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RESEARCH ARTICLE (Open Access)
Contents Vol 51(1)

Implementing implants: delivery efficiency, contraceptive efficacy and population outcomes in two overabundant kangaroo populations

Graeme Coulson https://orcid.org/0000-0001-9873-8203 A B * and Michelle E. Wilson A C
+ Author Affiliations
- Author Affiliations

A School of BioSciences, The University of Melbourne, Parkville Campus, Vic. 3010, Australia.

B Macropus Consulting, Carlton, Vic. 3053, Australia.

C Wilson Environmental, Coburg, Vic. 3058, Australia.

* Correspondence to: gcoulson@unimelb.edu.au

Handling Editor: Douglas Eckery

Wildlife Research 51, WR22170 https://doi.org/10.1071/WR22170
Submitted: 10 October 2022  Accepted: 24 June 2023  Published: 18 July 2023

© 2024 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

Abstract

Context

Overabundant native wildlife can pose serious challenges for managers. The most direct, immediate way to reduce density-dependent impacts is by culling, but lethal control often lacks public support. Fertility control offers a non-lethal management alternative.

Aims

We conducted two fertility-control projects on eastern grey kangaroos (Macropus giganteus), a common and widespread species in Australia. We aimed to evaluate three key components of an integrated fertility-control project, namely, delivery efficiency, contraceptive efficacy and population-level outcomes.

Methods

The two study sites on the urban fringe of Melbourne, Australia, were small reserves. Both had undergone an irruptive peak and subsequent crash, with negative impacts on animal health. Gresswell Forest (52 ha) is open forest habitat enclosed by a kangaroo-proof fence. Serendip Sanctuary (250 ha) is grassy woodland and retired pasture, with a boundary allowing kangaroo movement onto neighbouring properties. We captured kangaroos with a dart gun at night and treated all healthy adult females with subdermal levonorgestrel implants.

Key results

Delivery efficiency (catch-per-unit effort) was greater from a vehicle at Serendip Sanctuary than on foot at Gresswell Forest, with only a marginal decline over successive nights at both sites. Background fecundity was 91% at Serendip Sanctuary, but close to zero at Gresswell Forest. Treatment efficacy was high, being 86–100% infertility at Serendip Sanctuary and 96–100% at Gresswell Forest. At Serendip Sanctuary, the proportion of females treated never reached 75%, whereas only one female at Gresswell Forest remained untreated after 3 years. Population density at Serendip Sanctuary exceeded the target range 3 years after culling and fertility control. No culling occurred at Gresswell Forest, but fertility control apparently held population density at a moderate level, albeit much higher than the density target.

Conclusions

Future management at Serendip Sanctuary and Gresswell Forest will require a sustained fertility-control effort. Further culling may also be required to complement fertility control at Serendip Sanctuary, the larger, open site.

Implications

Contraceptive efficacy of levonorgestrel is high in kangaroos and implants can be delivered efficiently, but achieving control of a large, open population will be challenging.

Keywords: catch-per-unit effort, culling, fecundity, fertility control, kangaroo, levonorgestrel, overabundance, recruitment, reproduction.

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