Summary

Many vertebrates are believed to be capable of experiencing pain and as such should be treated so that they do not suffer unnecessarily. My conservative estimate suggests that the human control of wild animal populations affects at least 25 million vertebrates annually. Unfortunately, popular lethal methods of population control also inflict significant suffering on target animals. Rather than lethally reducing existing populations, an alternative is to artificially manipulate population growth such that target species reproduce at slower rates.

Introduction

Population control is the policy or practice of limiting growth in numbers of a population. The growth of animal populations may be managed in a wide variety of lesser-known ways. The most controversial method, culling, is considered by detractors as a cause of unnecessary suffering and by supporters as an effective way to regulate overabundant species and preserve biodiversity. An oft-mentioned alternative to killing overabundant animals is to slow their rate of population growth through, for example, contraception. In this paper, I consider a broad spectrum of methods and discuss their impact on individual beings with the aim of understanding which methods are most effective at reducing the unnecessary suffering of wild animals.

The paper begins by discussing population dynamics to build a foundational understanding of the factors that affect population growth. Section 4 considers how to define “humane” and which subjective experiences of animals are included in this definition. Following this, Section 5 provides a brief look at the numbers of wild animals culled annually within the populations of the species considered – African elephants, kangaroos, wild boar, feral pigs, red foxes, deer, European rabbits, house mice, and European carp. These were selected because they are some of the more widely targeted species for population control. While they do not represent all animals subject to population control, their experiences are largely generalizable to those within the same genus or family.

The lethal methods of population control are then broken down into hunting, chemical methods, and disease. Section 6 on hunting analyzes the different techniques involved in hunting and the amount of suffering they cause the target species. Section 7 on chemical control methods looks at the physiological effects of various poisons used in baits and fumigation. Section 8 on disease considers whether the systemic release of species-specific diseases, which is not common but highly effective, is more or less humane than the previously discussed non-biological methods. Lastly, section 9 presents two viable alternatives to population reduction in the form of artificial population regulation — immunocontraception and genetic modification — and concludes that more resources and more research should be dedicated towards understanding how to artificially regulate population growth to manage overabundance.

Population dynamics

Population dynamics refers to the shift in number and composition of populations of species over time. There are density-dependent and density-independent factors that play into population dynamics. Density-dependent factors include elements such as resource competition and predation which impact birth and mortality rates (Yarrow, Ecology, & Specialist, n.d.). For example, when populations are low relative to the maximum that an environment can support, per-mother birth rates are high. When the population is at or near the environmental carrying capacity, birth rates will slow, and mortality rates increase (Yarrow et al., n.d.). This encompasses a natural form of population regulation. Density-independent factors, including natural disasters or severe weather such as droughts, also impact birth and mortality rates predominantly by impacting the species’ ability to survive (“density-independent factor | biology,” n.d.).

There are cases where certain environments lack natural predators or present low resource competition, which allows populations to grow exponentially (Bowman, W.D. Cain, M.L. & Hacker, S.D., 2008). However, exponential growth is not indefinite. As the growth of populations begins to reach the environmental carrying capacity, the birth rates will decline as the environment is no longer capable of supporting the previous rate of growth. Mortality rates will rise as individuals struggle to find adequate food and shelter, and an increased rate of individuals will succumb to disease, parasitism and predation (Bowman, W.D. Cain, M.L. & Hacker, S.D., 2008).

