Mustela erminea (ermine)

Mustela erminea (the stoat) is an intelligent, versatile predator specialising in small mammals and birds. It is fearless in attacking animals larger than itself and adapted to surviving periodic shortages by storage of surplus kills. In New Zealand it is responsible for a significant amount of damage to populations of native species. This species has been nominated as among 100 of the "World's Worst" invaders.

The stoat (Mustela erminea) has the typical mustelid shape: a long thin body, a smooth, pointed head, short legs, and five toes on each foot, furred between the pads. The claws are sharp and non-retractile. The ears are short, rounded, and set almost flat into the fur. The eyes are round, black and slightly protruding; the whiskers are very long, and the muzzle is black and dog-like. The body fur is short, normally chestnut brown on the head and back, and white or cream (sometimes shading to yellow or even to apricot) on the underside. The tail is much longer than the extended hind legs, and always tipped with a conspicuous tuft of long black hair, which may be bristled out into a ‘bottlebrush’ at moments of great excitement (taken from King and Murphy, 2005).

Native range: Native to Holarctic region north of 40th parallel, ie northern parts of Eurasia and North America (King, 1983).
Known introduced range: New Zealand and some European Islands within the native range of the species (King, 2005). These islands include the Shetland Islands, north of Scotland; Terschelling Island, off Holland; and Strynoe Kalv, Denmark.

Stoats (Mustela erminea) are found anywhere they can find prey from beaches to above the treeline. They are found in all types of forest, grassland, agricultural land, dunes, scrubland and tundra. They are vulnerable to predation from other mammals and raptors so they tend to stick to cover in open country. In alpine areas stoats may spend most of their time in runs and burrows below the snow, this helps provide insulation against extremes in air temperature. Stoats do not avoid human settlemernts and can occasionally be seen in villages and suburban gardens (King, 1983; King and Murphy, 2005).

Introduced to New Zealand later than most other introduced predators (King 1984), after serious damage to native birds had already been done, stoats (Mustela erminea) contributed to the collective toll, especially in more remote areas of South Island (King and Murphy, 2005). M. erminea has been shown to be responsible for catastrophic losses of kiwi chicks in most years (see Apteryx australis; Apteryx haastii; Apteryx mantelli; and Apteryx owenii in IUCN Red List of Threatened Species) (Basse et al. 1999), and of hole-nesting forest birds in southern beech forests during periodic mouse irruptions (O'Donnell 1996). Once kiwi chicks reach a weight of around 800g they are able to defend themselves against stoats (McLennan et al. 2004) so kiwi nurseries have been set up where kiwi chicks are translocated to areas with heavy stoat control until they become large enough to defend themselves against stoats. Cost of research and management of stoats in New Zealand runs into millions of dollars a year.

Stoats (Mustela erminea) have been used to exterminate pest rodents and rabbits on small islands with few alternative prey (King 1989), but only in certain conditions which are hard to meet. Belief that they could control rabbits was the reason for bringing them to New Zealand, but the islands were too large and alternative prey too abundant (King, 2005).
Stoats were formerly an important source of white fur (ermine) harvested by trappers in Russia and Canada.

Preventative measures: Risk Assessment models for assessing the risk that exotic vertebrates could establish in Australia have been further explored by the Western Australia Department of Agriculture & Food (DAFWA) to confirm that they reasonably predict public safety, establishment and pest risks across a full range of exotic species and risk levels.
The Risk assessment for the Stoat (Mustela erminea), has been assigned a VPC Threat Category of EXTREME.
Mammals and birds were assessed for the pest risk they pose if introduced to Australia, by calculating Vertebrate Pests Committee (VPC) Threat Categories. These categories incorporate risk of establishing populations in the wild, risk of causing public harm, and risk of becoming a pest (eg causing agricultural damage, competing with native fauna, etc). The 7-factor Australian Bird and Mammal Model was used for these assessments.

Trapping is widely used to remove stoats (M. erminea) from game estates in UK and conservation reserves in New Zealand. Trapping is effective when very intense, but is rapidly countered by immigration (McDonald & Harris 2002). Leg-hold traps are still legal for the moment, but likely to be banned in the forseeable future; the first humane trap, the "Fenn", developed in UK in the 1950s, was better but does not meet current standards. New, more humane traps, are being developed. There are no poisons currently registered for use against stoats, but they are often killed by secondary poisoning after operations targetting possums and rats.

