Contraception Caps Bison Numbers on Catalina

On Santa Catalina Island, scientists have advanced the science of immunocontraception as a safe and effective tool to humanely manage the island’s bison.

In 1924, Hollywood brought 14 bison to Santa Catalina, a 22-mile-long island standing 22 miles off the coast of Los Angeles, to serve as backdrops in a silent film, The Vanishing American. The bison, however, never made it into the movie and when filming ended, never made it off the island—left to fend for themselves in what proved to be a relatively hospitable landscape.

By 1934, the herd had grown to 19, after several births offset the shooting deaths of some of the original implants. That same year, nine additional bison were imported to the island, bringing the population to 28. Over the next few decades, as the bison became a popular fixture on the island, the population swelled into the hundreds. By the 1990s, up to 600 bison roamed Catalina. As a non-native species occupying a limited space, however, they presented a problem. According to the Santa Catalina Island Conservancy, which manages most of the island,

the fauna of Santa Catalina Island did not historically include large grazing ungulates, which has led to significant recent concern regarding the potential ecological effects of bison on native and endemic plants and animals on the island. In the intervening years since bison were first brought to the island, however, this large ungulate has taken on important cultural and economic significance to island residents.

The Conservancy feels the island can comfortably support about 150–200 bison. (A 2003 study found the roughly 350 bison on the island at that time to be undernourished.) Without natural predators, from 1970 through 2009 the bison population was controlled through periodic roundups, with captured bison shipped to mainland livestock auctions, private dealers, and Native American tribes. This was an expensive undertaking that subjected the bison to significant stress (and in many cases, slaughter), and failed to address the growth in bison numbers between roundups.

In 2009, the Conservancy, working with scientists from California State University, Fullerton, and the Science and Conservation Center in Billings, Montana, initiated a bison fertility control program using the porcine zona pellucida (PZP) immunocontraceptive vaccine. Having been successfully used in captive bison, it was anticipated that the vaccine would safely and effectively control the bison herd’s growth. Indeed, as reported in a study published in the December 2013 edition of the Journal of Zoo and Wildlife Medicine, the bison calving rates declined from 67.4 percent in 2010 (before the vaccine took effect), to 10.4 percent in 2011, finally dropping to 3.3 percent in 2012.

The PZP vaccine’s success will allow bison to inhabit Catalina Island at numbers that are not as hard on the ecosystem—allowing the natural diversity of island plants to flourish and ensuring sufficient forage for other island species—while avoiding disruptive bison roundups in the future. As the scientific evidence documenting the safety and efficacy of immunocontraception builds, this is a technology that should be embraced as a non-lethal and humane method for managing wildlife when and where appropriate.