In reality, human population growth has had a significant impact on animal population dynamics already. Changes such as increased land cultivation and urbanisation have contributed to the population decline of many herbivores (e.g. the American bison) (“American bison,” 2016) and predators (e.g. the Tasmanian tiger) (“Thylacine,” 2016). In other cases, human interference has created an environment for the exponential growth of populations, such as deer, whose natural predators have largely been eliminated (Yarrow et al., n.d.). Currently, population regulation policies focus on species that interfere with agriculture (e.g. foxes killing livestock) or threaten the populations of native flora and fauna (e.g. the African elephant in Kruger National Park). Motivations for population control can be, and sometimes are, independent of whether species’ population growth rates indicate they may be nearing carrying capacity. However, if the population control methods do not cause additional suffering, these policies may, in fact, be reducing total wild animal suffering for two reasons. Firstly, the remaining individuals have greater access to resources which means they are less likely to experience resource competition (Tomasik, 2015). Secondly, a reduction in the aggregate number of animals may mean a reduction in the aggregate suffering of those animals, if we are inclined towards the view that the lives of the most numerous, short-lived wild animals may be net negative (Tomasik, 2015).

If we recognise the need to manage the population of certain species, whether it is to reduce their suffering or to prevent their interference with human populations, we should prioritise the suffering inflicted on the individual such that population control does not become a greater source of suffering than natural factors such as predation, parasitism, disease, starvation, and dehydration.

What does “humane” mean?

The term “humane” is not unambiguously or universally defined. However, there are certain criteria that can be applied to the term to understand how to classify human intervention in the lives of animals. A safe starting point is “a desire to avoid the infliction of unnecessary pain upon wild animals.” (T. Sharp & Saunders, n.d.) In relation to the population control of wild animals, this amounts to “the development and selection of feasible control programs and techniques that avoid or minimise pain, suffering and distress to target and non-target animals.” (T. Sharp & Saunders, n.d.) There are two elements to this definition: the physical suffering involved in control programs and the psychological distress immediately prior to and during the control program. A criterion for humaneness should consider not only the method of killing but also the experiences of the animal prior to killing and the experiences of the non-target animals inadvertently disrupted (T. Sharp & Saunders, n.d.). It is likely that most current lethal control methods involve some negative impact on animals. Therefore, a more humane method will have a reduced negative impact on the animal’s welfare (T. Sharp & Saunders, n.d.). This is how I will understand the relative humaneness of lethal control methods versus population regulation alternatives.

Wild animal numbers

There is very little data on the populations of so called “invasive” and overabundant species. The categorisation of invasive depends on which geographical region we assess. Overabundance categorisation faces similar difficulties. For a more comprehensive look at wild animal numbers, see (“How Many Wild Animals Are There? | Essays on Reducing Suffering,” n.d.).

Species Population numbers / density Country/Region No. killed annually African elephant 450 000 - 700 000 (“Basic Facts About Elephants,” 2012) Africa N/A Kangaroo 50 000 000 - 60 000 000 (Australian Wildlife Society, n.d.) Australia 5 200 000 (“Commercial kangaroo harvest in 2012 - Wildlife trade and conservation in Australia,” n.d.) Wild boar / Feral pigs 23 000 000 (“Feral Pigs,” n.d.) Australia ~ 3 450 000 5 000 000 (Gates, 2012) US >750 000 (Tompkins, 2013) ~ 5 000 000 (“European Warming Sends Wild Boar Numbers Soaring,” 2015) Europe 750 000 (Bathurst, 2012) Red fox 6 200 000 (“Red foxes in Australia,” 2016) Australia ~ 310 000 240 000 (IUCN, 2016) UK ~ 12 000 Deer 2 000 000 (Design, n.d.-a) UK 350 000 (Design, n.d.-a) European rabbit 200 000 000 (“The Rabbit Problem | Rabbit Free Australia,” n.d.) Australia > 10 000 000 House mouse 50 - >800 mice/hectare (Singleton et al., 2005) Australia > 25 000 000 (Singleton et al., 2005)

Hunting

The species featured in this paper do not comprise a comprehensive study of all animals culled by hunting. Instead, they represent a snapshot of the animals culled in large numbers. Their experiences can largely be generalised to other invasive or overabundant species sharing similar physiologies.