Management Information

Compiled by the IUCN/SSC Invasive Species Specialist Group (ISSG)

1. General Considerations
King (1984) concluded that stoat control is probably only worthwhile (in New Zealand) in specific situations, namely during the nesting season and only for the protection of a few endangered bird species that are most adversely affected by stoat predation (in Lawrence and O.Donnell 1999). This extract is edited from King (1994):
To plan a mustelid control or monitoring operation, it is necessary to:
Be familiar both with the conservation landowner’s general policy on pests, and also with the national strategy for mustelids
Have a clear understanding of the local problem, and with any native species to be protected
Set clear and achievable objectives, and
Choose appropriate field techniques that will serve the objectives and assess whether they have been met.
To operate the chosen plan, it is necessary to:
Develop experience in the efficient use and maintenance of the field techniques to be used
Keep systematic records on what has been done,
Note and think about any potentially significant observations made during the course of routine field work.
2. Monitoring / Tracking
Stoats (Mustela erminea) can be detected via tracking tunnels, which conservation managers and researchers at mainland sites throughout New Zealand are now using to index mustelid numbers (Gillies and Williams Unpub.).
3. Preventative measures
Clapperton and Day (2001) found that the costs of exclusion fencing for stoats using the Xcluder™ design of multi-species pest-proof fence is effective and cost-efficient in large reserves of 5000 ha or more and on peninsulas. In such locations the cumulative costs of conventional control methods exceeds the initial cost of a fence set up (plus maintenance costs) in as little as four years. Electric fences may also be used but may be prone to failure (Clapperton and Day 2001).
4. Physical Control
Trapping is a widely used method to control stoats in the United Kingdom and New Zealand. Intensive trapping is effective but may be rapidly countered by re-invasion from surrounding areas (McDonald and Harris 2002). Control via trapping was attempted in New Zealand to protect the takahe Notornis mantelli.  Traps were set in a line at 200 m intervals in Takahe Valley, however, they had little effect on the stoat population, and takahe numbers continued to decline (Lavers and Mills 1978, in Dilks et al. 1996).
Leg-hold traps are still legal in some countries but are likely to be banned. The Fenn is a relatively humane trap developed in United Kingdom in the 1950s. In fact, at least four kill traps tested have proven to be less than satisfactory at delivering a (relatively) humane death, namely the Fenn Mk IV, Fenn Mk VI and Victor Snapback (Warburton and Connor 2004) (see 10. Ethical Considerations).
The cost of stoat control is also likely to be prohibitive, however, perimeter trapping layouts may reduce costs. Perimeter tapping around a 100 hectare block has been proven experimentally to be as effective at achieving high stoat catch rates as setting traps in intensive 100-meter-spacing lines (Lawrence and O.Donnell 1999). However, this experiment was conducted in a situation of low stoat density and may not be applied to higher stoat densities.
Baits for traps are often based on the food source that is available to the target animal (Hamilton 2004). This can be the prey item of highest abundance within an area and can vary from season to season, eg: rabbit, possum or mouse (Hamilton 2004). Other food items such as eggs (Dilks et al. 1996, Dilks and Lawrence 2000, in Hamilton 2004), commercially mixed baits, fish paste, and pet food have been used in New Zealand. In many cases the choice of type of bait is more for ease of operation and the long-life properties of the bait than for its attractant capabilities (Hamilton 2004).
Locating natal stoat dens using trained dogs and controlling female stoats and young inside (by pellet use and sealing of holes) is a potentially effective control method which may reduce the summer influx of young stoats into an ecosystem. Theobald and Coad (2002) trialled this method in Trounson Kauri Park, New Zealand, to protect newly hatched kiwi chicks. However, the results of this study were not promising (possibly as not all entrances were sealed off or the pellets used were not appropriate).
5. Chemical Control
Sodium monofluoroacetate (1080) poisoning, which is described as causing death ‘from heart and lung failure within 12 hours’ is perceived as more acceptable method of stoat control by the public than Talon® (brodifacoum), which is described as ‘a common rat poison’ causing death ‘from internal bleeding within a week’ (according to a survey conducted by Fitzgerald, Fitzgerald and Wilkinson 2005). However while views about Talon® use may be clearer and more negative than about 1080 a large number of respondents (40%) still found 1080 to be an unacceptable stoat control method.
The use of cholecalciferol (registered in New Zealand as FeraCol®) to kill stoats was studied experimentally by Spurr et al. 2002 to test palatability of bait formulations Death by this route does not appear to be pleasant, with the toxin inducing the stoat to stop feeding after one day, lose weight and die via organ failure within about a week. The authors also suggest cyanide, diphacinone, zinc phosphide, or MNT.
6. Biological Control
