MANAGEMENT OPTIONS

4.1 Translocation

Translocation has been used as a conservation and popu­lation management strategy for koalas in southern Aus­tralia for over a century. It has been used as part of a management strategy for locally overabundant Proser­pine rock-wallabies on Hayman Is.

Following population collapses caused by overexploi­tation and habitat loss and fragmentation, koalas have been successfully re-established through translocation across parts of their range in southern Australia, in par­ticular Vic. Translocation has subsequently been used as a tool in managing locally overabundant koala popula­tions. Despite the success of translocation in re-establish­ing koalas in habitat from which they were formerly extirpated, translocation remains a controversial koala population management tool. Although some studies have demonstrated high survival post-translocation (Menkhorst et al. 2019), evaluation of the translocation of more than 3000 koalas over a 20-yr period from an island population revealed high rates of mortality (37.5%) and dispersal for sterilised and translocated individuals (Whisson et al.

Published accounts of macropod translocations are almost exclusively focused on conservation management and generally pertain to smaller species such as various wallabies, bettongs and potoroos. Although small num­bers of eastern grey kangaroos have been translocated in the face of urban development (Higginbottom and Page 2010), there are no published accounts of large-scale trans­locations for the purposes of managing overabundant populations. Such translocations are widely considered to be unsuitable because of the logistical challenges, includ­ing limited availability of suitable recipient sites; potential welfare implications associated with remote chemical cap­ture, transport and recovery of free-ranging kangaroos; post-release monitoring; and high resource implications (ACT Government 2010). Principal tenets in the argument against translocation of large macropods such as eastern grey kangaroos are that they are not threatened and trans­location is an ineffective population control strategy.

Common brush-tailed possums may reach high densi­ties in some urban and agricultural environments. Although local relocation of individual possums by licenced possum removalists or home owners is permis­sible, in most Australian states and territories transloca­tion to remote sites is not (Russell et al. 2013). High mortality (70%) has been demonstrated for translocated common brush-tailed possums (Pietsch 1994). Nonethe­less, illegal translocation of perceived problem possums with attendent animal welfare compromise has been documented (Wilks et al. 2008).

4.2 Contraception

The use of fertility control for population management of free-ranging wildlife populations has also been advo­cated as a humane alternative to culling (Adderton Her­bert 2004).

4.2.1 Surgical sterilisation

Techniques for surgical sterilisation have been described in detail elsewhere (Jolly and Spurr 1996; Duka and Masters 2005; Vogelnest and Portas 2008; see Chapter 10). Surgical options in males include castration and vasectomy, with vasectomy frequently preferred because of the retention of secondary sexual characteristics and reproductive behav­iours (Tribe et al. 2014). Surgical options in females include ovariectomy, ovariohysterectomy and oviducal ligation/ transection. Oviducal transection or cauterisation via lapa­roscopy has been used extensively in koalas as a population management tool at various locations in Vic. and on Kan­garoo Is., SA. Surgical sterilisation has also been used, but less extensively, in eastern grey kangaroos in locations where immigration and dispersal are limited such as in south-east Qld (Tribe et al. 2014) and the ACT. Laparo­scopic ovariectomy has been successfully used to manage an enclosed population of eastern grey and red kangaroos in NSW (Colgan and Green 2018) (see Chapter 10). More than 1400 kangaroos underwent laparoscopic ovariectomy utilising a novel approach with minimal complications.

Surgical sterilisation has the advantage of providing permanent contraception. Disadvantages include: high initial resource implications (capture, anaesthesia, surgi­cal expertise, specialised equipment); limited efficacy when males are targeted but not all individuals in a popu­lation are sterilised; limited efficacy where the immigra­tion of non-sterilised individuals is possible; and the potential for increased aggression between vasectomised males in polyoestrous species with continued cycling of females. Surgical sterilisation is recommended for ani­mals in managed care, populations where immigration and emigration is restricted (e.g. islands, fenced reserves, some urban parks) and situations where animals are to be translocated and a non-breeding population is to be established at the recipient site.

