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

AUGUST 2005 FINAL REPORT
Information Specific to the Marianas MRA Study Area—There is a low or unknown occurrence of
humpback whales from the coastline (excluding harbors and lagoons) in the Marianas study area
and vicinity (Figure B-4). These occurrence patterns are applicable throughout the year, except
June through September when this species is not expected in the area. The winter range of the
Western North Pacific stock of humpback whales extends, at least occasionally, into this region
(Taitano 1991; Darling and Mori 1993). Humpback whales in other locales during the breeding
season are typically found in insular shelf waters, but can also be found in deeper waters during
the breeding season. For example, humpback whale calls were detected from whales located to
the northeast and east of the Puerto Rican Trench over deep water (>6,000 m) and far from any
banks or islands (Swartz et al. 2002).
Reeves et al. (1999) suggested that the Marianas might be south of the normal breeding range,
but clearly some whales do move into the study area in the breeding season (Figure B-4). There
are several recent records of humpback whales in the Marianas Islands, at Guam, Rota, and
Saipan during January through March (Darling and Mori 1993; Eldredge 1991, 2003). February
and March are the months when humpback whales are most often sighted in the Marianas
(Anonymous 2004). The breeding season extends well into the spring; whalers took humpback
whales through the month of May in the southern Marianas (Eldredge 1991).
Behavior and Life History—Humpback whales are arguably the most social of all the baleen
whales. Group size can range from single individuals to large groups of up to 20 or more whales.
These groups are, however, typically small and unstable, with the exception of mother/calf pairs
(Clapham and Mead 1999). On the feeding grounds, relatively large numbers of humpbacks may be
observed within a limited area to feed on a rich food source. While large aggregations are often
observed, it is not clear if there are stable associations between individuals, or if this is simply a
reflection of a concentration of animals brought together by a common interest in locally abundant
prey (Clapham 2000). On the breeding grounds, small groups of males may occur, competing for
access to females (Tyack and Whitehead 1983; Baker and Herman 1984; Pack et al. 1998). On rare
occasions, competitive groups have been observed on the feeding grounds (Weinrich 1995).
Humpback whales feed on a wide variety of invertebrates and small schooling fishes. The most
common invertebrate prey are euphausiids (krill); the most common fish prey are herring, mackerel,
sand lance, sardines, anchovies, and capelin (Clapham and Mead 1999). These whales are lunge
feeders, taking in huge batches of prey items as they lunge laterally, diagonally, or vertically through
patches of prey (Clapham 2002). Feeding behavior is highly diverse, and humpbacks employ unusual
behaviors, such as bubble netting, to corral prey (Jurasz and Jurasz 1979; Weinrich et al. 1992). This
is the only species of baleen whale that shows some evidence of cooperation when feeding in large
groups (D’Vincent et al. 1985). Humpback whales are not typically thought to feed on the wintering
grounds; however, Salden (1989) observed apparent feeding by a juvenile humpback whale off
Hawai’i.
Female humpbacks become sexually mature at 4 to 9 years of age (Clapham 1996). Gestation is
approximately one year. Calves are weaned before one year of age. Calving intervals are usually 2 to
3 years, although occasionally females give birth to calves in successive years (Clapham 1996).
Males compete for access to receptive females by aggressive, sometimes violent interactions, as well
as vocal displays (Clapham 1996; Pack et al. 1998).
Humpback whale diving behavior depends on the time of year (Clapham and Mead 1999). In
summer, most dives last less than 5 min; those exceeding 10 min are atypical. In winter (December
through March), dives average 10 to 15 min; dives of greater than 30 min have been recorded
(Clapham and Mead 1999). Although humpback whales have been recorded to dive as deep as 500
m (Dietz et al. 2002), on the feeding grounds they spend the majority of their time in the upper 120 m
of the water column (Dolphin 1987; Dietz et al. 2002). Humpback whales on the wintering grounds
dive deeply; Baird et al. (2000) recorded dives deeper than 100 m.
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