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

AUGUST 2005 FINAL REPORT
coloration is distinct: dark gray dorsally, white beneath, with streaks of intermediate shades on the
sides (Stewart and Leatherwood 1985). The most distinctive light marking is a brilliant white band
across each flipper of northern hemisphere minke whales (Stewart and Leatherwood 1985).
Status—The IWC recognizes three stocks of minke whales in the North Pacific: one in the Sea of
Japan/East China Sea, one in the rest of the western Pacific west of 180º N, and one in the
remainder of the Pacific (Donovan 1991). Smith (2003) suggested that there are only three breeding
groups in the western Northern Pacific, and that at least one of these (group W) breeds in the vicinity
of the Bonin-Japan Trench during winter. This would presumably correlate to minke whales observed
around the Marianas, but it should be noted that this is still very speculative, and is based primarily on
modeling results (with few supporting data).
The minke whale is designated as near threatened on the IUCN Red List (Reeves et al. 2003). There
are no abundance estimates for this species in this area; Horwood (1990) noted that densities of
minke whales throughout the North Pacific are low.
Habitat Preferences—The minke whale generally occupies waters over the continental shelf,
including inshore bays and estuaries (Mitchell and Kozicki 1975; Ivashin and Vitrogov 1981; Murphy
1995; Mignucci-Giannoni 1998; Calambokidis et al. 2004). However, based on whaling catches and
surveys worldwide, there is also a deep-ocean component to the minke whale’s distribution (Slijper et
al. 1964; Horwood 1990; Mitchell 1991; Mellinger et al. 2000; Roden and Mullin 2000). Dorsey et al.
(1990) noted minke whales feeding in locations of strong tidal currents in inshore waters.
Distribution—Minke whales are distributed in polar, temperate, and tropical waters (Jefferson et al.
1993); they are less common in the tropics than in cooler waters. Minke whales are present in the
North Pacific from near the equator to the Arctic (Horwood 1990). The summer range extends to the
Chukchi Sea (Perrin and Brownell 2002). In the winter, minke whales are found south to within 2° of
the equator (Perrin and Brownell 2002). The distribution of minke whale vocalizations (specifically,
“boings”) suggests that the winter breeding grounds are the offshore tropical waters of the North
Pacific Ocean (Rankin and Barlow 2003). There is no obvious migration from low-latitude, winter
breeding grounds to high-latitude, summer feeding locations in the western North Pacific, as there is
in the North Atlantic (Horwood 1990); however, there are some monthly changes in densities in both
high and low latitudes (Okamura et al. 2001). In the northern part of their range, minke whales are
believed to be migratory, wherease they appear to establish home ranges in the inland waters of
Washington and along central California (Dorsey et al. 1983) and exhibit site fidelity to these areas
between years (Borggaard et al. 1999).
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. There is a low or unknown occurrence of the minke whale here from the
coastline (excluding harbors and lagoons) to seaward of the Marianas study area and vicinity.
Taken into consideration were both sightings in shallow waters in some locales around the world,
as well as the anticipated oceanic occurrence of this species (Figure B-8). Occurrence patterns
are expected to be the same throughout the year, but with a higher probability during winter and
the adjacent months from November to May (Jefferson personal communication).
Behavior and Life History—Minke whales are sighted alone or in small groups (Perrin and Brownell
2002). Mating is thought to occur in winter or early spring, but has never been observed (Stewart and
Leatherwood 1985). Stern (1992) described a general surfacing pattern of minke whales consisting of
about four surfacings interspersed by short-duration dives averaging 38 sec. After the fourth
surfacing, there was a longer duration dive ranging from approximately 2 to 6 min. Minke whales are
“gulpers,” like the other rorquals (Pivorunas 1979). Hoelzel et al. (1989) reported on different feeding
strategies used by minke whales. In the North Pacific, major food items include krill, Japanese
anchovy, Pacific saury, and walleye Pollock (Perrin and Brownell 2002).
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