Controlling Mouflon Sheep at the Kahuku Unit of Hawai`i Volcanoes National Park

Abstract

Hawai`i Volcanoes National Park acquired the 115,000-acre Kahuku Ranch Unit in 2003. The Kahuku Unit contains numerous exceptional natural resources including endemic plants and birds, many of which are federally listed threatened and endangered species. Eleven European mouflon sheep were introduced to Kahuku from 1968-1974 for private trophy hunting. Because Hawaiian plants did not evolve with mammalian herbivores, managers began to control the large mouflon population with a closely directed volunteer program. Since 2004, more than 1,900 mouflon have been removed through this program. We estimated that there were 2,586 ± 705 (90% CI) mouflon at Kahuku in November 2004, but by December 2006 the population decreased 30%. Although the ram:ewe ratio did not change significantly after this population decrease, the mean (±95% CI) overall ratio in 2004 was 1:2.4 (1:2.1-1:2.7) and 1:2.7 (1:2.4-1:3.1) in 2006. We found that 82.6% of adult ewes (n = 26) were pregnant with a single fetus in early 2007, and there was a significant increase in the number of lambs per ewe from 2005 to 2007 consistent with a density-dependent response. The maximum (±95% CI) number of lambs per ewe was 0.484 (0.412-0.558) in 2005 and 0.667 (0.587-0.750) in 2007. The directed volunteer program has been more successful in reducing mouflon abundance at Kahuku than species such as feral pigs elsewhere on Hawai`i Island, but some population-level responses such as increased reproduction could result from density decreases.