HONOLULU - The Hawaiian monk seal, the most endangered marine mammal in the United States, has a long list of threats - fishing nets, sharks and, particularly, humans. But for one group of seals, the biggest threat came from one of its own: a 400-pound brute named KE18 who killed two other seals and wounded at least 11, most of them helpless pups.

The Hawaiian monk seal is on course to disappear in 50 to 100 years, scientists say. But KE18 was en route to having his ticket punched much sooner due to his propensity for nudging his own species toward extinction.

"It's really disheartening when the species you're trying to protect is becoming the troublemaker," said Charles Littnan, the lead scientist for the Hawaiian monk seal research program at the National Oceanic and Atmospheric Administration.

The female seal KE18 killed would have likely given birth to four or five pups, he said.

Authorities had planned the drastic step of euthanizing KE18. He was spared when the NOAA team that planned to put him down traveled to Kure Atoll, where he attacked most of his victims, but wasn't able to find him.

KE18 turned up at Midway Atoll, a 55-mile journey from Kure, where NOAA officials decided to save him. They captured the 9-year-old seal on a beach, loaded him on a Coast Guard plane, and flew him 1,400 miles south to Honolulu and a temporary home at Waikiki Aquarium.

Jeff Walters, NOAA's monk seal recovery coordinator, said KE18 will be used in research on monk seal eating habits and calorie consumption at the University of California-Santa Cruz, where he'll go later this month.

Monk seals have lived in the waters off the Hawaiian archipelago for millions of years. At one time, their population was estimated at 15,000. It has dropped dramatically in recent years, however, and now totals about 1,100. More than 80 percent live in a nature preserve among dozens of small atolls northwest of the main Hawaiian Islands.

The seal is an important part of Hawaii's history. They're called Ilio holo I ka uaua in the Hawaiian language, which means, "dog that runs in rough water." They are called "monk" seals because they are solitary - like monks - and the soft folds of fur around their necks look similar to the cowls worn by monks.

Stretching 7 feet, the seals are remarkable divers, plunging up to 500 feet to find food and remaining submerged as long as 20 minutes. While they spend most of their time at sea, they come ashore occasionally, notably to give birth. Monk seals don't react well to human contact - some mothers will abandon their offspring, which helps explain why the survival rate for pups has declined sharply as man has encroached on native habitats.

Sharks are another threat - they prey on pups. And commercial fishing has left more danger from old nets that can entangle and suffocate the seals.

KE18 became a concern in 2010, when Hawaiian monk seal research program workers saw him bully other seals at Kure. Last year he escalated his attacks, scratching, biting and holding other seals under water. Most of his victims were pups just weaned from their mothers.

"It was hard to break his attention when he was on top of a seal," Littnan said. Monk seal researchers at Kure tried to distract him by throwing chunks of coral in the water but failed. "They'd be yelling, and he would continue to hold pups under water or wrestle with them," Littnan said.

NOAA staff would scare KE18 away from one victim only to see him move down the beach and begin bullying another.

Ten of the 13 pups born at Kure last year had wounds from KE18. Scientists suspect him in two pup deaths. He also injured three juvenile seals.

NOAA officials aren't sure why KE18 did this. Because the attacks happened during the breeding season, and KE18 didn't have mates, there's a possibility he was acting out of misplaced breeding aggression. Since male monk seals don't generally become dominant until they're 14 or 15, the 9-year-old KE18 may have attacked vulnerable pups for years to come if not removed from the wild.

In extreme cases, NOAA will euthanize an aggressive seal. The last time that happened was 1991. KE18 would have met the same fate if not for the fortuitous confluence of space opening at the aquarium and him swimming to Midway, a National Wildlife Refuge and former Navy base that's easy to reach because it's equipped with a runway.

Miyoko Sakashita, a lawyer for the Center for Biological Diversity, an environmental group that petitioned the government to expand critical habitat for monk seals, said these types of management actions may be necessary when one seal poses a risk to a population that can't afford to lose any individuals.

"I think this is really just a symptom of the overall problem with the monk seals edging their way toward extinction," Sakashita said.

Michael Hutchins, the executive director of the Wildlife Society, said wildlife managers are increasingly being forced to make difficult ethical decisions to protect species as humans encroach on wildlife habitat.

"Because of our own activities, we've driven many species to the brink of extinction," Hutchins said. "Emergency room ethics are what we are talking about here."

After he's finished in California, KE18 is expected to return to Hawaii and spend his days at Sea Life Park outside Honolulu, one of just four institutions authorized to house Hawaiian monk seals.

Meanwhile, he is eating 10 pounds of fish a day at Waikiki Aquarium and swimming around in his own pool. He's in quarantine, but for KE18, it was almost much worse.