2. Mink Control in Scotland

2.1. Introduction

13. Seabirds have a number of natural predators distributed across their range. Natural predators generally pose a low risk to breeding seabirds as they have co-evolved with predation pressure and have mechanisms or behaviours to withstand it. Seabirds primarily use avoidance to counter such predation. This is why they often select nesting areas like cliffs, offshore islands, or secluded boulder fields or beaches where the threat of predators is minimal or non-existent (Furness and Birkhead, 1984). When mammals, which would not typically be present, are introduced into these habitats, the consequences for bird populations globally can be severe (e.g. Courchamp et al., 2003; Jones et al., 2008; Russell et al., 2005; Towns et al., 2011).
14. Invasive mammalian species influence colonies by (depending on the species) predating eggs, chicks and adults, changing the distribution of breeding colonies and changing nesting habitat. There are many species that have been introduced into sensitive island and mainland ecosystems within the UK and the Channel Islands, with a number of offshore islands around the UK and the Channel Islands having established populations of invasive mammals, originating from mainland Britain (e.g., escapees from fur farms) or from further afield (e.g. through stowaways or shipwrecks) (Thomas et al., 2017; Stanbury et al., 2017).
15. The American mink Neovison vison (hereafter mink) is a non-native species established across much of the UK and Ireland. In the past century, the fur farming industry has caused mink to artificially spread from its native range in North America, across the globe. Mink are now prevalent in 28 countries across Europe, Asia, and South America, making them one of the most widely distributed and destructive invasive species in the world (Bonesi and Palazon, 2007; Fasola et al., 2021).
16. The concept of this compensation measure is to continue, enhance and intensify the current Scottish Mink Control Project (MCP) (which cover trapping and invasive habitat management) in partnership with Scottish Invasive Species Initiative (SISI). The MCP operates across large areas of Scotland, protecting native Scottish wildlife, including razorbill and kittiwake, from invasive mink. The MCP currently has funding in place until March 2026 however, without support from Ossian, it has no current funding to support the continued existence of the project after this date.
17. The following sections of this report outline the evidence conveying the significance of mink predation to seabirds across the colonised range of the invasive species.  Further detail of how the compensation measure would be secured and delivered in partnership with SISI is provided within the Compensation Plan (appendix 2). The Compensation Plan also includes information on scale, location, design, monitoring and adaptive management.

2.2. Evidence

18. Mink have been documented as a threat to seabird colonies in every part of their invasive range (Spatz et al., 2022; López et al., 2023; Bonesi and Palazon, 2007; Hipfner et al., 2010). The Scott Islands in British Colombia has historically supported the largest population of breeding seabirds in the eastern Pacific Ocean, south of Alaska (Hipfner et al., 2010). Fur farmers introduced mink to the islands in the 1930’s. They have since had negative impacts on seabird populations and mink removal has been considered a primary conservation priority (Hipfner et al., 2010). Similarly, a study in the Cape Horn Biosphere Reserve in Chile showed seabirds’ susceptibility to mink predation, particularly on nests on shores with rocky outcroppings and on highly concealed nests (Schüttler et al., 2009).
19. In Iceland, mink colonised islands over 10 km from the coast by ‘island hopping’, and have had an adverse impact on Icelandic seabird populations, particularly Atlantic puffin Fratercula arctica (hereafter puffin), black guillemot Cepphus grille and guillemot, with 200 guillemot chicks found in a single mink den in one example (T. Björnsson pers. comm in Clode and Macdonald, 2002; Björnsson and Hernsteinsson, 1991; Johannesson and Gudjonsdotti, 2007; Stefansson et al., 2016). Mink are also the reason for the decline of the only two remaining puffin colonies in France, at Ouessant and Baie de Morlaix (Harris and Wanless, 2011).
20. Mink have spread widely throughout Europe since their introduction in the 1920s (Macdonald and Harrington, 2003). Mink that escaped from fur farms began spreading through the Western Isles of Scotland in the 1950’s (Boyd and Boyd, 1990). The prevalence of mink across Scotland, particularly along the coasts, has been a reason behind a complete or near-complete loss of breeding seabirds from many Scottish archipelagos, sea lochs, firths and sounds (Craik, 1997; Fraser et al., 2015). They have contributed to 34 whole colony extinctions of terns, gulls, storm petrels Hydrobates spp., Manx shearwater Puffinus puffinus and puffin (Mitchell and Daunt, 2010).
21. Mink distributions in the Western Isles of Scotland were highly correlated to that of seabird colonies, and in areas of high mink presence breeding success is lower or in many cases fails altogether (Clode and Macdonald, 2002; Craik, 1995). Between 1989 and 1995, they led to extensive breeding failures that eventually led to whole colony failures among black-headed gulls Chroicocephalus ridibundus, common gulls Larus canus, and common terns Sterna hirundo in colonies on small islands along a 1,000 km stretch of mainland coast in west Scotland (Craik, 1997).