Elephants

Descriptions (“Elephant Management — National Geographic Magazine,” n.d.) of elephant culling in South Africa depict a helicopter tracking elephants using a GPS, and a professional with a rifle aiming one shot to the head of the matriarch, followed by a series of shots to the remainder of the herd. Death is described as immediate, and the elimination of the herd is described as humane (“Elephant Management — National Geographic Magazine,” n.d.). If this were the reality, the accuracy of aerial shooting and the immediacy of death would, in fact, limit the suffering of the elephant herd. However, footage (“cull video,” n.d.) of elephant culling suggests a starkly different experience. A helicopter flies above, distressing the elephants who are then shot by hunters on the ground. Shots at such close range will not always hit the head and kill immediately. It takes many more shots to end the elephant’s life.

Between 1966 and 1994 a total of 16,027 elephants were culled in Kruger National Park (KNP), South Africa, as part of a population reduction program (Owen-Smith, Kerley, & Page, 2006). The target of the cull was to bring elephant populations under a ceiling of 7000 (Owen-Smith et al., 2006) and only adults were targeted. The orphaned young were trapped and translocated as part of a conservation program. Decades later, a study into how these culling practices affected elephant social systems found that elephants disrupted by a cull “performed poorly on systematic tests of their social knowledge, failing to distinguish between callers on the basis of social familiarity… [and] showed no evidence of discriminating between callers when age-related cues simulated individuals on an increasing scale of social dominance, in sharp contrast to the undisturbed population where this core social ability was well developed.” (Shannon et al., 2013)

In 1995 a ban was placed on culling, and by 2006 elephant numbers were estimated to be 13 000 in KNP. This raised fears that the population of elephants had exceeded the environmental carrying capacity and that in the face of irreparable ecological harm , culling must be reintroduced. However, a panel of scientists researching the issue for the Minister of Environmental Affairs and Tourism concluded (amongst others):

The previously maintained ceiling of around 7000 elephants in KNP should not be construed as a carrying capacity.

The case for active intervention is stronger in smaller reserves, but other measures could reduce the need for culling.

Management interventions need to be backed by sufficiently informative monitoring of the consequences. (Owen-Smith et al., 2006)

Despite these findings, the culling of elephants was reintroduced between 2008 – 2013 in South Africa. It finally ended with the release of the Elephant Management Plan which focuses predominantly on minimising the ecological impact of elephants but does so by strategically manipulating access to water so they are encouraged to migrate. (“Confidential,” n.d.)

Kangaroos

The culling of kangaroos to protect native flora and reduce resource competition with other grazing animals has been conflated with the commercial harvesting of kangaroos. The Australian government sells licences to kill kangaroos for commercial purposes which are said to support “management goals based firmly on principles of sustainability.”(“Commercial kangaroo harvesting fact sheet – 2012,” n.d.) The Australian Government permits the harvest of four kangaroo species in four states: Queensland, New South Wales, South Australia, and Western Australia (“Commercial kangaroo harvesting fact sheet – 2012,” n.d.). Quotas are set annually as a proportion of estimated populations (“Commercial kangaroo harvesting fact sheet – 2012,” n.d.). In 2012, the quota was more than 5.2 million kangaroos across Australia (“Commercial kangaroo harvest in 2012 – Wildlife trade and conservation in Australia,” n.d.).

The Code of Practice stipulating the most humane way to cull kangaroos requires adult kangaroos be shot in the head by a single bullet (Purposes, n.d.). This is a mandatory requirement bolstered by the regulation that kangaroos showing evidence of bodily trauma other than to the head cannot be sold commercially. However, shooting generally occurs at night, and it is carried out from distances of 50 m to 100 m using a single-shot, high-power rifle. These factors and the small target size of the kangaroo’s head mean that many may not be accurately shot. Consider, for example, this description from a former commercial shooter: “[t]he mouth of a kangaroo can be blown off and the kangaroo can escape to die of shock and starvation. Forearms can be blown off, as can ears, eyes and noses. Stomachs can be hit expelling the contents with the kangaroo still alive. Backbones can be pulverized to an unrecognisable state, etc. Hind legs can be shattered with the kangaroo desperately trying to get away on the other or without the use of either.” (Nicholls, 2005). An assessment of compliance with the Code, carried out by Animal Liberation NSW between 2005 and 2008, identified that an average of 40% of kangaroos hunted in New South Wales and Queensland were not fatally shot (Ben-Ami et al., 2014). More concerning is that when female kangaroos are shot, the most humane way to kill orphaned ‘in pouch’ joeys is described as stunning followed by decapitation or clubbing to death (Purposes, n.d.). Meanwhile, ‘at foot’ joeys that escape are more likely than not to die prematurely of starvation, dehydration, or exposure.

