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Abstract
High-density free-ranging animal populations have potential to cause ecological and economic problems often necessitating active management to reduce numbers or to limit population growth. Fertility control is a more ethically acceptable management alternative to lethal methods, and is particularly attractive when the target population is a native species and eradication is undesirable.
Some high-density koala populations, particularly those occupying habitat isolates with limited dispersal opportunities, have the potential to cause extensive damage to their preferred habitat. Without intervention this can result in death of food trees, welfare impacts for the resident koala population and destruction of habitat for other species. Clearly these populations need managing. This thesis examines potential and current management options for controlling the introduced and overabundant koala population on South Australia’s Kangaroo Island. I examined the impact of these techniques on individual animals, with particular focus on trialling the commercially available GnRH-agonist contraceptive implant (Suprelorin®, Peptech Animal Health, Sydney) and comparing the outcomes to the current management practice of surgical sterilisation and translocation. An assessment of the reproductive characteristics of the population was also undertaken as management on Kangaroo Island has, to-date, been based on reproductive parameters derived from Victorian populations.
Suprelorin implants are simple to deliver in the field, provide a contraceptive effect in female koalas for 1-2 years and have no apparent adverse impacts on health or ranging behaviour of treated individuals. Likewise, surgical sterilisation, whilst costlier and more invasive, had no adverse effects on the behaviour or health of female koalas if animals were released back at the site of capture. However, increased mortality was observed in surgically sterilised koalas in the first 12 months after translocation to mainland South Australia.
Due to the relatively short duration of contraception provided by Suprelorin and the difficulty and large cost of capturing koalas on Kangaroo Island, it is not currently considered practical for large-scale koala population management at this site. Suprelorin implants have potential for smaller-scale management programs where capture is straightforward or in situations when permanent sterilisation is not desirable. Development of remote delivery techniques would significantly increase Suprelorin’s efficacy for management.