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

AUGUST 2005 FINAL REPORT
and weigh 170 to 240 kilograms [kg], with females growing slightly larger than males (Gilmartin and
Forcada 2002). Other than this size difference, there is little noticeable sexual dimorphism. Coloration
of Hawaiian monk seals is drab, generally a yellow-brown to silvery gray color with slight
countershading and some small ventral white patches.
Status—The Hawaiian monk seal is listed as endangered under the ESA and depleted under the
MMPA (Ragen and Lavigne 1999). The Hawaiian monk seal population is a NOAA Fisheries strategic
stock (Carretta et al. 2004). Hawaiian monk seals are managed as a single stock, although there are
six main reproductive subpopulations at French Frigate Shoals, Laysan Island, Lisianski Island, Pearl
and Hermes Reef, Midway Island, and Kure Atoll (Ragen and Lavigne 1999; Carretta et al. 2004).
Genetic comparisons between the Northwestern and main Hawaiian Islands seals have not yet been
conducted, but observed interchange of individuals among the regions is extremely rare, suggesting
that these may be more appropriately designated as separate stocks; further research is needed
(Carretta et al. 2004).
The best estimate of the total population size is 1,304 individuals (Carretta et al. 2004). In 2001, there
were an estimated 52 seals in the main Hawaiian Islands (Baker and Johanos 2004; Carretta et al.
2004); the vast majority of the population occurs in the Northwestern Hawaiian Islands. The trend in
abundance for the population over the past 20 years has mostly been negative (Baker and Johanos
2004; Carretta et al. 2004). A self-sustaining subpopulation in the main Hawaiian Islands may
improve the monk seal’s long-term prospects for recovery (MMC 2003; Baker and Johanos 2004;
Carretta et al. 2004).
Critical habitat for the Hawaiian monk seal is designated from the shore out to 37 m (20 fathoms) in
10 areas of the Northwestern Hawaiian Islands (NMFS 1988) (Figure 3-2).
Habitat Preferences—Breeding habitat for the Hawaiian monk seal primarily consists of short, sandy
beaches, although hard substrate beach areas and exposed reef are used at some islands (Gilmartin
1983). Dry sand areas above the high tide line are preferred (Kenyon 1973; Gilmartin 1983). The
breeding beaches are characterized by shallow, protected water adjacent to the shoreline, which may
limit access by large predatory sharks (Westlake and Gilmartin 1990). Foraging habitat is commonly
restricted to waters of less than 100 m in depth near the breeding atolls and seamounts (Parrish et al.
2000; Stewart 2004). The inner reef waters adjacent to the islands are critical to weaned pups
learning to feed, as the pups move laterally along the shoreline, but do not appear to travel far from
shore during the first few months after weaning (Gilmartin 1983). Feeding has been observed in reef
caves (Gilmartin 1983). Recent studies have shown that adult seals at French Frigate Shoals forage
at depths of 300 to 500 m in deepwater coral beds (Parrish et al. 2002), and 2 to 3 year-olds feed at
much shallower depths of 10 to 30 m (Parrish et al. 2005). However, juveniles feed in water sand
fields with a bottom depth of 50 to 100 m on the atoll’s outer slope (Parrish et al. 2005).
Local environmental conditions affect trends in abundance of specific monk seal subpopulations, with
areas sharing similar ecological characteristics having similar trends (Schmelzer 2000). Climatedriven
changes in oceanic productivity could be decisive in reducing (or enhancing) monk seal
reproduction and survival. Juvenile seals feed over deepwater sand fields, and oceanographic
conditions are thought to influence survivorship of these seals (Parrish et al. 2005). Polovina et al.
(1994) speculated that climate changes in the central North Pacific cause declines in lobster
recruitment, which contributed to declines in the numbers of monk seal pups. More recent work
suggests that El Niño conditions may actually benefit Hawaiian monk seals by making prey more
available and resulting in weaned pups with larger axillary girths (and therefore, presumably, better
nutritional condition) (Antonelis et al. 2003). Further research is needed to fully understand the
implications of climate change on this species.
Distribution—The Hawaiian monk seal occurs only in the central North Pacific. Until recently, this
species occurred almost exclusively at remote atolls in the Northwestern Hawaiian Islands where six
major breeding colonies are located: French Frigate Shoals, Laysan and Lisianski Islands, Pearl and
3-22