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

AUGUST 2005 FINAL REPORT
Gannier (2002) reviewed cetacean occurrence in various locations of the tropical Pacific and provided a
general assessment. Areas without a significant continental shelf domain do not shelter large densities of
bottlenose dolphins. A stable component of four oceanic or semi-oceanic species is present in all areas:
spinner and pantropical spotted dolphins, Risso’s dolphin, and pilot whale. Spinner and pantropical
spotted dolphins are among the two most frequent species in all areas. A general rule is that in most
tropical areas, spinner dolphins and pantropical spotted dolphins are the most common cetacean species
(Jefferson personal communication).
3.1.2.1 Marine Mammal Occurrences
As noted earlier, marine mammal occurrence in this region is not well-documented. Appendix A-3
discusses the paucity of marine mammal data for this area, and Chapter 6 provides recommendations on
how to address these data gaps, particularly in offshore waters such as W-517. The distribution of
available marine mammal records is presented for the dry (December through June) and rainy seasons
(July through November) in the maps in Appendix B. An occurrence record does not reflect the number
of marine mammals; due to the social nature of cetaceans, multiple individuals of a species are often
sighted at the same time at the same location. It should be noted that the number of marine mammal
observations in this area is a function of the level of effort to collect this information, rather than just the
actual marine mammal abundance in the area. This is evident in viewing the relatively few available
sighting and stranding records for marine mammals in the Marianas study area (Figure B-1).
A listing and description of data sources used to determine each species’ occurrence in the Marianas
study area is found in Appendix A-3, while the process used to create the map figures is described in
Section 1.4.2.2. On the map figures, various types of shading and terminology designate the occurrence
of marine mammals in the Marianas study area. “Expected occurrence” (area shaded in dark blue) is
defined as the area encompassing the expected distribution of a species based on what is known of its
habitat preferences, life history, and the available sighting and stranding data. “Concentrated occurrence”
(area shaded in green) is the subarea of a species' expected occurrence where there is the highest
likelihood of encountering that species; the designation is based primarily on areas of concentrated
sightings and preferred habitat. “Low/unknown occurrence” (light blue area) is where the likelihood of
encountering a species is rare or not known. “Occurrence not expected in the Marianas study area”
(white, unmarked area) is the area where a species encounter is not expected to occur.
In this MRA, the 200 m isobath was used as a surrogate for the location of the shelf break surrounding
the islands. The oceanic waters surrounding the Marianas study area do not contain a true continental
shelf, and therefore no true shelf break, the region in which there is a sharp break in the slope of the
island shelf (Kennett 1982; Thurman 1997). Rather, the Mariana archipelago is composed of a series of
volcanic islands that have broken the sea surface (Eldredge 1983) where the offshore bottom topography
is steep and slopes rapidly reaching the ocean basin floor (4,000 m) within tens of kilometers (see
Figures 2-1 and 2-2).
Each marine mammal species is listed below with its description, status, habitat preference, distribution
(including location and seasonal occurrence in the Marianas MRA study area), behavior and life history,
and information on its acoustics and hearing ability. Species appearance within the text begins with
threatened and endangered marine mammals, while the remaining species follow the taxonomic order as
presented in Table 3-1.
3.1.2.2 Threatened and Endangered Marine Mammals of Guam and the Commonwealth of the
Northern Mariana Islands
There are eight marine mammal species that are listed as endangered under the ESA with confirmed or
possible occurrence in the study area: North Pacific right whale, humpback whale, sei whale, fin whale,
blue whale, sperm whale, Hawaiian monk seal, and the dugong. Most of the cetacean species are
expected to occur in the study area. There are very rare sightings of the dugong here and no confirmed
records for the North Pacific right whale or Hawaiian monk seal.
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