Hawaiian Monk Seal
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Hawaiian monk seals are the only true seals to be found year-round in tropical waters. After the annual moult, this monk seal is a silvery grey colour on the back, with cream colouring on the throat, chest and underside. Over time the coat looks brown above and yellow below; males, and some females, turn almost black with age. Certain individuals may have a red or green tinge or spots due to algal growth. Pups measure about one metre at birth and have a silky black coat, which moults after around a month into the silvery adult-like fur.
Hawaiian monk seals occur throughout the Hawaiian Island chain. Their six main reproductive sites are in the northwestern Hawaiian Islands at Kure Atoll, Midway Atoll, Pearl and Hermes Reef, Lisianski Island, Laysan Island, and French Frigate Shoals (National Marine Fisheries Service 2007). While sightings were previously rare in the main Hawaiian Islands, monk seals are now regularly seen there and births have been documented on all of the major islands (Baker and Johanos 2004). Sightings outside of the main range have occurred at Johnston Atoll, Palmyra Atoll, and Wake Island (Ragen and Lavigne 1999).
Hawaiian monk seals are non-migratory, and tend to remain near the atoll where they were born. However, some seals will relocate temporarily or permanently to other sites in the island chain, and long distance wanderers have been recorded.
Adult Hawaiian monk seals reach lengths of about 2.1 to 2.4 m and weigh 170-240 kg with females being somewhat longer and heavier than males. Pups are about 1 m and 16-18 kg at birth and when weaned 6 weeks later they weigh 50-100 kg (Kenyon 1981, Antonelis et al. 2003).
Monk seals are generally solitary, both on land and at sea. Even when seals gather together on land, they are not normally gregarious and only mothers and pups and recently weaned seals regularly make physical contact. On land, Hawaiian monk seals haul-out and breed on substrates of sand, coral or volcanic rock. Sandy beaches with shallow protected water near shore appear to be preferred for pupping (Westlake and Gilmartin 1990).
At-sea movements and habitat use of Hawaiian monk seals have been investigated using satellite-linked dive recorders and animal-born video cameras that have been put on seals in the northwestern Hawaiian Islands. Results show that they forage within atolls, in the shallow waters surrounding atolls and islands and farther offshore at submerged banks and reefs (Stewart et al. 2006). Seals carrying cameras searched for and preyed on benthic fish and invertebrates in areas of rubble and consolidated bottom material, along areas of transition of benthic habitat types, and also in deepwater coral beds (Parish et al. 2000, 2002).
Most dives that have been recorded have been less than 150 meters deep, although some individuals dove to more than 550 m (Stewart et al. 2006). Monk seals are known to eat a variety of fishes, eels, cephalopods and crustaceans (Goodman-Lowe 1998).
While the habitats of the Hawaiian monk seal are distributed over thousands of kilometers, the terrestrial habitat available for their use in the northwestern Hawaiian Islands is very limited. The total area of emergent land is only about 13.5 km² and only a fraction of that is suitable for use by seals (Ragen and Lavigne 1999).
Hawaiian monk seals have been on the United States Fish and Wildlife Service’s endangered species list since November 23, 1976 and are listed as critically endangered on the IUCN Red List of Threatened Species. They have been in decline for over 20 years, and as of 2007, only 1200 individuals remained in the wild. Experts estimate that fewer than 1000 individuals will remain in the wild by the end of 2012. Efforts by the National Marine Fisheries Service to stabilize population numbers include keeping tourists away from known reproductive sites, moving aggressive males to new breeding grounds, and implementing a captive care program, which provides females with nutritional supplements. The goal of the captive care program is to increase the survival rate of female juvenile seals, which have an extremely low survival rate. Hawaiian monk seals are vulnerable to introduced disease, inbreeding depression, low genetic diversity, human disturbance, and competition with fisheries. In addition to the U.S. Fish and Wildlife Service’s Endangered Species Act, they are also protected under the Marine Mammal Protection Act.
Hawaiian monk seals are intolerant of human disturbance on beaches used for birthing and resting, and pup mortality is often higher at disturbed sites. Hawaiian monk seals began declining in the mid to late 1800’s, when they were hunted for their meat and skins. Currently, populations are declining due to over fishing and seals becoming hooked or entangled in fishing gear (Antonelis et al., 2003).
Current threats to monk seals are thoroughly reviewed and analyzed in NMFS (2007). The most crucial threats in the northwestern Hawaiian Islands at this time are: 1) food limitation that could be due to changes in oceanographic conditions, competition with fisheries, or competition with other predators; 2) entanglement in marine debris, largely fragments of net and line discarded by North Pacific fisheries; and 3) predation by sharks, especially on pre-weaned and recently weaned pups. An emerging threat in this region may be the loss of terrestrial habitat due to sea level increases resulting from global warming (Baker et al. 2006). The situation in the main Hawaiian Islands is somewhat different, with the main threats there being: 1) interactions with recreational fishing gear especially hookings and entanglements in gillnets; 2) possible transmission of diseases from domestic pets and livestock to seals; and 3) disturbance of seals that haul out on beaches heavily used by people.
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