Habitat loss and non-native species are considered the two most important threats to global biodiversity . Invasive carnivores have caused substantial biodiversity loss, particularly on islands . The American mink Neovison vison is a widely distributed invasive carnivore, occurring in 28 European countries, The eradication of invasive Mustelids in general can be challenging , and most mink control operations in Europe are long-term control operations rather than eradications, for example in Iceland or at a local catchment level in England .
Mink populations were established on the Outer Hebrides of Scotland when animals escaped, or were deliberately released, from fur farms in Carloway, Dalmore and Steinish on the Isle of Lewis in the 1950s . Mink then spread steadily southwards through Harris, and although attempts were made to stop them from colonising the Uists [North Uist, Benbecula and South Uists , they were found in North Uist in the 1990s and a population was discovered in South Uist in 2002. Thus, they had successfully established populations across the entire archipelago of 2800 km2 within 50 years of their initial release.
The Outer Hebrides support internationally important habitats and bird populations. Mink have been reported to have severe impacts on bird populations , in particular on ground-nesting species, and fish populations . In addition to direct impacts on biodiversity, there are financial concerns in the Outer Hebrides as eco-tourism, aquaculture, game fishing and crofting are important elements of the local economy to the estimated value of up to £30 million annually (
The Hebridean Mink Project was established with the objective of removing mink from North Uist, Benbecula and South Uist and to reduce mink density from neighbouring South Harris to minimise recolonisation of the Uists. . The control site in the Uists comprised approximately 356 islands and skerries totalling 850 km2. The area contained a complex mixture of freshwater and saltwater habitats, with 1116 km of coastline, 2416 km of loch shore and 189 km of rivers and streams, all of these are key habitats for mink, particularly along the west coast of Scotland and its offshore islands .
Prior to start of the project, a review of existing data on the key mink life history variables was undertaken, and a simple population model constructed including biologically plausible assumptions where direct information was absent . These included recorded mink densities and literature on reproductive rates and survival . This review formed the basis of a successful bid for EU LIFE funding, and guided the initial planning and logistics of the control programme. However, the project began without detailed understanding of the number of mink present in the areas, the effort required to achieve their eradication or the most effective methods. This work therefore adopted an adaptive management approach , analysing and interpreting the results as the work progressed to identify possible refinements.
Trapping began in November 2001 while searching for denning animals using dogs was added as a secondary method from spring 2003. A total of 2545 live capture cage traps was dug into the ground during the first 3 months of the project, although a total of 10 % was open at any one time, with the remainder left locked shut to prevent captures in traps that were not being set and monitored daily. Unset traps with closed doors were also easier to find than those with doors locked open, when they were set later . This approach reduced the manpower needed to repeatedly set, lift and relocate traps, relying instead on large numbers of pre-located traps being used in rotation. The location of each trap was recorded using GPS. Traps were set and baited and were in use for a 1 or 2 week period, when they were checked daily. On average each trap was in use during four or five separate periods per year. A total of 100,824 trap nights of effort was deployed during the project. Starting in spring 2003, three trained dogs (collies and spaniels) were used to locate active den sites, followed by intensive trapping in the immediate vicinity of the breeding den. Approximately 500 days of effort were put into dog based searches although effort was not formally recorded as they were used opportunistically, and covered large areas by quartering the land. The time spent on the ground, and the area covered, was highly variable; although effort was most concentrated during the spring and summers of 2003–2005, dogs accompanied handlers throughout the year, and mink presence was verified by handlers interpreting their behaviour. Trappers also recorded all presence of sign such as scats and prey remains, especially along the edge of water courses. The effort involved in the collection of these additional data could not be recorded as it was carried out as trappers moved from trap to trap along their daily routes. Sightings recorded by the public were also recorded and traps in the area were often opened to catch these animals. These provided additional data on mink presence/absence.
This programme relied on traps and dog searches. Other techniques were considered or have become available subsequently. Techniques have been available for tracking mustelid movements for many decades . These provide the basis for techniques, such as mink rafts which have proved to be effective in some habitats, for example slow moving waters . Given the fast flowing or tidal habitats on the Western Isles, we did not use this method. Since the completion of this phase of the campaign, other tools have emerged, such as self-reporting traps . These systems enable trappers to set large numbers of traps and service only those that have made a capture, greatly enhancing efficiency at landscape scales.
Captured animals were humanely dispatched with a shot to the brain stem using .22 calibre air pistols. They were then sexed and aged broadly into juveniles and adults based on tooth wear combined with the presence or absence of milk teeth . For the purposes of simple categorization as adult and juvenile, this was deemed sufficient. A total of 228 mink was captured during the course of the Uist project, 191 during standard trapping and 37 (including 20 dependent young) during trapping associated with dog searches. The last capture occurred on 23 March 2005. After that date, a total of 5567 further trap nights and a summer of dog searches did not produce further captures or signs of mink, in particular we found no evidence of young being produced which would have suggested the presence of trap shy breeding individuals. This supports a conclusion that the programme had been successful in removing all breeding activity, but with the possibility that isolated individuals may have remained, for example on offshore islets. This work was followed by a more extensive eradication programme and expending effort to improve the confidence in local eradication at the end of this pilot study would not have been a productive use of effort. In South Harris, 248 mink were captured, of which 230 derived from trapping. The Harris data did not demonstrate a decline in captures as occurred in the Uists, as would be expected as this region was intended to act as a buffer zone bordering a large, untrapped population to the north.
The trapping effort was between 20,000 and 31,000 trap nights per year during each of the 4 years of the project in the Uists. However, catch per unit effort rose between the first and 2 years before falling away in subsequent seasons (Table 1). It was mainly during the first 2 years of the work that active steps were taken to improve trap efficacy. As Uist mink numbers were expected to have declined in each year of the project, the observed increase in catch per unit effort thus suggests an improvement in trap efficiency as the work progressed, at least between the first and second years, rather than an increasing mink population. The rising capture rate in the buffer zone in Harris also suggested an improvement in trapping efficacy. The reduction seen in capture rates, and the changes seen in population age and sex structure are likely to reflect a decreasing population caused by the campaign. This was corroborated by records of scats and signs collected by trappers and associated sighting reports collected from the public. Captures made as a result of sightings resulted in seventeen captures in 2003, and four in 2004. All reported sightings resulted in a capture. As further evidence, all carcasses were aged and sexed and researchers looked for clusters of juvenile captures or placental scars and corpora lutea in females to provide evidence of breeding events and therefore presence of males. There was no evidence of breeding events from the Uists recorded beyond February 2004, although there was in Harris.