4.2.2 Hormonal implants

Synthetic progestins

Levonorgestrel - The principal contraceptive action is via reduction of FSH and LH release from the pituitary, resulting in the inhibition of follicular development and ovulation (Poiani et al. 2002). Secondary mechanisms also inhibit fertility and include changes to cervical mucus volume and viscosity, which impedes migration of spermatozoa, and interference with endometrial develop­ment inhibiting implantation. The effects of levonorg­estrel implants have been evaluated in tammar wallabies (Nave et al. 2000; Hynes et al. 2007), eastern grey kanga­roos (Poiani et al. 2002) and koalas (Middleton et al. 2003; Hynes et al. 2010; Ramsey et al. 2021). Adverse effects associated with the use of synthetic progestins have been described in placental mammals. No adverse effects associated with levonorgestrel have been reported in marsupials, but there have not been specific or long­term studies to examine potential adverse effects.

Contraceptive efficacy and duration of effect in east­ern grey kangaroos appear to be dose-related, with higher infertility rates and longer duration of action in kanga­roos treated with 210 mg versus 140 mg levonorgestrel (Coulson et al.

Behavioural changes observed in female eastern grey kangaroos implanted with levonorgestrel were limited to treated females spending increased amounts of time in open areas compared with control animals. Males were more likely to associate with untreated than treated females, leading the authors to suggest this may promote greater ranging behaviour by males, placing them at increased risk of vehicular trauma (Poiani et al. 2002).

In two separate studies, female koalas implanted with 70 mg of levonorgestrel were effectively treated for the duration of both studies: 4 and 6 yr, respectively (Middle­ton et al. 2003; Hynes et al. 2010). Removal of the implants resulted in a return to fertility the following breeding season in all animals. Examination of vaginal cytology suggests that koalas (the only marsupial known to be an induced ovulator) may still ovulate if mated when implanted with levonorgestrel, but will fail to conceive.

Despite the fact that capture and restraint are required for administration of levonorgestrel, and the high cost of the implants, its long duration of effect offsets the high initial resource requirement. However, the potential adverse effects associated with long-term use have not been adequately investigated in native mammals at this time.

Etonogestrel - Implants administered at either 34 mg or 68 mg doses were ineffective for reducing fertility in female koalas (Hynes et al. 2010). Etonogestrel has not been investigated in other marsupial species and its effi­cacy in other marsupial species is unknown.

GnRH agonists

Deslorelin acetate - A synthetic gonadotropin-releasing hormone (GnRH) agonist available as a biodegradable slow-release SC implant in either 4.7 mg or 9.4 mg doses (Suprelorin, Peptech Animal Health Australia) (Fig.

Although it is an effective contraceptive in female marsupials, deslorelin is ineffective in male marsupials. Administration of deslorelin at a range of doses in male tammar wallabies elicited acute elevations in LH and tes­tosterone, but failed to affect basal concentrations of these two hormones in the longer term compared with untreated controls (Herbert et al. 2004b). Although sperm parameters were not assessed, testicular size was unaffected by deslorelin administration, suggesting that fertility was maintained. In male common brush-tailed possums, deslorelin caused significant declines in FSH and testosterone concentrations, but testicular size was unaffected, with treated males siring as many offspring as control males (Eymann et al. 2007). Fertility was main­tained in two male koalas following administration of 5-mg deslorelin implants (Herbert et al. 2001).

There are published reports of deslorelin use for fertil­ity control in the common brush-tailed possum (Eymann et al. 2007), eastern (Herbert et al. 2006; Woodward et al. 2006; Wilson et al. 2013; Tribe et al. 2014; Wilson and Coulson 2016) and western grey kangaroos (Mayberry 2010), tammar (Herbert et al. 2004a; Herbert et al. 2005; Herbert 2007; Herbert et al. 2013) and black-footed rock­wallabies (Willers et al. 2015) and the koala (Herbert et al. 2001; Carlyon 2013). Deslorelin has also been used for population control in a range of other marsupials, in fly­ing-foxes and in seals held in managed care.

