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One of the most significant threats to global biodiversity is the introduction and establishment of an alien species. Therefore the management and prevention of these invasive animals are of crucial importance to global conservation (Atkinson 1996(Genovesi 2005, Myers et al. 2000). Native Irish Mustelid species are restricted to the badger Meles meles, the pine marten Martes martes, the stoat Mustela erminea and the otter Lutra lutra (Dayan and Simberloff 1994). Due to the lack of competition facing the American mink Mustela vison (see figure 1), as an alien introduced species, it is difficult to predict its impacts on the Irish fauna (Stokes et al. 2004).
In this review the status of the American mink in Ireland and its impacts on the Irish fauna and flora are examined. Strategies and actions of control of the invasive mink in Europe is evaluated and the methods of predator control in New Zealand's mainland islands and their applicability to Ireland are investigated. Finally an estimation of the potential cost of mink control in Ireland is calculated.
1.1. Legislation and policy
The Republic of Ireland is home to many wildlife populations of international importance that are required to be preserved under European Union Law. Various Special Protected Areas (SPAs) have, as a result, been created for the protection of bird species and their habitats under the EU Birds Directive. Special Areas of Conservation (SACs) have been established for the protection of other wildlife species and their habitats under the EU Habitats Directive. The establishment of these directives was as a result of The Berne Convention (1979) for the protection of wildlife and habitats, The Bonn Convention (1980) for the protection of migratory species, and The Ramsar Convention (1994) for the protection of wetland birds. Sites delegated as Natural Heritage Areas (NHA) provide protection under national legislation and can be observed in figure 2. Under the Convention on Biological Diversity (1992) the ROI is required to regulate invasive species through methods of control, prevention and eradication as outlined by Stokes et al. (2004). The effective management of invasive species as a vital aspect in the conservation of biodiversity is outlined in the above conventions (Genovesi & Shine, 2004)
Directions for the regulation of invasive species are provided by the Council of Europe (COE) and the International Union for the Conservation of Nature (IUCN). The management of invasive species in isolated or island ecosystems for the conservation of any indigenous threatened species was put forward by the Convention on the Conservation of European Wildlife and Natural Habitats. Reinforcement of the Convention on Biological Diversity has resulted in the prevention, control and eradication of introduced species that have detrimental effects on native and migratory species, ecosystems and habitats.
1.2 American mink as an invasive species
The American Mink is native to North America and is one of five invasive species that are now firmly established in Europe (Jeschke and Strayer 2005). The establishment of these feral populations result from escapes from captivity, in cases of fur farming, and preconceived release in Russia and other European countries (Mitchell-Jones et al. 1999). The American mink have reportedly naturalised in parts of South America and Asia also (Medina 1997, Previtali, Cassini and Macdonald 1998). As an introduced and invasive species the American mink has had a destructive effect on both native species and economic activities (Harrison and Symes 1989, Moore et al 2000, Macdonald and Harrington 2003). Therefore it is critical to identify the ramifications of the establishment of the American mink in Europe and the methodologies for the control of these populations.
1.2.1 The establishment of mink in Ireland.
The establishment of the American mink in Ireland arose following numerous escapes of mink from fur farms as a result of which feral populations were self-sustaining by the late 1980s (Smal 1991). Six fur farms still remain in regulated operation in the ROI (Stokes et al. 2004). The first record of wild mink was detected by a spread from eastern fur farm locations to the west in 1961 (Deane and O' Gorman 1969). The wild mink still favour an easterly distribution as seen in figure 4 (Bailey and Rochford 2006, Chapman and Chapman 1982, Dean and O' Gorman 1969, Preston et al. 2001, Smal 1994). Early detection of feral mink were within the vicinity of 24 functioning fur farms. However the spread of the mink has led to its inhabitation in many SPAs, SACs and NHAs designated sites in the east and further inland (Roy et al. 2009).
Although the American mink is found throughout Ireland (Smal 1988) its numbers are sparse (Smal 1991) and this seems to be in correlation with those of the white-clawed crayfish Austropotamobius pallipes. Species and taxa that are under threat from the mink in Ireland consist of island-nesting birds, terns and waterfowl listed under Annex 1 of the Birds Directive. Vulnerability of these species is mainly due to their size and ground nesting habits (Roy et al. 2009). Due to the localised nature of the impacts local control only has been enforced (Smal 1991).
