alone or uproot the entire plant and consume the
leaves, roots, and rhizomes, leaving distinct feeding scars (Preen 1995a; Marshall et al. 2003).
Seagrasses of the genera Halophila and Halodule are preferred (Marsh 2002). Marine algae are also
eaten. In northern Australia, Whiting (2002) documented long-term feeding by dugongs on algalcovered
rocky reefs in the intertidal zone. It is not known whether algae form a minor part of their diet
or if dugongs only feed on algae when seagrasses are scarce (Whiting 2002). Dugongs are known to
occasionally supplement their herbivorous diet with macro-invertebrates (Preen 1995b). The highly
specialized dietary requirements of the dugong suggest that only certain seagrass meadows may be
suitable as dugong habitat (Preen 1995a). Maximum recorded dive depth for a tagged dugong is 20.5
m, although as noted earlier, maximum dive depths of up to 33 m are inferred from feeding scars in
seagrass meadows (Chilvers et al. 2004). Maximum dive time at depths greater than 1.5 m is 12.3
min (Chilvers et al. 2004). Typical mean dive duration for depths less than 3 m from the water’s
surface is 2.7 + 0.17 min (Chilvers et al. 2004).
Acoustics and Hearing—Dugongs produce complex sounds such as chirp-squeaks, barks, and trills
(Anderson and Barclay 1995). Chirp-squeaks are in the 3 to 18 kHz range and last approximately 60
msec (Anderson and Barclay 1995). Trills last as long as 2,200 msec and are within the 3 to 18 kHz
frequency band (Anderson and Barclay 1995). Barks are broadband signals of 500 to 2,200 Hz
lasting 30 to 120 msec (Anderson and Barclay 1995). No hearing data are available for this species
(Ketten 1998).
3.1.2.3 Non-Threatened and Non-Endangered Marine Mammal Species of Guam and the Northern
Mariana Islands
There are 21 non-endangered/non-threatened marine mammal species: two baleen whale species, 18
toothed whale species, and one pinniped species. For most of the marine mammal species occurring in
the Guam and the Northern Mariana Islands, there are few occurrence records. Many species in this area
are known only from stranding records or are suspected to occur in the area. For species with few
records, this may be a result of difficulty in visually detecting the species, identifying it, or due to
extralimital occurrences of the species.
♦ Minke Whale (Balaenoptera acutorostrata)
Description—The minke whale is the smallest balaenopterid species in the western North Atlantic,
with adults reaching lengths of just over 9 m (Jefferson et al. 1993). The head is extremely pointed,
and the median head ridge is prominent. The dorsal fin is tall (for a baleen whale), recurved, and is
located about two-thirds of the way back from the snout tip (Jefferson et al. 1993). The minke’s
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