Wild boar / Feral pigs

Wild boar / feral pig populations are artificially regulated in a number of countries, including the United States, the United Kingdom, and Australia. Attempts at population reduction for feral pigs have proven ineffective despite the fact that they have been hunted for centuries (“TPWD: Feral Hogs,” n.d.-a). As a result, lethal population control of wild boar / feral pigs is likely to involve multiple methods, including fencing, baiting, and hunting. In the UK, US, and Australia culling is managed by states or local communities. This means, there is little to no legal protection requiring wild boar / feral pigs to be killed humanely. In fact, after raising concerns regarding the welfare of wild boar, the UK Department for Environment, Food and Rural Affairs notes “there is no legislative vehicle under which specific welfare protection for feral wild boar can be introduced.” (“feralwildboar.pdf,” n.d.)

Aerial shooting is usually employed by states in Australia because large populations of feral pigs can be killed in a relatively short period of time. Skilled spotters ensure shots are lethal and the distress involved in the cull program is short. A helicopter sweep can result in 3000 feral pigs killed (“HCPSL Shooting Hunting of Feral Pigs Booklet.indd,” n.d.). However, there is a possibility that not every feral pig will be fatally shot and that some may be left to a protracted and painful death. Aerial shooting is also used in the US; however, environmental conditions often do not allow for effective sweeps (Nordrum, n.d.). While aerial sweeps can be effective, they offer only a short-term and localised solution.

Ground shooting is performed both professionally and commercially in the US, UK and Australia. Guidelines on the most humane way to hunt wild boar are to shoot at night and target the chest (heart and lungs) or head (“WildBoar_shooting.pdf,” n.d.). Factors such as the small target size of the boar’s head, the thick skull and hide, and the pig’s intelligence, speed and ferocity, make it very hard to locate and accurately make these shots from a safe distance. There is little regulation to ensure wild boar are ground shot as humanely as possible. Given the difficulty in tracking wild boar, the use of dogs also known as pig-dogging is permitted in the US, UK, and Australia (“ABC OPEN: The bloody truth about pig dogging,” n.d., “TPWD: Feral Hogs,” n.d.-b, “WildBoar_shooting.pdf,” n.d.). Pig dogging is an especially brutal method of hunting. It involves the use of dogs to track wild boar until they pick up their scent. The dogs pursue and attack the pig, sometimes mauling them to death while the hunters catch up and shoot or slaughter the pig with a knife (“ABC OPEN: The bloody truth about pig dogging,” n.d.). This method of culling puts dogs and humans at risk, and it causes unnecessary suffering to the captured pig.

The continued practice of pig dogging is all the more inexcusable given the effectiveness of ground shooting feral pigs by setting traps (Choquenot, Mc Ilr, & Korn, n.d.). This is particularly useful if poison baiting is not possible and hunting is impractical. Traps essentially lure feral pigs with feed baits and contain feral pigs within an enclosure (Choquenot et al., n.d.). Feral pigs are then shot at close range, which means shots have a higher likelihood of accuracy. If checked regularly, this control method reduces both physical and psychological distress associated with hunting and shooting (“pig trapping worksheet,” n.d.). However, if traps are not checked daily, feral pigs caught in a trap are left to the elements with no food or water and could experience extremely painful deaths due to exposure, starvation, and dehydration (“pig trapping worksheet,” n.d.).