1.2.2 The economic impacts of mink
The establishment of the American mink as an alien, invasive species can have disastrous effects on both native species and economic activities. Feral mink have been cause of economic damage in Wales and England with damage reports submitted through surveys by trout farmers and by the Ministry of Agriculture and Fisheries and Food advisors (Harrison and Symes 1989). The most frequent problems applied to damage to poultry by hunting mink. Incidents were minor, however predation of farmed game birds were the most costly loss. 46% of trout farmers, who reported incidents with feral mink, complained of damage to fish cages and that scratching and stress of the trout were of concern. It was also noted that damage was most frequent in late summer due to predation by dispersing juvenile mink (Gerell 1970). Although the mink do not appear to have a serious economic effect at present, without appropriate management their numbers will increase.
1.3 Mink control in Ireland.
The National Parks and Wildlife Service (NPWS) has put a small scale control programme in place in numerous locations throughout the ROI in an attempt to protect terns and corncrakes from ground based predators such as the mink (Warner and Cormacan 2008). Areas of interest are:
1. Black Islands, Lough Ree and Carrownure Bay, Lough Ree.
2. Frans Callow, Tower Callow and Burranagh Callow, River Shannon.
3. Crolly Lough, Co. Donegal.
4. Inch Lough, Donegal.
5. Carrowmore Lough, Co. Mayo.
6. Loughs Conn and Cullin, Co. Mayo.
7. Cross Lough, Co. Mayo.
8. Baltray, Meath.
9. Wexford Wildfowl Reserve.
10. Lady's Island Lake, Wexford.
11. Kilcode beach, Wicklow.
Over 49 feral mink were trapped over 2944 trap nights in 2008 in the above locations. Live capture traps were baited with scent glands and fish. An average of 0.044 mink per trapnight was calculated with a maximum of 0.26 mink per trapnight in Black Islands, Lough Ree and Carrownure Bay. The lowest number of mink per trapnight was 0.0028 in Crolly Lough (Warner and Cormacan 2008).
The feral mink were easily caught within the first 2-3 days of baiting and were seen to be active throughout the summer period. This may be due to the positioning of traps in colonial ground bird nesting areas which were rich in food sources and therefore trappings did not cover a largely varied biogeographically area. Female mink seeking food for young in these areas during the summer months may contribute to the steady number of mink captured in this area as this has been seen in similar projects (Harrington et al. 2009). Due to the nutritionally rich area, caught mink were continuously replaced by colonising individuals thus remaining trappable throughout the summer. This vacuum effect has been observed in many species removal programmes (Efford, Warburton and Spencer 2000). Although high numbers of mink can be trapped within a small area, therefore making it a cheap and effective method of protecting native species in specific areas, it does not however have a permanent effect on the mink population. Trapped mink are steadily replaced by outsider animals hence contributing to the large number of trapped individuals resulting in misleading eradication results (Warner and Cormacan 2008).
2. The current status of mink control in Europe
2.1 The status of mink in European countries
Figure 5 outlines the status of the feral American mink as an invasive species in twenty eight European countries.
2.2 Strategies and actions
Depending on dispersal behaviour there are three foremost strategies that can be implemented for the control of invasive species:
1. Early detection
3. Management and evaluation of established species (Wittenberg and Cock 2001).
Four scenarios can be assembled dependant on the current and potential abundance of invasive American mink present in European countries:
1. Countries which are absent of mink fur farms and established mink populations, e.g. Slovenia, Malta.
2. Countries in which mink fur farms are present and small, localised mink populations are established, e.g. Spain, Italy.
3. Countries in which mink fur farms were once present and activity has ceased though mink populations are widespread, e.g. Estonia, UK.
4. Countries in which mink fur farms are present and widespread mink populations are established, e.g. Sweden, Denmark.
Prevention is a method of avoiding the establishment of mink populations in countries which are absent of fur farms and established colonies (Wittenbery and Cock 2001). This is particularly important in regions which contain native species that are vulnerable to either predation or competition. Small islands are of great concern as native animals are less likely to endure competition caused by alien species (Courchamp, Chapuis and Pascal 2003). A dynamic approach would involve the positioning of future fur farms in areas that would be unfavourable for the establishment of mink populations, e.g. in areas with little water (Linscombe et al. 1982) and scarce in food source (Birks and Dunstone 1985).
In countries in which fur farms are present though mink populations are not yet established, early detection, intervention and eradication are the best resort (Wiitenberg and Cock 2001). During the early establishment of populations, or at lag phase, reduction and control is most effective in preventing the growth of mink populations (Byers et al. 2002).