Because of continuing concerns about the impact of stoats on the nation’s biodiversity, especially its avifauna, the New Zealand Government created, in 1999, a special five-year fund for research into the control of stoats. The project considered biological control methods such as canine distemper virus, potential mustelid-specific diseases, and possible techniques for controlling the fertility of stoats which could require the genetic modification or ‘engineering’ of organisms for use in biological control. However in a public survey on attitudes toward stoat control Fitzgerald, Fitzgerald and Wilkinson (2005) found a lack of support for this method. This was probably due to negative attitudes towards and public debate over the genetic modification of organisms. Despite this animal welfare is an important factor in the selection process for possible biological control options (Fitzgerald Wilkinson and Saunders 2000; Lenghaus et al. 1994, in Littin and Mellor 2005).
McDonald and Lariviere (2001) reviewed the literature on the diseases and pathogens of stoats, and closely related mustelids, with a view to identifying potential biological control agents. They found that Aleutian disease virus, mink enteritis virus, and canine distemper virus hold promise as agents of lethal control, though the risks to non-target species posed by these viruses are serious. Host-specific ectoparasites such as Tnchodectes ermineae, nematodes such as Skrjabingylus nasicola, and bacteria such as Hehcobacter mustelae and Bartemella spp could have a role as vectors for the transmission of fertility control agents (McDonald and Lariviere 2001).
7. Information Access
An international bibliographic database of 1213 papers, notes, books and unpublished reports on stoats and two related species (Mustela erminea, weasel M. nivalis and common weasel M. frenata) dating from between 1877 and 2004 is available as a simple reference list at http://bio.waikato.ac.nz/staff/bibliography.pdf. It is also available in a searchable, interactive form on the following Australian based website: www.feral.org.au (which also provides information on other invasive species). It is interesting to note that New Zealand is now the world centre for research on mustelids, especially stoats
8. Integrative Management
Stoat control often goes hand in hand with monitoring recovery rates of threatened or native species. At Dart Valley, New Zealand, a site of southern beech (Nothofagus spp.) forests populations of threatened mohua (yellowhead) Mohoua ochrocephala are at risk from the stoat population. Lawrence and O’Donnell (1999) recommend measuring increases in the breeding success of mohua to detect positive outcomes of stoat control (decreased predation rates and increased nesting success).
9. Research
Research into the use of new baits and lures is substantial (a few examples are discussed below). However, primary data is lacking and in the population biology of stoats with the notable exception of stoat populations in southern beech Nothofagus spp. mast cycles (New Zealand) (McDonald and Lariviere 2001). Understanding the relationships between stoat density and bird survival is needed for the development of novel biotechnological stoat control methods (McDonald and Lariviere 2001) and, perhaps, to justify such research. Such studies may contribute to an understanding of the effects of new stoat control methods/agents on predator (ie: stoat) and prey (ie: bird species) populations. After initial data is collated computer simulation modelling may prove a powerful tool in determining the impact of a range of control strategies on stoat populations and the birds they prey on (Courchamp and Sugihara 1999, in McDonald and Lariviere 2001).
More attractive lures and acceptable baits are required to improve the efficacy of stoat control in New Zealand (and elsewhere). Apparently traps with bright covers (yellow and red) are more attractive to stoats than those with duller covers (green and black) (Hamilton 2004). A series of preference trials by Robbins and colleagues (2007) on captive stoats to determine the potential of various visual, textural, and movement cues in lures and baits found the following: (i) bait colour does not affect bait consumption, (ii) mouse-shaped bait with realistic eyes are more readily eaten than disk-shaped bait, (iii) possum fur increases the palatability of wax baits, but not of already palatable bait, (iv) concave mirrors are attractive lures, however, the presence of egg lures is more important, (v) red light-emitting diodes do not increase investigatory responses and, finally, (vi) moving baits attached to a pendulums may direct the attention of stoats towards baits/control devices.
Improved, cost-effective stoat control methods are needed to reduce stoat predation on New Zealand wildlife. Clapperton and colleagues (2006) tested the attractiveness and palatability of a range of lures and baits on captive stoats. Prey-based lures were the most attractive. The authors failed to find additional odours that would enhance lure success. Highly palatable baits included gelatine-injected freeze-dried mice and a wax/tallow/rabbit meat mix. These effective baits can be used as alternatives to egg-based baits to increase variety and counter bait-shyness (Clapperton et al. 2006). The addition of sodium meta-bisulphate to bait produces attractive, palatable baits with a field life of at least one week (Clapperton et al. 2006).
10. Ethical Considerations
Stoats are cute. Many people are horrified at the thought of killing these animals. This poses an ethical dilemma. Sometimes certain values must be compromised in order to preserve other values such as the conservation of the environment. In this case reducing predation by stoats is clearly essential for the survival of several endemic species on the mainland of New Zealand, such as kiwi Apteryx spp., kokako Callaeas cinerea, takahe Porphyrw hochstetteri, mohua Mohoua ochrocephalus, kaka Nestor meridionalis and kakariki Cyanoramphus spp. (McDonald and Lariviere 2001; Dilks et al. 1996). This is because the irregular but severe predation by stoats on native birds has been implicated in their decline (Elliott and O'Donnell 1988, Donnell 1996, in Dilks et al. 1996).