Deer

The culling of deer is subject to extensive guidelines in the UK through the Deer Initiative. The Deer Initiative estimates 350,000 deer are culled each year (Design, n.d.-a) and provides guidelines supported by legislation on where, how, when, and which species of deer may be culled (“guide_legislation.indd,” n.d.). The ecology of the six species of deer involved in cull programs is monitored (Design, n.d.-b), and a guide on humane shooting states that “good bullet placement is to induce unconsciousness as swiftly as possible, rendering the animal insensitive to pain, and for this to be quickly followed by death.” (“161.pdf,” n.d.) Deer populations breed rapidly, and the Deer Initiative provides considered guidance on reduction and maintenance culling programs which demonstrate a good understanding of deer population dynamics (Objectives, n.d.).

However, other than legislation on the use of firearms and other slaughtering instruments, there is no legally binding requirement to follow the Deer Initiative shooting guidelines (“guide_legislation.indd,” n.d.). It is, of course, unlikely that deer are shot accurately every single time, and the suffering involved in a stray shot is likely to be protracted and immense, comparable to the experiences of both wild boar and kangaroos. Whilst it is not possible to remove all negative impacts of a hunting program, the depth of research, monitoring, and evaluation of deer populations by the Deer Initiative is an example of an effective and considered approach to managing populations by lethal means.

Chemical control methods

Baiting

Next to hunting, baiting is one of the most common methods of lethal population control. Baiting is a cost-effective way to eliminate a large percentage of an invasive species quickly, and it is a core component of integrated control programs. The baiting methods featured below are not meant to comprise a comprehensive study of all baiting techniques. Rather, they represent a snapshot of the most commonly used methods.

1080

In Australia, 1080 is recommended to manage populations of “pests” such as rabbits, feral pigs, and foxes (“1080 PCO fact sheet | NSW EPA,” n.d.; Fluoroacetate & Fluoroacetate, n.d.). 1080 is made of a naturally occurring enzyme to which native Australian animals have built a tolerance (Fluoroacetate & Fluoroacetate, n.d.). Its selective susceptibility makes it an efficient, species-specific pesticide, and it is described as a humane pesticide (Fluoroacetate & Fluoroacetate, n.d.). However, in a report commissioned by the New Zealand Parliamentary Commissioner for the Environment death by 1080, it is described as resulting from the “inhibition of energy production cells resulting in vital functions of the body to stop. Herbivores usually die of heart failure, whereas carnivores are more likely to die of respiratory failure.” (“PCE-1080.pdf,” n.d.) Death by 1080 poisoning is one of the least humane methods measured (“Humaneness Assessment – PestSmart Connect,” n.d.). Whilst 1080 is perhaps the most targeted poison in Australia, it can and has been ingested by nontarget animals. Depending on the size of the animal and the size of the dose, it can cause serious illness or death (“RAB002_ground-1080-baiting.pdf,” n.d.).

Anticoagulants

Anticoagulants, either single-dose or multiple-dose (“Rodenticide,” 2016) are used on rabbits and rats (“Pindone Chemical Review,” 2014). In Australia, where the environment doesn’t allow for the use of 1080, such as residential or semi-rural areas, the alternative is Pindone. Pindone is described as a “first-generation anticoagulant that acts by blocking the synthesis of vitamin K-dependant clotting factors, which causes fatal haemorrhages in susceptible animals.” (“Ground baiting of rabbits with pindone,” n.d.) The time of death is 10 to 14 days after ingesting the dose in rabbits (“Ground baiting of rabbits with pindone,” n.d.). The US uses similarly acting anticoagulants such as warfarin, chlorophacinone, and diphacinone. They may require multiple doses over several days to be lethal (“Rodenticides,” n.d.-a). This is likely to prolong the suffering of the target species.

