Previous reports have demonstrated gradual reductions of white-tailed deer (Odocoileus virginianus) populations through immunocontraception, with stabilization occurring after 2–4 yr of treatment, and subsequent reductions of 6–10% annually. These studies employed porcine zona pellucida (PZP) vaccines that required two initial treatments and annual retreatments. From 2005 to 2010, 258 adult and yearling female deer on Fripp Island, South Carolina, were treated with one of several PZP preparations designed to produce 2 yr of effective contraception with a single treatment. These included several preparations of SpayVac® and of native PZP-adjuvant emulsion plus PZP and QA-21 in timed-release pellets. Deer were chemically immobilized, ear-tagged, and administered initial treatments by hand in February–March. Some treated deer were boosted remotely with PZP-adjuvant emulsion 1.5–4.5 yr after initial treatments. Ground-based distance sampling was used to estimate deer population density at Fripp Island, a resort community, and at a relatively undeveloped neighboring control site, Hunting Island. Most vaccine preparations tested reduced fawning rates by 75% to 95% for at least 1 yr. From 2005 to 2011, deer density on Fripp Island declined by 50%, from 72 deer/km2 to 36 deer/km2, an average annual reduction of 11%. In contrast, population density on the Hunting Island control site fluctuated between 2005 and 2011, averaging 23 deer/km2 (range, 19–28 deer/km2). Population declines on Fripp Island were associated with an increase in the proportion of treated females and with a progressive decrease in winter fawn:doe ratios, from 1.21 fawns/doe in 2005 to 0.19 fawns/doe in 2010. Winter fawn:doe ratios averaged 1.36 fawns/doe (range, 0.84–1.62 fawns/doe) at the Hunting Island control site. Annual survivorship averaged approximately 79% among ear-tagged females. The rate at which deer populations diminished in association with PZP treatments on Fripp Island was higher than that seen at other study sites, although the reasons for the more rapid decline on Fripp Island are not well understood.
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