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

AUGUST 2005 FINAL REPORT
range for this species. Identification to species for Tursiops can be difficult for the inexperienced
observer. However, any occurrence of bottlenose dolphins in the Marianas study area would
likely be of the common bottlenose dolphin (Jefferson personal communication; Perrin personal
communication). Miyashita (1993) reported that all his sightings of bottlenose dolphins in the
western Pacific were of a larger, unspotted type (presumably the common bottlenose dolphin, as
opposed to the Indo-Pacific bottlenose dolphin). The Indo-Pacific bottlenose dolphin is considered
to be a species associated with continental margins, as it does not appear to occur around
offshore islands great distances from a continent, such as the Marianas (Jefferson personal
communication). However, since the Indo-Pacific bottlenose dolphin occurs directly west and to
the south of the Marianas study area, there is the possibility of extralimital occurrences of this
species.
Bottlenose dolphins are expected to occur from the coastline to the 2,000 m isobath (Figure B-
13), which takes into consideration the known habitat preferences of Tursiops globally. Individuals
are expected to occur in both harbors and lagoons based on observations worldwide in similar
habitats. There is a low or unknown occurrence of the bottlenose dolphin seaward of the 2,000 m
isobath (Figure B-13). This pattern takes into account possible movement by bottlenose dolphins
between the CNMI chain, as well as sightings globally in deep waters. Occurrence patterns are
expected to be the same throughout the year.
Interestingly, there are no stranding records available for this species in the Marianas study area
and vicinity, and only a mention by Trianni and Kessler (2002) that bottlenose dolphins are seen
in coastal waters of Guam. It is possible that bottlenose dolphins do not occur in great numbers in
this island chain. Gannier (2002) attributed the fact that large densities of bottlenose dolphins do
not occur at the Marquesas Islands to the fact that the area does not have a significant shelf
component. A similar situation could be occurring in the Marianas MRA study area and vicinity.
Behavior and Life History—Tursiops are very gregarious; they are typically found in groups of 2 to
15 individuals, although groups of up to 100 or more have been reported in some areas (Shane et al.
1986). Based on photo-identification techniques using dorsal fin shapes and markings (Würsig and
Würsig 1977; Würsig and Jefferson 1990), it is well known that Tursiops has a fluid social
organization (Connor et al. 2000). Habitat structure, in terms of complexity and water depth, is
generally a major force that shapes Tursiops groupings (Shane et al. 1986). Shallow-water areas
typically have smaller group sizes than open or oceanic areas (Wells et al. 1980). Open coastlines,
however, differ in habitat structure and prey distribution from more protected areas. Protected areas
have been found to foster relatively small school sizes, some degree of regional site fidelity, and
limited movement patterns (Wells et al. 1987). In contrast, semi-open habitats often sustain larger
school sizes, diminished levels of site fidelity, and more expansive home ranges (Defran and Weller
1999). In waters of the eastern tropical Pacific, group size estimates range into the thousands, and
herds of over 100 are not uncommon (Scott and Chivers 1990). In the tropical Pacific, bottlenose
dolphin groups around offshore islands and near coastlines average between 72 and 94 individuals,
and groups in more offshore locations average around 40 to 44 dolphins (Scott and Chivers 1990).
Along the Atlantic coast of the U.S., where the majority of detailed work on bottlenose dolphins has
been conducted, male and female bottlenose dolphins reach physical maturity at 13 years, with
females reaching sexual maturity as early as 7 years (Mead and Potter 1990). Bottlenose dolphins
are flexible in their timing of reproduction. Seasons of birth for bottlenose dolphin populations are
likely responses to seasonal patterns of availability of local resources (Urian et al. 1996). For the
same central U.S. Atlantic coast areas, Hohn (1980) reported one (spring) and possibly two calving
seasons (spring and fall), whereas Mead and Potter (1990) reported a prolonged calving season with
a spring peak. There is a gestation period of one year (Caldwell and Caldwell 1972). Calves of
bottlenose dolphins typically remain with their mothers for 3 to 6 years (Wells et al. 1987). In the
Pacific, there is clearly much geographical variation among various coastal and offshore populations
(Walker 1981; Kasuya et al. 1997). In Japanese waters, calves are born at a length of around 128
cm, with a calving peak in June (Kasuya et al. 1997). Sexual maturity appears to be reached at
similar ages as in the Atlantic (5 to 13 years for females and 9 to 11+ for males; Kasuya et al. 1997).
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