2.2.1. Impacts on Razorbill

22. Razorbill have been shown to be extremely vulnerable to nest predation by mammals at breeding locations, and have well-documented instances of substantial mink predation events (i.e. Thomas et al., 2017 and Nordström et al., 2003). Predation can result in adult mortality and low mean chick survival rates and productivity. Nesting colonies have also been known to redistribute to potentially less favourable locations that are more inaccessible to predators (Barrett, 2015, Booker et al., 2018).
23. The Baltic Islands host several important seabird colonies. However, since the arrival of mink, razorbills in particular have suffered considerable declines (among other species including black guillemot) (Olsson, 1974; Hario et al., 1986; Jönsson and Rosenlund, 1990; Hagemeijer and Blair, 1997; Nordström et al., 2003) as these species often breed in accessible crevices, with adults also at high risk of predation from mink (Nordström et al., 2003).
24. The presence of mink across seabird breeding colonies in southeastern Finland resulted in a reduction in breeding pairs of razorbill with a 60% reduction in pairs, and a 78% decline in the number of razorbill colonies (i.e., localised extinctions) between 1973-1974 and 1994 (Miettinen et al., 1997). As mink inhabited the locations from the 1970s (Kauhala, 1996, 1998) it’s suggested that the occurrence of mink is the main cause of impact (Hario et al., 1986).
25. Nordström and Korpimaki (2004) suggest that since the introduction of mink in 1973-1974 at the locations in southwest Finland (and considered in Nordström et al., 2003), razorbill had become extinct as a breeding species in historic breeding locations, with the remaining populations having redistributed their breeding locations to more isolated islands (noting that overall the number of breeding pairs had reduced by 60% during that time period (Miettinen et al., 1997). A very similar issue was documented by Andersson (1999) in Baltic Sweden where mink also eliminated many small seabird colonies including razorbill (and other species), eventually causing the concentration of the remainder onto inaccessible islands to the mink. Additionally, a mink control programme in the Finnish Baltic Sea removed the species from several small islands and found increases in the breeding densities of seabirds. Razorbill and black guillemot were both extinct from the islands, but recolonised following the mink eradication (Nordström et al., 2003, Banks et al., 2008).
26. Barrett (2015) recorded exceptionally low razorbill chick mean survival rates as a result of high mink predation rates at Hornøya, northeast Norway. Productivity of puffin was also impacted at that site due to mink predation (Fayet et al., 2017).
27. Within almost all the aforementioned examples of mink impacts on razorbill distribution and population, black guillemot is also mentioned to undergo similar, if not more drastic results as a result of mink presence at breeding colonies. This is not surprising given the very similar and often overlapping nesting preferences for the species within secluded crevices. Razorbill actually have two nesting strategies; they will either lay an open nest on vertical cliffs or, in the absence of cliffs, in an enclosed cavity (which is the preference for black guillemot (Mitchell et al., 2004). Both nesting strategies are vulnerable to predation where their distribution overlaps with invasive mammalian species, such as mink (Booker et al., 2019). Furthermore, both species provision their young within crevices for several weeks (the adopted nesting strategy for razorbill) adding to the vulnerability of the species. Examples show black guillemot declining as a result of mink predation, which can indicate similar impacts on razorbill as a result of their nesting strategy. 
28. This theory is supported by the authors of the Seabird Populations of Britain and Ireland (JNCC) (Mitchell et al., 2004) who suggest it is likely to be more than just a coincidence that razorbill and black guillemot have undergone large scale population declines where their nesting habitat coincides with mink present along the north-west mainland coast of Scotland (from Lochaber to north Caithness).
29. Examples of mink predation of razorbill are limited by both the difficulty in accessing or even observing razorbill nesting locations, as well as the practice of mink to cache their prey in dens, which are difficult to find and access. For example, Birks and Dunstone (1984) recorded guillemot and razorbill cached within mink dens on the Galloway Coast, Scotland, and in one study, 200 guillemot chicks were found in a single mink den (T. Björnsson pers. comm in Clode and Macdonald, 2002). It is therefore highly likely that mink predation of razorbill is under-represented when compared with more visible nesting species such as gulls and terns.