In regions in which mink have established populations there are numerous methods which can be employed. Table 1 depicts a list of programs of control methods carried out in Europe.
2.3 Restoration and manipulation of habitat
The ability of native species to successfully compete with invasive species is dependant on the environment in which the two species occur. Habitat management is a prospective movement in the minimisation of the impact of mink on native species. It has been established that the vulnerability of the water vole Arvicola amphibius to mink predation is intensified by habitat loss (Barreto et al. 1998). However, under certain habitat alignments, the co-existence of the water vole and mink is viable. This has been seen in areas in which reedbeds, which provide shelter for prey species, have been restored in attempt to the conserve the water vole in wetland parts in the UK (Carter and Bright 2003). Habitat fragmentation has played an important role in the verification of water vole populations and their preservation in specific areas (Lawton and Woodroffe 1991). Observations demonstrate that species distribution and variety of habitat are key factors in the restoration and managment of habitat in order to increase the viability of threatened indigenous species.
2.4 Promotion of native mink competitors
The establishment and preservation of invasive species is often dependant on the presence of prey species and separate prey contenders with similar trophic niches (Birks 1989). The Eurasian otter Lutra lutra is the only known mustelid that has an adverse effect on mink populations in Europe (Kauhala 1996). By excluding mink from specific habitats (Sidorovich 1997) the Eurasian otter is capable of mink population reduction and colonisation prevention (Ruiz-Olmo et al. 1997). The promotion of otter recovery is favourable for the management of mink numbers, however, as opportunistic feeders, this may cause a shift in the feeding habits of the mink from aquatic to terrestrial prey therefore causing complications for threatened terrestrial species (Clode and Macdonald 1995).
The management of abundant prey species, in order to reduce mink population, may prove effective. Invasive species, such as rabbits in the UK and the American crayfish Procambarus clarckii in Spain (Santiago Palazon, personal observation), have contributed and supported the circulation of mink populations. The management of these viable invasive species may have an indirect effect on the colonisation and establishment of alien mink populations, however, one must remember that mink are opportunistic feeders. In areas in which invasive mink have been removed competitors and prey species can be introduced such as the water vole (Macdonald et al. 2002) or European mink Mustela lutreola (Maran 2003).
2.5 Prevention of further escapes and rapid release response
It is recommended by the European Strategy on Invasive Alien Species that prevention and rapid response of escapees through managed actions of fur farms is central in the inhibition of feral mink populations (Genovesi and Shine 2004). By decreasing the number of mink accidently released and the frequency of escapes there is a reduced probability of the establishment of wild populations (Kolar and Lodge 2001). This aspect of prevention is of major importance considering that 79% of feral mink captured, in Denmark, originated from fur farms (Hammershøj et al. 2005). Piero Genovesi is currently developing a series of procedures to enhance rapid response after mink release in collaboration with the Italian Wildlife Institute and the Italian Association of Mink Farmers (Bonesi and Palazon 2007). The guidelines aim to confirm an immediate response to mink farm escapes through the initiation of coherent communication procedures, interpreting an authorized plan of action for all necessary procedures, and refining the technical capabilities of the authorities to carry out the relevant actions.
3. Predator control in New Zealand's mainland islands
The islands of New Zealand, with the exception of bats, were unoccupied by terrestrial mammals. Since their recent introduction, specifically those of a predatory nature, mammals have contributed to the extinction of 40% of New Zealand's bird species. Remaining bird species inhabit offshore islands which remain free of mammalian predators. New Zealand's history in the management of invasive mammalian predators, such as the weasel and stoat, make them a leading example in invasive species control on offshore island communities. Management techniques include live and destructive trapping, and direct, indirect and secondary poisoning (Parkes and Murphy 2004). Biological control agents such as mustelid toxins and diseases have also been developed and utilised (Parkes et al. 2004). Procedures and skills that have been augmented in island predator extermination have been implemented on New Zealand's mainland and involve large area management, low density animal detection and immigration control (Saunders 1990). The mainland is divided and managed, like islands with virtual boundaries, to a near-zero predator density. It is agreed that eradication is not an achievable goal and immigration of predators to surrounding areas is a possibility. New Zealand currently manages six mainland island reserves as displayed in figure 6.
Boundaries of New Zealand's six mainland islands are specified as:
â€¢ Natural geographical boundaries such as mountains, rivers and peninsular landforms. These form natural divides which obstruct animal movement without preventing immigration following the removal of the predator.
