Hawaiian monk seal

 

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Classification:

Global Population

Description and Natural History

Physical Description
Few adult monk seals have been measured or weighed and values in the literature vary. Adult female Hawaiian monk seals reportedly reach a length of 2.3 meters (7.5 feet) and weigh 205 – 273 kilograms (452 – 602 pounds). Adult males are slightly smaller, measuring up to 2.1 meters (6.9 feet) and weighing 170 – 230 kilograms (375 – 507 pounds).

Adults are silvery-gray on the back and fade to cream on the throat, chest and belly. Additional light patches may also be found on the body. Over time, the coat appears brown on the back and yellow on the sides and belly. Males, and some females, turn dark brown or black as they age. Some adult Hawaiian monk seals exhibit a red or green tinge due to algal growth on their coat. Pups are born with a long, woolly, black neonatal coat. They weigh approximately 14 – 17 kilograms (31 – 37 pounds) and are about one meter (3.3 feet) long at birth.

Natural History
The Hawaiian monk seal is found on the sandy beaches and in the surrounding subtropical waters of remote islands and atolls in the northwestern Hawaiian Islands known as the Leeward Chain: Kure Atoll, Midway Atoll, Pearl and Hermes Reef, Lisianski Island, Laysan Island, and French Frigate Shoals. These islands and atolls extend more than 2,000 kilometers (1,240 miles) to the northwest of the main Hawaiian Islands.

Small populations also occur at Necker and Nihoa Islands, although these islands lack the beaches and shallow near-shore areas preferred by pupping females. Some animals also live around the main Hawaiian Islands and, on rare occasions, monk seals have been seen at Johnston Atoll.

Information on the monk seals’ natural history is known mainly from their activities on land; little is known of their offshore habits or distribution. Monk seals tend to be solitary, both on land and in the water.

Females reach sexual maturity at four to eight years of age, depending on location, and give birth to one pup a year. Age at sexual maturity is unknown for males but it is thought to be the same as in females. Pupping occurs year-round but is more common between December and mid-August, with peaks in March and early April. Most pups are born at French Frigate Shoals, Laysan Island, Lisianski Island, Pearl and Hermes Reef and Kure Atoll. Midway Atoll used to be an important pupping site but was abandoned by monk seals in the late 1960s after the Atoll was occupied by the United States Navy. Midway is now being re-colonized following the departure of the Navy.

The majority of monk seals remain at their place of birth, though some movement between sites does occur. The pups’ black neonatal coat is shed at about six weeks of age, about the same time they are weaned. Limited data on monk seal diet shows that they feed on a variety of fish, cephalopods (including octopus and squid), and crustaceans (including lobster). Hawaiian monk seals may live to be 20 – 25 years old.

Due to a number of factors including space, food availability, and, possibly, ambient air temperatures, Hawaiian monk seals were probably never very abundant, perhaps numbering in the thousands rather than tens or hundreds of thousands. Current population estimates range from only 1,300 to 1,400 individuals. Between 1958 and 1996, mean beach counts of the main reproductive populations declined by 60 percent. The population continues to decline; between 1985 and 1996 the rate of decline was about four percent per year.

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Status of the Species

Status
Hawaiian monk seals were listed as Endangered in 1976 under the U.S. Endangered Species Act (ESA). They are also listed as Endangered on the International Union for the Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species, and are listed on Appendix I of the Convention on International Trade in Endangered Species (CITES).

Threats to the Species
Hawaiian monk seals were heavily exploited by early hunters, whalers and fishers. By the late 1800s sightings were already rare. Today, threats include incidental capture in fishing gear, intentional kills, a decrease in food availability for some sub-populations (e.g. French Frigate Shoals), and ingestion of discarded fish from the kahala (Seriola dumerii) fishery, which may contain high levels of ciguatoxins, a toxic algae.

Like the other remaining monk seal species (the critically endangered Mediterranean monk seal, M. monachus, and the extinct Caribbean monk seal, M. tropicalis), the Hawaiian monk seal is very sensitive to human disturbance and habitat loss.

These factors, along with shark attacks, an inherently slow reproductive rate, and mobbing (a behavior in which females and immature individuals of both sexes are wounded, sometimes fatally, by the aggressive sexual behavior of some males) continue to threaten the remaining population.

Conservation strategies include: protection of critical habitat, identification of main reproductive habitats, research on the survival of various age and sex classes, rehabilitation and release of undersized pups, removal of marine debris, and the mitigation of human disturbance. A captive breeding program has also been advocated by some scientists.

International Trade
Listed on CITES Appendix I which prohibits international trade.

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Authors and Sources

Sources
CITES. 2001. Hawaiian monk seal. http://www.cites.org.

IMMA. 2000. The Hawaiian monk seal. The Monachus Guardian. www.@monachus.org.

IUCN. 2001. The 2000 IUCN Red List of Threatened Species. http://www.redlist.org.

Jefferson, T.A., S. Leatherwood and M.A. Webber. 1993. Marine Mammals of the World. FAO Species Identification Guide. United Nations Environment Programme. Rome.

Lavigne, D.M. 1999. The Hawaiian Monk Seal: Management of an Endangered Species. In J. R. Twiss Jr. and R. R. Reeves (eds.). Conservation and Management of Marine Mammals. Smithsonian Institution Press. London. 471pp.

Leatherwood, S, B.S. Stewart and R.R. Reeves. 1992. The Sierra Club Handbook of Seals and Sirenians. Sierra Club Books, San Francisco. 359pp.

Ragen, T. J. & D. M. Lavigne. 1999. The Hawaiian Monk Seal: Biology of an Endangered Species. In J. R. Twiss Jr. and R. R. Reeves (eds.) Conservation and Management of Marine Mammals. Smithsonian Institution Press. London. 471pp.

Reijnders, P. S. Brasseur, J. van der Toorn, P. van der Wolf, I. Boyd, J. Harwood, D. Lavigne and L. Lowry. 1993. Status Survey and Conservation Actioan Plan: seals, fur seals, sea lions, and walrus. IUCN/SSC Seal Specialist Group. 88pp.

Rice, D.W. 1998. Marine mammals of the world: systematics and distirbution. Special Publication Number 4. The Society for Marine Mammalogy. 231pp.

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