Sodium Nitrite

Research has identified the success of sodium nitrite as a poison bait for feral pigs because of their susceptibility to methaemoglobin-forming compounds (Cowled, Elsworth, & Lapidge, 2008). Sodium nitrite “acts by blocking oxygen-binding pathways in the pig” (Hopper, n.d.), which leads to unconsciousness and death within two hours (Hopper, n.d.). If eaten by non-target animals it is unlikely to cause death, since feral pigs are unique for their low levels of methaemoglobin reductase (Cowled et al., 2008). In addition, its effects are potentially reversible (Hopper, n.d.). It is considered the most humane poison for use on feral pigs (Hopper, n.d.), and it is currently being trialled for use in the US by the USDA (“USDA testing sodium nitrite to poison feral hogs, which do $800M damage a year to US farms,” 2014).

Zinc Phosphide

Australia faces mouse plagues on average once every 3.5 years (Singleton et al., 2005). A mouse plague is defined as an increase from 1 mouse ha-1 to 1,000 mice ha-1 over a period of 12–18 months (Singleton et al., 2005). High densities of mice are particularly damaging to crops (Singleton et al., 2005). To combat the effects of a mouse plague, zinc phosphide baits are used in high mouse-density regions. Zinc phosphide produces a phosphine gas when it comes into contact with the water and acid in the mouse’s stomach. This prevents the body’s cells, particularly the heart, brain, kidney, and liver from making energy, and the cells die (“Rodenticides,” n.d.-b). The symptoms of ingestion are described as “convulsions, paralysis, coma, and death from asphyxia.” (Whisson, 1996). It is unclear how long it takes from the point of ingestion to death. However, one source suggests death will occur “overnight.” (Whisson, 1996).

Fumigation

Fumigation is recommended as a lethal population control method for rabbits and foxes, the purpose of which is to kill large numbers simultaneously in their respective shelters – warrens and dens. This is particularly used during breeding seasons (“Rabbit Control | NSW Department of Primary Industries,” n.d.). Fumigation is the introduction of poisonous gas into a warren or den (“Integrated fox control for urban and semi-urban areas | Established Invasive Animals | Invasive animal management | Pest animals | Pests, diseases and weeds | Agriculture | Agriculture Victoria,” n.d.-a).

Phosphine

Phosphine is a systemic poison which depresses the central nervous system and respiratory function (“rabbit fumigation of warrens with phosphine worksheet,” n.d.). Rabbits are rounded-up into their warrens, and a phosphine tablet is thrown in. From the time the tablet enters the warren, death takes on average 225 minutes (“rabbit fumigation of warrens with phosphine worksheet,” n.d.). However, from the onset of symptoms – collapse, gasping, convulsions and paddling – it is on average 30 minutes to death (“rabbit fumigation of warrens with phosphine worksheet,” n.d.). If the concentration of the phosphine is not high enough, the duration of suffering is longer, and death is caused by pulmonary edema – the abnormal buildup of fluid in the lungs (“rabbit fumigation of warrens with phosphine worksheet,” n.d.).

Carbon monoxide

Carbon monoxide is used on foxes during August – October (in Australia) when vixens are confined to their dens with their cubs (“Integrated fox control for urban and semi-urban areas | Established Invasive Animals | Invasive animal management | Pest animals | Pests, diseases and weeds | Agriculture | Agriculture Victoria,” n.d.-b). Their burrows are sealed and carbon monoxide is pumped into the den. Carbon monoxide kills by binding “to haemoglobin in the red blood cells, with an affinity 250 times that of oxygen. This results in reduced oxygen-carrying capacity and altered delivery of oxygen to cells. Hypoxia – the reduction of oxygen supply to the tissues – eventually leads to unconsciousness and death.” (P. by T. Sharp & Crc, n.d.-a) If delivered in the right dosage and in a properly sealed space, then carbon monoxide can lead to a loss of consciousness within minutes (“FOX004_den-fumigation_web.pdf,” n.d.). This means it is likely to be a relatively more humane form of chemical population control than the others discussed in this section.

