TRENTON, N.J. (AP) – How do you keep bears from procreating?

Very carefully.

Encounters between New Jersey’s black bears and humans have increased in recent years, with bear sightings in some urban and southern areas as well as the bruins’ traditional home in the state’s northern woods.

So state Environmental Protection Commissioner Bradley Campbell approved a plan Tuesday that includes a number of approaches, most controversially a Dec. 5-10 bear hunt – only the second such hunt in New Jersey since the Nixon administration.

Officials also advocate the use of bear-proof trash cans in certain regions, as well as more public education about how to react when encountering the animals.

But perhaps the most ambitious idea in the plan involves bear contraception, in which doctors inject the animals with chemicals to ward off reproduction.

The technology is still in its infancy, but two studies by a team of scientists are under way at an animal park in Jackson.

Dr. Patricia Sertich, a consultant to the studies and clinical director at the University of Pennsylvania’s Hofmann Research Center for Animal Reproduction, said she’s found very little academic research about the reproductive systems of Ursus americanus.

“You do a little bit of work with bears and you’re fascinated by them,” Sertich said. “But there’s a pretty big void in the literature.”

So far the state has paid for a review of that published material, to be completed early next year. The studies at Six Flags’ Wild Safari animal park are mostly volunteer efforts by research veterinarians, with the park providing the animals, anesthetics and staff support.

Black bears typically mate in the summertime, but an embryo lives in the female’s uterus until late fall before implanting itself in the uterine wall and beginning its two-month gestation. In the meantime, the would-be mother’s body has been sussing out the nutritional environment: If food has been abundant, she will produce more cubs. Litters can range from one to six.

A healthy female black bear can weigh more than 200 pounds, but she produces newborns that are no bigger than kittens.

Dr. Allen Rutberg, a research professor at Tufts University’s Center for Animals and Public Policy in Grafton, Mass., said his team began testing a contraceptive called PZP on six female bears in fall October 2004. The vaccine, made from pig tissue, prevents sperm from attaching to an egg.

Since female bears give birth every other year, it will be February before the doctors know whether the vaccine has worked.

“So far there’s been nothing discouraging, but nothing terribly encouraging either,” Rutberg said. His work is partly funded by the Humane Society of the United States.

Burlington County veterinarian Dr. Gordon Stull is overseeing the injection of some male bears at the animal park with Neutersol, a chemical that in long-term tests on dogs suppresses sperm production but not testosterone.

The researchers think male bears will continue to mate, causing their female partners to ovulate and their bodies to believe, incorrectly, that they are pregnant.

That’s doubly useful because females won’t accept other suitors when pregnant.

“We’ve created a temporary contraceptive,” Stull said. “She’s out of reproductive commission for however long she stays pseudo-pregnant.”

Translating the results of either study into practice is at least a few years off. While opinions among the scientists about next month’s hunt vary, Stull said he was sorry to see it approved.

He said that stricter garbage control would better reduce the number of unwanted encounters between bear and human.

The physiology of the female bear underscores Stull’s belief that getting humans in bear country to be more conscientious about securing their trash is key to reducing human-bruin contact.

“That’s why it’s so important to control garbage in areas where bears live,” he said. “Not only is it the source of many nuisance complaints, but it increases their level of nutrition to the point where they’re much more prolific.”