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10/24/21

Hawaiian Monk Seal

The Hawaiian monk seal is endemic to Hawaii, mostly the Northwestern islands, although about 200-300 can be found on the main islands. They are a monk seal species, along with the Mediterranean monk seal and the extinct Caribbean monk seal. Their Hawaiian name, ilio-holo-i-ka-uaua, means dog running in rough water, because they look a bit like dogs. These monk seals are in fact closely related to canines. Their common name, monk seal, comes from the fold of skin on their neck that some say looks like a monk’s robe.

Female Hawaiian monk seals have one pup each spring or summer, which they will stay with for 5-7 weeks. Their mother doesn’t eat while nursing, meaning they end up losing a third of their body weight. Meanwhile, the pups gain 80 kg (175 lbs)!

Hawaiian monk seals, when born, weigh only about 11-16 kg (25-35 lbs), and have an all black fur coat that they shed as they grow. By the time they reach adulthood, they will have a dark grey back that gets darker as they age, a light belly, weigh 180-270 kg (400-600 lbs) and have a length of 2.1-2.2 m (7-7.5 ft).

Every year, sometime between April and December, all of the Hawaiian monk seals go through a ‘catastrophic molt’. During this time, lasting a little more than a week, the seals shed all of their hair and a layer of their skin. Seals that spend a lot of time underwater may grow algae on their fur and look green, which the molt can help with.

Hawaiian monk seals will eat many types of food, depending on what they can find. This includes fish, cephalopods, crustaceans, squids, and eels. They particularly prefer things they can find at the sea floor, and interestingly don’t target a lot of popular game fish found in the area. To find their prey, these monk seals are able to dive over 270 meters (900 ft). They are only able to do this because of a feature called bradycardia, which is when their hearts slow down nearly eight times less than usual, reducing the amount of oxygen needed and allowing the seal to stay underwater longer.

Hawaiian monk seals can live up to 30 years old, but most don’t.

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Image courtesy of NOAA

Hawaiians monk seals are one of the rarest marine mammals, and are listed as endangered by the U.S and Hawaii’s endangered species lists, as well as under the Marine Mammal Protection Act. There are about 1,100-1,400 Hawaiian monk seals in the wild, the populations of which declined at 4% per year for a long time.

Natural threats to these monk seals includes predation from tiger sharks (mostly of pups) and aggression from adult male monk seals. The biggest threat is from humans, including loss of habitat because of environmental change, entanglement in fishing gear and plastic rings, beach disturbance, overfishing, invasive species, coral bleaching, ocean acidification, sea level rise, intentional killing, and diseases. A large population of the Hawaiian monk seals was lost from the 1950s to very recently, mostly because of young seals not getting enough food because of competition and changes related to climate change. But after 2013, things started to look up with the population increasing at 2% per year.

Another threat is canine diseases, such as toxoplasmosis, a disease first spotted in Hawaiian monk seals in 2004. There have been at least 13 deaths from toxoplasmosis since then, with a large cluster of deaths together in both 2018 and 2020.

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Image courtesy of James Watt

Image courtesy of Lifegate

Image courtesy of NOAA

Image courtesy of the National Wildlife Federation

Further Resources

Works Cited

NOAA Fisheries. (n.d.). Hawaiian monk seal. NOAA. Retrieved October 17, 2021, from https://www.fisheries.noaa.gov/species/hawaiian-monk-seal.

Hawaiian monk seal - hawaiian islands - U.S. fish and wildlife service. U.S. Fish & Wildlife Service. (n.d.). Retrieved October 17, 2021, from

Hawaiian monk seal. National Wildlife Federation. (n.d.). Retrieved October 17, 2021, from https://www.nwf.org/Educational-Resources/Wildlife-Guide/Mammals/Hawaiian-Monk-Seal.

Cover Image courtesy of James Watt