2000-2006: Mount Eccles/Budj Bim Koala surgical sterlisation/hormone implants

Hunted, marooned, re-introduced, contracepted: a history of Koala management in Victoria

Peter Menkhorst

“…At a meeting of the Australian and New Zealand Environment and Conservation Council (a forum of State, Commonwealth and New Zealand Environment Ministers) held in May 1996, culling was rejected as a management tool. Consequently, it was not considered during preparation of the National Koala Conservation Strategy published in 1998 (ANZECC 1998).

Counter balancing concerns for individual animal welfare is the increasing concern amongst land managers and conservationists about the ecological damage resulting from Koala over-browsing (Koala Management Task Force 1996; ANZECC 1998; Masters et al. 2004; Menkhorst 2004).The search for an alternative to translocation began in earnest in 1995 when the Department of Natural Resources and Environment commissioned a review of fertility control options (Middleton 1996a). The outcome of the review was a recommendation to conduct separate field trials of the effectiveness of two techniques – slow-release implants of a progestin hormone or oestradiol to females, and vasectomy of males (Middleton 1996b). Implementation began in late 1996, despite opposition from some quarters because it was feared that the program would divert money away from other urgent wildlife research and management programs, and because of doubt about the efficacy of male sterilisation (eg. Anderson 1996). So began a decade of intensive research and adaptive management trials by the Victorian and South Australian wildlife management agencies to develop methods for in-situ population control (Table 2).In Victoria, these trials have been guided by an expert advisory committee, the Koala Technical Advisory Committee, convened jointly by the two Government agencies with primary responsibility for Koala management, the Department of Sustainability and Environment and Parks Victoria. The committee’s role is to advise the two Government agencies on technical matters relating to Koala management. Its primary focus in recent years has been to advise on adaptive-management trials to assess a range of fertility control techniques for their efficacy, ethics and cost-effectiveness. 

“A six-year field trial of subdermal implants containing either the synthetic progestin levonorgestrel, or low doses of oestradiol, applied to female Koalas, began at Tower Hill Game Reserve in 1997. This trial indicated that a contraceptive rate of 100% could be maintained for up to six years using levonorgestrel implants, representing at least 60% of the reproductive life of a female Koala (Middleton et al. 2003; DSE unpublished data). A trial of the impact of vasectomy of male Koalas was also conducted at Red Bill Creek on French Island between November 1996 and October 1998. By vasectomising all males captured on the study site (the proportion of treated males on site varied over time because the population was not a closed one) this program reduced the proportion of females carrying pouch young from 87% at the beginning of treatments to 36% over two breeding seasons (DSE unpublished data).”

Meanwhile, severe over-browsing problems were emerging on Snake Island and at Mt Eccles National Park. Because the hormone implant trials had not been completed, surgical sterilisation of females, by transection and bipolar cautery of the distal oviduct, as undertaken by the South Australian Government on Kangaroo Island (Masters et al. 2004; Duka and Masters 2005), was initiated on Snake Island in 1999, and at Mt Eccles National Park the following year. In both these Victorian cases it was found that the combination of surgical sterilisation and immediate translocation could result in high levels of mortality (up to 90% in one treatment group) (Parks Victoria 2003a) and the practice was abandoned…

Based on the results of the hormone implant trial (Middleton et al. 2003), and the animal welfare concerns associated with surgical sterilisation, a large-scale trial of the efficacy of hormone implants at the population level was begun at Mt Eccles National Park in 2004. The Koala population there was estimated at 11 000 animals (Wood 2004) with a sex ratio a little below parity and female fertility rate of 38% (Chlamydia is present in the population) (McLean 2003). Therefore, it was estimated that there were about 2100 fertile females present. Over the three years to 2006, 2450 females were implanted (Figure 8), a level of treatment that is approaching the 75% of fertile females required to produce a significant population decline (N. McLean unpublished). This trial gives hope that most of the Koala populations currently causing significant defoliation can be held at sustainable population densities via a determined contraception program using levonorgestrel implants. Fortunately, three of the four current over-browsing populations (Tower Hill Game Reserve, French Island, Raymond Island) are considerably smaller than that at Mt Eccles. The Mt Eccles program also included the development of a koala-forest model to help evaluate the long-term consequences of different levels of fertility control on both the Koalas and their food supply (the Manna Gum forest) (Todd et al. in press). The model allows the assessment of the most ecologically and financially desirable target population size for the National Park (1000 adults).”