In female black-footed rock-wallabies contraception was achieved for a period of 27 mo with no difference in duration or variability observed between the administra­tion of one or two 4.7-mg implants (Willers et al. 2015). Timing of administration is important, because if there is both PY and an embryo in diapause (which will subse­quently develop) the duration of contraception will be reduced to ~14 mo.

Administration of 9.4 mg deslorelin in eastern grey kangaroos held in managed care in conjunction with PY removal resulted in contraception lasting at least 510 d (Woodward et al. 2006). No behavioural or body mass changes were observed in treated animals, but a behav­ioural oestrus was induced 3 d post-administration. Sev­eral studies examining deslorelin use in free-ranging female eastern grey kangaroos have demonstrated con­traceptive efficacy of 100% in the first year post-adminis­tration with a gradual return to fertility in the subsequent 3 yr (Herbert et al. 2006; Wilson et al. 2013).

The efficacy of deslorelin in small numbers of female koalas has also been evaluated (Herbert et al. 2001; Car- lyon 2013). Duration of effect for 4.7-mg implants is a minimum of 12 mo and up to 25 mo in some individuals. Efficacy is less than 100%, but the reasons for this are unknown. Females maintained and increased their body­weight and exhibited long-distance ranging behaviour (correlated with the absence of back young) following deslorelin administration (Carlyon 2013).

Among free-ranging female common brush-tailed pos­sums administered 4.7 mg deslorelin, 80% were rendered infertile for an average of 381 d and up to 483 d in one indi­vidual (Lohr et al. 2009). In another study, females housed in managed care implanted with either 4.7-mg or 9.4-mg implants remained infertile for at least one breeding season.

Fig. 6.2. Subcutaneous implantation of a deslorelin implant for contraception of a female swamp wallaby (Wallabia bicolor).

Duration of return to fertility was variable in two females from which the implants were removed and in those in which implants were retained (Eymann et al. 2007). Body condition of females increased following implantation and the seasonal fluctuations observed in untreated controls were not observed in treated individuals.

Major limitations to the widespread use of deslorelin for fertility control in overabundant free-ranging popula­tions include the requirement for capture and restraint for administration, cost and variable duration of effect between and within species. Its relatively limited dura­tion of action further limits its usefulness in free-ranging seasonally breeding species. Best results are achieved if treatment is timed to coincide with anoestrus.

4.2.3 Immunocontraceptive vaccines

a. GonaCon™

GonaCon™ is a non-commercial immunocontraceptive vaccine developed by the US Department of Agriculture’s Wildlife Services’ National Wildlife Research Centre as a single-dose treatment to produce multi-year reproductive suppression in females of a range of species including ungulates, carnivores and rodents. Although the vaccine has been tested in males, it is less efficacious in terms of duration of effect and has greater welfare implications (Fagerstone et al. 2010; Miller et al. 2013). The vaccine consists of a synthetic GnRH peptide conjugated to a mollusc protein carrier, which is emulsified in the adju­vant AdjuVac™. It induces production of antibodies against GnRH, which bind to circulating endogenous hormone and prevent GnRH from binding to pituitary receptors and, in turn, prevent the release of FSH and LH. The disruption in FSH and LH release downregulates the production and release of oestrogen, progesterone and testosterone, resulting in infertility. IM vaccine adminis­tration induces a granuloma or sterile abscess at the injec­tion site, although such responses may be necessary in inciting an appropriate immune response (Fig. 6.3). The investigation of GonaCon_™ for fertility control in Aus­tralian native mammals is currently limited to tammar wallabies, Proserpine rock-wallabies, eastern grey kanga­roos and common brush-tailed possums.