â€¢ Man-made boundaries such as fencing. Predators are removed from fenced areas which are then retained as predator free islands. However, the use of fencing, particularly electric fencing, is costly and therefore this course of action is best applied to smaller areas or large areas where there has been a reduction in costs for trapping etc. (Clapperton and Day 2001). Maintanence costs for fences include repairs and supervision. Combination of natural geographical boundaries and fencing in large areas is favourable as this allows for the natural movement of indigenous animals and the successful removal of predators species.
â€¢ Virtual boundaries as practised in Trounson Kauri Park (Gillies et al. 2003). The predator population within these virtual mainland islands are controlled at low levels so as to protect terrestrial nesting birds which are monitored as an indicator species. Both predator and indicator species are monitored to distinguish how each responds to the displacement of the other. Such organisation of mainland islands requires good conservation operation and continued management. Management of various predator species in Trounson Kauri Park is carried out through secondary poisoning and trapping (Gillies and Pierce 1999). Dogs are used for the detection of predators at low densities.
3.2 Applicability of mainland island concepts to Ireland
Methods of New Zealand's mainland island concepts that could be applied to Ireland include:
â€¢ The technique of mainland island development could be applied to SPAs, which are inhabited by feral mink, through the use of geographical boundaries.
â€¢ Virtual boundaries would work well in areas where the protection of native bird species from predatory mink is of priority.
â€¢ Methods of maintenance and development associated with mainland islands would also be applicable in an Irish context. Various methods of trapping and predator detection at low densities over great areas with the use of dogs and tracking tunnels. Rafts, used for identification of mink, have already shown effectively in the detection of populations at low densities (Reynolds, Short and Leigh 2004).
However, some concepts applied to mainland islands would not be applicable in the management of feral mink populations in Ireland. Erecting fences, particularly on wetland, would be expensive and, due to the semi-aquatic nature of the mink, they would be capable of swimming around fencing in coastal or freshwater areas. Fencing would also prohibit the movement of other native animals. Poisoning is another technique that would not be recommended as previous studies outline that it is not always a receptive technique (Roy et al. 2009.
4. The potential costs of mink control in Ireland
4.1 Modelling populations
Roy et al. 2009 devised a schematic depicting a simple population model within a catchment area consisting of 800km2. This is shown in figure 7. Criteria were assembled from published data and results from former studies. The schematic was primarily used for population assessments of specific areas and how these may change depending on the various levels of control effort applied to them. Yearly removal of 25%, 50% and 75% were used as control levels. The reason for density dependence, in this population model, is in order to reduce encounter rates that have led to a reduction in female mink as the population declines. This is an occurrence that was observed in the Hebridean Mink Project (Moore et al. 2003).
Figure 8 exhibits the dramatic decrease in mink populations, over a time period of 2-3 years, following the removal of 75% of the juvenile population. This is then pursued by long-term preservation of a small number of individuals. An estimation of 10-15% for immigration is taken into account as observed in South Harris (Helyar 2005).
4.2 Estimating the cost of control
Table 2 represents an estimated figure of the required expenses for the management of the 800km2 catchment, over a 5year period, to obtain a 75% yearly control. The table outlines only basic estimates of the requirements and costs that would be expected in order to depict the order of magnitude of costs involved in the management of this sort of project. This estimate can be scaled upwards to predict costs for widespread and localized mink control. According to table 2 the yearly decrease of the mink population by 75%, in the given control area and time frame, is calculated at approximately â‚¬1,000- â‚¬1,350 per km2. The costs of larger projects may decrease per km2 as pooling of resources may be taken into account.
Information on the American mink as a foreign invasive species in European countries and methods of reducing its impacts is constantly being accumulated. Although the mink poses many threats to the Irish native fauna, in particular island-nesting birds and waterfowl, economically they are not of major concern. Due to the piscivorous diet of the mink and their current unprotected status in Ireland they can be beneficial and utilised as an indicator species for Pseudamphistomum truncatum, a biliary parasite, recently found in Cyprinid fish species in Ireland (Hawkins et al. 2010). The most costly aspect of the American mink is the management and energy that is exerted in the reduction of the feral mink population. Therefore it is important that an efficient and cost effective control programme be put in place in order to protect vulnerable native species from the predation and competition of the mink. New Zealand's methods of control have been successful in their management of invasive species and the application of their mainland island concept in Ireland may be an effective technique. However, it is important to consider welfare issues in cases involving the culling of mink. Investing in the prevention of established feral mink populations is the most longterm cost effective method from a financial and environmental aspect.