Factors that determine whether commercial exploitation of introduced mammals in New Zealand provides a useful method for reducing their densities and therefore their impacts on native biota are examined. The history of commercial harvesting of three introduced species, red deer Cervus elaphus, Himalayan thar Hemitragus jemlahicus and possums Trichosurus vulpecula is described. It is then assessed why the conservation outcomes of this harvesting have differed for these three species and an attempt is made to define some general rules about where and when commercial exploitation is a useful pest control tool. Commercial harvesters of red deer for game meat and byproducts have harvested over 2 million deer since 1960 and reduced the national population from over 1 million to a current population size of ca 250,000 deer, a 75% reduction overall. Current annual harvests average ca 20,000 deer, with annual variations explained largely (r2 = 0.89) by the price of venison. Commercial harvesting of thar for game meat between 1971 and 1982 killed at least 39,000 thar and reduced the population by over 90% to <5,000 animals. After the peak harvests before 1976, low annual harvests of only a few hundred animals were able to be sustained as thar were killed as bycatch of the deer industry - but the harvest was stopped between 1983 and 1994 because of pressure from recreational hunters. Commercial exploitation of possums for fur began in 1921, with over 56 million skins being exported. The annual harvest is correlated with the price of furs. Compared with deer or thar, the prices paid per possum are low, and possums are much more abundant (ca 60 million) and ubiquitous pests. The annual harvests of possums have therefore been variable and never sufficient to have more than locally significant effects on population densities.
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Vol. 2 • No. 3