In adult male tammar wallabies, GonaCon_™ adminis­tration produced rapid testicular atrophy and infertility for a minimum of 2 yr in association with the production of significant GnRH antibody titres (Snape et al. 2011). Immature male wallabies failed to undergo puberty for at least 2 yr following vaccination. In adult female tammar

Fig. 6.3. Typical granulomatous response following IM hand injection of GonaCon™ in a free-ranging female eastern grey kangaroo (Macropusgiganteus). Photo: Claire Wimpenny

wallabies, 100% of animals were rendered infertile for 6 yr following vaccination. Lactation was unaffected by treat­ment, but the viability of diapausing embryos was dis­rupted within 3 mo (L Hind pers. comm.). In female common brush-tailed possums administered a single injection of GonaCon™, greater than 70% of animals were infertile for a period of at least 2 yr (Cross et al. 2011).

In a study of sub-adult female eastern grey kangaroos, 16 animals administered GonaCon™ via hand injection were all infertile for a period of 3 yr and failed to undergo normal peripubertal development (Snape 2012). At 8 yr post vaccination, 82% of the kangaroos still alive (11 ani­mals) remained infertile (ACT Government 2017). As with other contraceptive options, the requirement to capture animals for administration is a significant impediment to broadscale and cost-effective application of GonaCon_™. Assessment of the efficacy of GonaCon_™ administration via single dart delivery in eastern grey kangaroos is currently being undertaken and funded by the ACT Government in collaboration with CSIRO. Since 2015, 142 female kangaroos across five sites in the ACT have been treated with GonaCon_™ administered IM either by hand injection or remotely by dart in order to compare the efficacy of the two methods. Results to date indicate high efficacy, with only 13% of hand injected and 21% of dart-delivered animals producing a young in the year following treatment. In the second year, none of the hand injected kangaroos produced a young. Over 90% infertility was achieved in female kangaroos in the third year post dart delivery (Wimpenny et al. 2021).

Initial observations have shown that dart delivery results in both SC and IM deposition of the vaccine com­pared with deep IM deposition following hand injection. Comparison of hand injection versus dart delivery in white-tailed deer (Odocoileus virginianus) demonstrated consistent deep IM deposition when the vaccine was hand injected and greater vaccine efficacy (Evans et al. 2015). Nevertheless, the potential for GonaCon™ to be used for fertility control in small populations of macropods with limited dispersal and immigration is high.

b. Zona pellucida vaccines

The mammalian oocyte is surrounded by a matrix of several glycoproteins, the zona pellucida (ZP), which plays a crucial role in the binding of spermatozoa with the oocyte during fertilisation. The ZP is composed of between two and four sulfated glycoproteins (ZP1-4) that are highly conserved across species, albeit with some interspecific variation (Duckworth et al. 2007). Vaccina­tion with ZP proteins results in the production of anti­bodies that reversibly inhibit binding of spermatozoa with the oocyte. ZP vaccines, typically derived from por­cine proteins, have been used to inhibit reproduction in a range of ungulate, primate and carnivore species. Two vaccinations at least 1 mo apart are required. Oestrus may be disrupted and welfare concerns have been raised in cases of repeated cycling in females following concep­tion failure. Additionally, prolonged treatment may result in permanent infertility.

Both porcine ZP proteins and non-glycosylated recom­binant common brush-tailed possum ZP3 emulsified in Freund’s complete adjuvant have been evaluated in common brush-tailed possums, eastern grey kangaroos and koalas (Kitchener et al. 2009a; Kitchener et al. 2009b). Antibody responses were observed in koalas treated with either preparation, but fertility was reduced only in ani­mals immunised with porcine ZP. In contrast, both vac­cines were effective in producing infertility in eastern grey kangaroos. Recombinant ZP2 and ZP3 are also effective in common brush-tailed possums and identification of an infertility relevant epitope of the ZP2 protein raises the possibility of species-specific ZP vaccines for marsupials (Duckworth et al. 2006; Duckworth et al. 2007).

There is currently no ZP vaccine registered for use in Australia and experimental studies have used laboratory preparations with varying levels of quality control for limited numbers of animals. Additionally, inflamma­tion, necrosis and pain may be associated with the use Freund’s complete adjuvant, leading to animal welfare concerns.