Whatever ones’ views on this issue, there is one thing everyone should agree about: if an animal is to be killed it should be done humanely and, if possible, expertly. Some of the trapping methods discussed in the literature are probably far from humane and more research needs to be conducted in this area. For example, kill traps have been assessed against the specifications that target animals must be rendered unconscious within three minutes, and results indicate that most kill traps currently in use fail the test (NAWAC 2000, in Warburton and Connor 2004).
Any planned mink control project should endeavour to gain public support through educational means (eg: informing the public about the negative impacts of stoats on native bird life). They should also be based around firm humane guidelines and legal methods of disposing of the animals. Interestingly, the large majority of respondents (96%) in a public survey responded that the least acceptable option (out of 13 stoat control methods including not controlling stoats at all) was not controlling stoats at all. This suggests that when weighing up the costs of damage done to the environment by stoats against the costs (to stoats) of stoat control respondents preferred the options of controlling stoats and were not prepared to allow stoats to remain uncontrolled in New Zealand (Fitzgerald, Fitzgerald and Wilkinson 2005).
Under New Zealand legislation a non-target species must be killed as quickly and humanely as possible. Animals must only be captured and killed in ways that fulfil legal obligations under the Animal Welfare Act 1999 (New Zealand) (Ragg and Clapperton 2004). See www.maf.govt.nz/biosecurity/animal-welfare act to download a guide to the Animal Welfare Act. For further legislation for pest control in New Zealand, Australia, Europe and the United Kingdom please see Littin and Mellor (2005) (this document can be downloaded from: https://www.oie.int/eng/publicat/rt/2402/PDF/littin767-782.pdf).
Furthermore, before any animal-research projects can proceed (eg: captive trials to determine the toxicity of poisons, the efficacy of fertility control agents, and welfare impacts of poisons and traps) in New Zealand they must receive approval from institutional Animal Ethics Committees (AECs) (Warburton and Connor 2004). This is usually done by assessing the ethical cost to the experimental animals (ie: pain and suffering) in relation to the end benefits of the study (i.e. biodiversity conservation and control of zoonoses) (Warburton and Connor 2004). However, it must be noted that this process is often only achieved with vague benefits being provided (eg: to develop more cost-effective protection of an endangered species), and weighed against equally vague costs to welfare, as it is a difficult and subjective task to evaluate and quantify the costs and benefits in these situations (Warburton and Connor 2004).
Further information
The Department of Conservation (New Zealand) DOC Technical Series has a wealth of information on this topic. This can be accessed via a search on Mustela erminea at:  http://www.doc.govt.nz/templates/page.aspx?id=39160. In particular King (1994) and King, O'Donnell and Phillipson (1994) have prepared a comprehensive guide to the monitoring and control of mustelids on conservation lands. This guide is available in two parts (Part 1: Planning and Assessing an Operation and Part 2: Field and Workshop Guide), which can be accessed at the aforementioned website or directly by using the websites in reference list below.
For more details on the effectiveness of trapping stoats using different bait types, tunnel designs, and trap positions please see Dilks et al. 1996, which can be accessed at: http://www.rsnz.org/publish/nzjz/1996/109.php.
To access an international bibliographic database of stoat information please go to the searchable, interactive Australian based website at: www.feral.org.au. Alternatively, to access a simple reference list go to: http://bio.waikato.ac.nz/staff/bibliography.pdf.
For a discussion on the ethics of stoat and other animal control please see Littin and Mellor (2005) Strategic Animal Welfare Issues: Ethical and Animal Welfare Issues Arising From the Killing of Wildlife for Disease Control and Environmental Reasons. This may be accessed at: https://www.oie.int/eng/publicat/rt/2402/PDF/littin767-782.pdf.

References
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Dilks, P.J., O'Donnell, C.F.J., Elliott, J.P. and Phillipson, S.M. 1996. The Effect of Bait Type, Tunnel Design, and Trap Position on Stoat Control Operations for Conservation Management, New Zealand Journal of Zoology 23: 295-306. [Accessed 20 March 2007, from: http://www.rsnz.org/publish/nzjz/1996/109.php]
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