The small population of about 1,400 individuals is threatened by human encroachment, very low levels of genetic variation, entanglement in fishing nets, marine debris, disease, and past commercial hunting for skins.

[7][8][9] Known to native Hawaiians as ʻIlio-holo-i-ka-uaua, or "dog that runs in rough water", its scientific name is from Hugo Schauinsland, a German scientist who discovered a skull on Laysan Island in 1899.

[11] Its grey coat, white belly, and slender physique distinguish them from the distantly related harbor seal (Phoca vitulina).

[3] The monk seal's physique is ideal for hunting its prey: fish, lobster, octopus and squid in deep water coral beds.

[12] When it is not hunting and eating, it generally basks on the sandy beaches and volcanic rock of the Northwest Hawaiian Islands.

[13] The Hawaiian monk seal is part of the family Phocidae, being named so for its characteristic lack of external ears and inability to rotate its hind flippers under the body.

The hair, generally dark gray on the dorsal side and lighter silver ventrally, gradually changes color through the year with exposure to atmospheric conditions.

Sunlight and seawater cause the dark gray to become brown and the light silver to become yellow-brown, while long periods of time spent in the water can also promote algae growth, giving many seals a green tinge.

In an influential 1977 paper, Repenning and Ray proposed, based on certain unspecialized features, that they were the most primitive living seals.

Evidence points to monk seals migrating to Hawaii between 4–11 million years ago (mya) through an open water passage between North and South America called the Central American Seaway.

[18] Berta and Sumich ask how this species came to the Hawaiian Islands when its closest relatives are on the other side of the world in the North Atlantic and Mediterranean Sea.

Monk seals spend much of their time foraging in deeper water outside of shallow lagoon reefs at sub-photic depths of 300 metres (160 fathoms) or more.

[20][21] Hawaiian monk seals breed and haul-out on sand, corals, and volcanic rock; sandy beaches are more commonly used for pupping.

Hawaiian monk seals have a broad and diverse diet due to foraging plasticity which allows them to be opportunistic predators that feed on a wide variety of available prey.

[23] Hawaiian monk seals mate in the water during their breeding season, which occurs between June and August.

On June 29, 2017 monk seal #RH58 popularly known as "Rocky" gave birth to a pup on Kaimana Beach fronting Kapiolani park.

[31] Natural factors threatening the Hawaiian monk seal include low juvenile survival rates, reduction of habitat/prey associated with environmental changes, increased male aggression, and subsequent skewed gender ratios.

[33] Mobbing leaves the targeted individual with wounds that increase vulnerability to sepsis, killing the victim via infection.

In the nineteenth century, large numbers of seals were killed by whalers and sealers for meat, oil and skin.

[8] This low genetic variability was allegedly due to a population bottleneck caused by intense hunting in the 19th century.

Monk seals can be affected by the toxoplasmosis pathogen in cat feces that enters the ocean in polluted runoff and wastewater, a new phenomenon.

[7][32][40] Although the WWII military bases in the northwestern islands were closed, minimal human activities can be enough to disturb the species.

NOAA is funding considerable research on seal population dynamics and health in conjunction with the Marine Mammal Center.

[44] The task is to identify a manner of alleviation that is possible, cost-effective, and likely to maximize the organic return (in terms of growth potential) until much time has passed and natural conditions allow scientists to observe the effects.

[32] One key natural factor affecting the seal populations is the male-biased sex-ratio, which results in increased aggressive behaviors such as mobbing.

A headstarting project began in 1981, collecting and tagging female pups after weaning and placing them in a large, enclosed water and beach area with food and lacking disturbances.

[46] Identification and mitigation of these and other possible factors limiting population growth represent ongoing challenges and are the primary objectives of the Hawaiian monk seal conservation and recovery effort.

[47] In 2011, the National Marine Fisheries Service issued a controversial draft programmatic environmental impact statement intended to improve protections for the monk seal.