4.3 Lethal control

There is growing sociopolitical opposition to the use of lethal control measures for managing overabundant native wildlife, particularly in peri-urban areas. Lethal control is not supported as a management tool for koalas at a federal or state level, but is commonly used for the control of locally overabundant macropod species at vari­ous locations around Australia. Three broad options for lethal control of native mammals exist: chemical eutha­nasia, poisoning and shooting.

Chemical euthanasia requires restraint (physical or chemical) and administration of a lethal injection, typi­cally IV barbiturates. The effectiveness of this approach is limited by the logistical and resource implications associ­ated with application to large numbers of animals, the potential for antemortem stress and carcass disposal issues. Nonetheless, chemical euthanasia remains appli­cable in some urban and peri-urban situations where shooting is not appropriate.

A variety of poisons are known to be lethal to native Australian mammals and have been used for population control in both Australia and New Zealand. Sodium monofluoroacetate (1080), strychnine and capsulated cyanide have been used as primary control agents in a range of species considered to be agricultural pests, including red-necked wallabies (N. rufogriseus), Tasma­nian pademelons (Thylogale billardierii) and common brush-tailed possums. 1080 remains in use under licence on agricultural and private forestry land in Tas. Signifi­cant welfare concerns have been raised regarding the use of 1080 and alternatives such as capsulated cyanide have been investigated (Eason et al. 2010).

Shooting of overabundant macropods remains in widespread use across several Australian states and terri­tories for the purposes of commercial harvesting, damage mitigation or biodiversity conservation. Both commercial and non-commercial shooting of macropods is governed by relevant state and territory legislation and two national codes of practice (National Code of Practice for the Humane Shooting of Kangaroos and Wallabies for Com­mercial Purposes and National Code of Practice for the Humane Shooting of Kangaroos and Wallabies for Non­Commercial Purposes) are designed to ensure appropriate animal welfare outcomes. Nonetheless, welfare concerns have been raised in relation to the animals shot, any PY or young at foot that may be present and non-target conspe- cifics (Descovich et al. 2015; Hampton and Forsyth 2016). A review of the extent of compliance with the National Code of Practice for the Humane Shooting of Kangaroos (subsequently replaced by two new codes for commercial and non-commercial shooting of macropods) identified welfare concerns in both the commercial and non-com- mercial sectors (RSPCA Australia 2002). Greater welfare compromise was noted with non-commercial (damage mitigation) shooting because of the limited oversight in this sector.

Two recent reviews of a peri-urban macropod culling program for biodiversity management in the ACT dem­onstrated appropriate animal welfare outcomes and high levels of compliance with the relevant code of practice (ACT Government 2013; Hampton and Forsyth 2016). Although some welfare issues associated with the eutha­nasia of dependant young were noted, this program was considered to produce minimal stress through the timing of the cull, habituation of target animals to human pres­ence and shooting methodology (Hampton and Forsyth 2016). Additionally, a review of a culling program to manage an urban, fenced population of western grey kan­garoos (M. fuliginosus), which incorporated public engagement before commencement, demonstrated that shooting control programs can be publically acceptable, humane and cost-effective (Mawson et al. 2016).

4.4 Dispersal

Dispersal of urban flying-fox roosts using a range of deterrents, including noise, smoke, light, roost vegetation modification and hazing by helicopter, has been attempted on numerous occasions. Reviews of dispersal attempts revealed that dispersed animals did not aban­don the local area; dispersed animals typically moved short distances and established additional roosts nearby; repeat dispersal action was required; human-wildlife conflict was often not resolved; and dispersal attempts were associated with high financial cost (Roberts et al. 2011; Roberts and Eby 2013). Additionally, analysis of urinary cortisol from flying-foxes undergoing dispersal demonstrated an association between the level of distress and the nature and timing of the dispersal activity (Edson et al. 2015). These findings suggest careful consideration should be given to recommending dispersal as a manage­ment option for urban flying fox camps and to the disper­sal techniques used.

5.