Disease

European rabbit

The use of disease as a lethal method of population control is only used on a few species. In Australia, rabbit haemorrhagic disease (RHD) has been effective in keeping rabbit populations low across most of the country (Cooke, n.d.). It is now released through inoculations and baits, the purpose of which is to cultivate an outbreak. Inoculation requires trapping rabbits (“rabbit inoculation of rabbits with RHDV worksheet,” n.d.) which, if the rabbits are not checked regularly or if placed in unsheltered regions, could lead to starvation, dehydration, and suffering or death from exposure. In addition, handling rabbits and the process of injecting them with RHD is likely to cause some level of psychological distress (“rabbit inoculation of rabbits with RHDV worksheet,” n.d.). The symptoms are described as “a rise in body temperature lasting 18 to 24 hours, followed, in around 70–90% of cases, by death up to 48 hours after the onset of fever.” (“rabbit bait delivery of RHDV worksheet,” n.d.) “The disease appears typically as a necrotizing hepatitis with associated haemorrhaging, and death occurring as a result of generalised organ dysfunction.” (“rabbit bait delivery of RHDV worksheet,” n.d.) However, the fever can last for up to two days and in some cases, death can takes as long as 114 hours if the rabbit is inoculated (“rabbit inoculation of rabbits with RHDV worksheet,” n.d.) and 150 hours if baited. (“rabbit bait delivery of RHDV worksheet,” n.d.) This leads to the potential for additional suffering due to loss of appetite, lethargy, and fatigue (“rabbit bait delivery of RHDV worksheet,” n.d.). “During an outbreak, a limited number of rabbits (5–10%) may show a chronic or subclinical form of the disease. These animals often die 1 or 2 weeks later, probably due to a liver dysfunction.” (“rabbit bait delivery of RHDV worksheet,” n.d.)

European carp

The use of the carp herpesvirus is currently in consideration to control the overabundance of carp in Australia. Current methods of controlling carp populations include the use of poison baits. However these are not species-specific and will kill all fish in a particular location (“PFFS1.pdf,” n.d.). Another method is the establishment of physical barriers to contain fish incursions (“PFFS2.pdf,” n.d.). So far, both of these have provided only very localised and time-intensive solutions. Carp herpesvirus is favoured for its specificity, as the virus has no impact on nontarget species (St J Crane, n.d.). Carp herpesvirus is very contagious, and its symptoms are described as “reddening of the gills, excess mucus on the gills and skin, darkening of the skin, and eventually patches of skin necrosis (tissue death). Signs of disease occur within 7-14 days of infection (depending on water temperature), and death then occurs within a day or so. Carp that survive infection are infected for life, and, when stressed, may die or show signs of disease again.” (“Carp herpesvirus as a biological control for carp in Australia – PestSmart Connect,” 2012) It is as yet unclear how much suffering carp experience from the point of infection till death.

The alternative: regulating population growth

It seems unlikely that any form of lethal population control will have no negative impact on the welfare of the target species, and in fact, many methods discussed above may potentially cause more suffering than natural causes of death, although a thorough study of various degrees of suffering would be required to determine this definitively. Rather than address the overabundance of animal populations through lethal means, it is possible that regulating population growth proves to be a more humane and more environmentally sustainable approach. There are a number of viable ways to artificially regulate population growth. Immunocontraceptives are currently used on invasive and overabundant species such as elephants and deer. However, this still requires a certain amount of interference with individuals which imposes a degree of psychological distress on the individual and their companions. A more sophisticated method being researched is genetic modification, which may prove to dramatically reduce the suffering of wild animals and solve issues arising from their overabundance.

