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Contraceptives helping control California bison herd

Published December 22, 2013 6:30 pm

Wildlife • Study finds that limiting population is more effective than relocating the animals.
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Woodland Hills, Calif. • A contraceptives program launched in 2009 on Catalina Island has been deemed an effective and more humane way of controlling the bison population than relocation, according to a new study by the Catalina Island Conservancy that will be published this month.

The program has allowed wildlife biologists to keep population levels that are both beneficial to the herds of bison as well as the ecological health of the island.

"It's a long-term program," said Calvin Duncan, a wildlife biologist who co-led the contraceptives effort. "The conservancy is looking to find a balance. The bison have been really ingrained into the culture and the residents love them. Hopefully, this is something that can be considered in other bison herds."

Bison were introduced to Catalina Island in 1924 for a movie that was never made, but have become somewhat of a historical icon indicative of the island's wildlife. Over time, the herd grew to more than 600 animals, creating a need to remove bison off the island.

The conservancy had been shipping bison to auction houses and Native American reservations, or rounding them up to administer large doses of hormones as a form of contraception.

"As a conservation organization, our board did a multiyear scientific study on what impacts the bison," said Ann Muscat, president and CEO of the Catalina Island Conservancy. "From that, we learned that if we chose to have a herd, we should reduce the number to 150 since it's better for the bison and it's better for the habitat.

"They're of great cultural and historical value to the island, and we're very pleased that the program is working out as well as it is."

The new program is a form of immuno-contraception, using porcine zona pellucida (PZP) applied in a very small dose administered by a dart or syringe.

"It follows the same rules as a vaccine," Duncan said. "The bison are doing what they were doing before and it's not causing any difference in hormones or behavior."

The contraceptive has been used for fertility control in zoos, wild horses and white-tailed deer. The females are administered two doses their first year of the program and then one every year after that. This program is more cost-effective and efficient for the biologists, who no longer have to corral the huge animals or find them across the island. The population now is already stabilized at 159 animals.

"It's something we can more easily manage and also keep the cultural and economic value," Muscat said. "It's one of a number of important wildlife management programs that we have and we take very seriously the responsibility to manage the wildlife on the island."

The contraceptives program is the most beneficial to the female bison, which could be pregnant and nursing new calves for most of their adult lives — up to 20 years — because of the lack of predators, said Julie King, a senior wildlife biologist. With the contraceptive, it allows the females to take a break and recover from the constant pregnancy and nursing.

In addition to benefits for the bison, the program is beneficial to the island as a whole.

"Ecologically for the big picture, it's an advantage because the population of bison here, without predators or disease, continues to expand to well over what the environment can sustain them with," King said. "They're not going to have enough to eat, so with a smaller population and the contraceptive, their numbers will be smaller, allowing more food, and it keeps their social structure intact."

The research was presented at the International Conference on Fertility Control in Wildlife in 2012 attended by scientists and researchers who come together to explore effective methods of wildlife population management.

"The unique part of Catalina is that it's a living laboratory," Duncan said. "A lot of the programs we do, we approach them with a lot more data than we would with normal research. We're mixing management and research in a way so that someone can replicate it somewhere else. It's part of the great things we're doing here."