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

AUGUST 2005 FINAL REPORT
generally return to the same areas to molt (Stewart and Yochem 1984; Stewart and DeLong 1995).
The different age and sex classes have somewhat differing annual cycles and migration patterns
(Stewart 1997). After weaning their pups in late winter, adult females forage at sea for about 70 days
before returning to land to molt their pelage. Following one month ashore, the females return to sea
for eight months (coincident with gestation), before returning to the rookery to give birth. Elephant
seals do not necessarily return to the same beaches for breeding and molting. For example, Huber et
al. (1991) found that female northern elephant seals often molt on one island and breed on another.
Adult males spend approximately four months at sea following the breeding season, returning to
shore in summer to molt. After one month ashore, they return to sea for four months before returning
to the rookery for the breeding season.
In December, male elephant seals haul out for the breeding season; many individuals remain there
continuously until March. In January, after many males have been on land for several weeks, the
adult females come ashore, give birth, suckle their young for about 27 days, breed, and depart (Le
Boeuf and Peterson 1969; Stewart and Huber 1993). Gestation is about 11 months, but there is a two
to three month period of delayed implantation. During the breeding season, elephant seals
congregate in large numbers on their breeding rookeries. Animals of all ages and both sexes are
present on these beaches, although yearlings generally do not return during the breeding season,
and are rare at rookeries. Large rookeries, such as those on Año Nuevo Island and peninsula and the
Channel Islands, may contain thousands of seals, which mostly arrange themselves in harems
consisting of up to several dozen breeding females, a single dominant (alpha) male, and the newborn
pups. Other animals, especially other bulls seeking to challenge the alpha male or sneak copulations,
often surround the harems.
Males reach sexual maturity at about 6 or 7 years, but do not reach “social maturity” until 9 or 10
years. Most adult males do not have high enough social status to do much breeding – a few high
ranking males called “alpha males” actually do the vast majority of the fertilization of the females (Le
Boeuf 1974). Both males and females lose a large proportion of their body mass while fasting during
the breeding season, and they must feed intensively after returning to sea to regain weight.
During the molting period, which is at different times of the year for different age classes, seals lose
their fur in large patches with the underlying epidermis. This is called a “catastrophic molt” and
molting seals look very ragged (Stewart and Huber 1993). Adults return to land between March and
August to molt, with males returning later than females (Carretta et al. 2004).
Elephant seals are probably the deepest and longest diving pinnipeds; few marine mammals can
match their abilities. Adults dive continuously, day and night, during their feeding migrations (DeLong
and Stewart 1991; Le Boeuf et al. 1986, 1989). Elephant seals may spend as much as 90% of their
time submerged (DeLong and Stewart 1991); this year-round pattern of continuous, long, deep dives
explains why northern elephant seals are rarely seen at sea and why their oceanic whereabouts and
migrations have long been unknown (Stewart and DeLong 1995). The average diving cycle consists
of a 23 min dive, followed by a 2 to 4 min surface interval (Le Boeuf et al. 1986, 1989; DeLong and
Stewart 1991). The longest known dive is 77 min (Stewart and Huber 1993). Dives average between
350 and 550 m in depth, with dives as deep as 1,561 m (females) and 1,585 m (males) (Stewart and
Huber 1993). Males and females pursue different foraging strategies. Females range widely over
deep water, apparently foraging on patchily distributed, vertically migrating, pelagic prey, whereas
males forage along the continental margin at the distal end of their migration, and they may at times
feed on benthic prey (Le Boeuf et al. 2000).
Northern elephant seals feed primarily on cephalopods, hake, and other epipelagic, mesopelagic, and
bathypelagic fishes and crustaceans, such as pelagic red crabs (Condit and Le Boeuf 1984; DeLong
and Stewart 1991; Stewart and Huber 1993; Antonelis et al. 1994). Most significant prey species
make vertical migrations and are part of the deep scattering layer (Antonelis et al. 1994).
Acoustics and Hearing—The northern elephant seal produces loud, low-frequency in-air
vocalizations (Bartholomew and Collias 1962). The mean fundamental frequencies are in the range of
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