Immunocontraception

Immunocontraception is the “use of an animal’s immune system to prevent it from fertilizing offspring.” (“Immunocontraception,” 2016) The Humane Society of the United States is currently sponsoring research into a synthetic version of PZP (porcine zona pellucida), which is a naturally occurring protein in pig ovaries. “Zona pellucida (ZP) proteins surround the unfertilized eggs of all mammals. Sperm must attach to ZP before an egg can be fertilized. When pig ZP (PZP) is injected into a female animal, her body produces antibodies to it. These antibodies attach to her own ZP proteins, preventing sperm from attaching and blocking fertilization.” (“Questions and Answers about Immunocontraception : The Humane Society of the United States,” n.d.) Currently, the preferred form of delivery is via a dart fired from a rifle or a helicopter (“Questions and Answers about Immunocontraception : The Humane Society of the United States,” n.d.). In the treatment of white-tailed deer “PZP typically reduces pregnancy rates by 80-90%” (“Deer PZP Fact Sheet 1-10.doc,” n.d.). While it is promising in its universal application to mammals and its effectiveness in trials, it is time-intensive to apply. The PZP dose requires two injections in the first year followed by annual boosters (“Questions and Answers about Immunocontraception : The Humane Society of the United States,” n.d.). That said, recent research has demonstrated successful immunisation for two or more years with new, one-shot PZP vaccines (“Questions and Answers about Immunocontraception : The Humane Society of the United States,” n.d.). The Humane Society notes that more work must still be done “to determine where, to what extent, and how fast PZP can reduce wildlife populations.” (“Questions and Answers about Immunocontraception : The Humane Society of the United States,” n.d.)

Genetic modification

There are various forms of genetic modification for population control including sex-specific sterilisation, sex-specific lethality construct, and trojan genes. For an assessment of their effectiveness, see the study by (Bax & Thresher, 2009). Below I discuss the most promising method: daughterless genes (Bax & Thresher, 2009).

Daughterless genes

Daughterless genes alter the genes of “offspring to develop as specified sex irrespective of sexual genotype.” (Bax & Thresher, 2009) Currently being tested to manage the population of carp in Australia, the synthetic gene introduced into female carp prevents the enzyme ‘aromatase’ from being produced which converts testosterone into oestrogen, thus producing female embryos (“Gene technology to be used in carp control plan,” n.d.). The absence of oestrogen in embryos means they will, by default, develop into males. In addition to affecting the pregnant female carp, the altered gene reproduces in the offspring (Hobart, n.d.), which allows the natural breeding habits of carp to spread the synthetic gene throughout the population. Over enough generations, the population of carp are expected to “crash.” (Hobart, n.d.).

The Commonwealth Scientific and Industrial Research Organisation (CSIRO) has successfully tested the daughterless gene on zebrafish and has reduced egg production by 70-90% (Hobart, n.d.). While still being tested on carp populations, the CSIRO is optimistic that the release of enough carriers of the daughterless gene, monitored over a number of generations, will effectively manage carp populations (Hobart, n.d.). In an assessment on the potential of 9 recombinant methods, daughterless technology proved to be the most robust in both frequency of genetically modified females displacing wild-type females and the elimination of viable females (Bax & Thresher, 2009). The study further concluded that “[i]mportantly, the results also show that control can be achieved by adding the construct at a small enough percentage of wild-type recruitment (<5%) that the approach may be logistically feasible for a variety of pest species.” (Bax & Thresher, 2009)

Conclusion

I began this paper by establishing that a definition of “humaneness” has two elements: the physical suffering involved in control programs and the psychological distress that is inflicted immediately prior to and during the control program. It seems clear, from a purely humane perspective, that a population control method that focuses on preventing animals from being born with a minimal amount of continued interference from humans is preferrable to one that requires the systematic killing of animals. However, immunocontraception and genetic modification are technologies still in early stages of development. They are promising in their humaneness, but not tested on a large scale and they are likely to be significantly more costly than lethal population control. With more resources and research dedicated to understanding and developing these and similar techniques we may be able to reduce the harm infliced on wild animals through population control practices in the future.

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