AVALON, Calif. (AP) -- Birth control for bison?

That's what conservationists are giving a herd on Santa Catalina Island in an attempt to reduce the population. Females over the age of 2 will be injected with a contraceptive under a five-year experimental program that begins Friday.

Some of the feral animals were corralled Thursday.

The bison, commonly called American buffalo, are descendants of 14 animals that were shipped to the island in 1924 to make a Western. They weren't used in the movie and were simply left behind.

At one point, as many as 600 of the shaggy beasts roamed the island 25 miles off the Los Angeles coast. There were about 350 in 2003 when a study found that they had poor nutrition and health.

The conservancy believes the herd should be kept at 150 to 200 to preserve the environment and the animals' health. To control the numbers, bison were shipped out for slaughter, but in recent years, the animals have instead been sent to Indian reservations for breeding.

Last month, 150 bison were deported at a cost of about $100,000, conservancy President and Chief Executive Ann M. Muscat said. The $200,000 contraception program is considered cheaper and less stressful for the animals.

The birth control vaccine, called PZP, is derived from pig eggs and must be renewed annually. It doesn't affect the animal's hormones or behavior, said Carlos de la Rosa, chief conservation and education officer for the Catalina Island Conservancy, a nonprofit group that owns much of the island.

"Bison will continue to be bison," De la Rosa said. "Males will continue to compete for females, and females will continue to go into heat. The only difference is that we can control how many calves they have."

"For bison in love, this means romance without responsibilities," he said with a laugh.

Copyright 2009 The AP.