2.2.2. Impacts on Kittiwake

30. Kittiwake are often able to avoid mammalian predation due to their nesting habits, but have been documented as being particularly vulnerable to mink predation on the Scottish east coast where both kittiwake and mink ranges overlap in some locations when not covered within the MCP project coverage. Furness et al., (2013) notes two counts of mink predation at British kittiwake colonies, one of which was at St. Abbs head, Scotland, where the individual mink predated half of the kittiwake colony during one breeding season. Additionally, fully grown kittiwake chicks at Troup Head in north-east Scotland (part of the Troup, Pennan and Lion’s Head SPA) were predated by mink, with large numbers (more than 50) of carcasses reported (X Lambin, pers. comm.). Additionally, in northern Norway, Dunstone (1993) reported mink to have decapitated kittiwake chicks.
31. The images in Figure 2.1   Open ▸ depict mink approaching a kittiwake colony (bottom left) where they are easily able to access kittiwake nests (bottom right) which are usually inaccessible to mammalian predators. The top two photos show mink predating both kittiwake chicks and adults. Personal accounts from Terje Kolaas at Ekkerøya Bird Cliff in the Varangerfjord, Norway report that a pair of mink had their den in close proximity to the pictured kittiwake colony in Figure 2.1   Open ▸ During just four hours of observation, 18 kittiwake chicks and two adults were predated and taken back to the minks’ dens (T. Kolaas, pers comm.).

Figure 2.1: Images of mink predating kittiwake. Top left: mink eating adult kittiwake. Top Right: mink eating kittiwake chick. Bottom left: mink approaching a kittiwake colony. Bottom right: mink reaching into a kittiwake nest. Images by Terje Kolaas (Kolaas, n.d)

2.2.3. Mink Dispersal and Colony Access

32. The highly mobile nature of mink and the predicted probability of mink occurrence in Scotland imply a substantial threat to seabird colonies ( Figure 2.2   Open ▸ ). Numerous studies observe a vastly greater-than-expected innate dispersal ability for mink when compared to similarly-sized carnivorous mammals (Melero et al., 2018; Fraser et al., 2015). In one study, 77% of mink dispersed and settled into non-natal patches, with 20% of mink dispersing > 80 km from their natal patch (Melero et al., 2018). Female mink typically give birth to a litter of three to six kits each year, though larger litters of 10 and 12 kits have been recorded (Melero et al., 2015).  
33. Landscape heterogeneity and a lack of traversable waterways is not a barrier to mink dispersal; in one study, 32% of recaptured mink were caught in different river catchments from their natal patch, implying overland dispersal independent of waterways (Oliver et al., 2016).
34. It can be difficult to predict mink incursion due to the confounding influence of current control programmes (Lieury et al., 2015; Oliver et al., 2016). However, multiple studies using sophisticated population modelling note that the long-range dispersal ability of mink requires a large spatial scale for effective control and a buffer exclusion area of at least 30km based on average dispersal distances (31 km for females and 38 km for males), which range from 4 km to 100km (Oliver et al., 2016). Furthermore, even with such an exclusion area, study authors note that there would be a requirement for ongoing vigilance as a small proportion of mink disperse much further than these distances, and even low numbers of mink can cause substantial seabird mortality at seabird colonies (Oliver et al., 2016).

Figure 2.2: Model Predictions for Probability of Occurrence of Mink in Scotland. Green Areas Indicate a Very High Probability of Mink Occurrence, White Areas Indicate an Extremely Low Probability of Mink Occurrence. Figure Taken from Fraser et al. (2015)

35. In geographical terms, mink dispersal and subsequent incursion risk cannot reliably be predicted by habitat suitability or quality. This is evident particularly in coastal areas where incursion has not decelerated despite decreasing availability of suitable habitat (Fraser et al., 2015).  Available observation data for Scotland repeatedly reports a preference of mink for coastal habitats, independent of landscape heterogeneity and habitat quality (Fraser et al., 2015).  This suggests that mink will actively colonise areas of suboptimal habitat suitability where intraspecific competition is reduced. Again, this highlights a credible risk of mink incursion to seabird colonies where mink have not yet been reported.
36. There is evidence to suggest that mink originating from inland areas preferentially disperse to coastal habitats. Stable Isotope and scat analysis studies in Iceland (Magnusdottir et al., 2013), the Outer Hebrides (Helyar, 2005; Bodey et al., 2010), Argentinean Patagonia (Previtali et al., 1998) and Spain (Delibes et al., 2004) have demonstrated that the diet of coastal living mink is dominated by marine-based prey. In one Scottish study investigating how stable isotope signatures change at the population level of mink over time in response to an eradication programme, isotope profiles signifying marine prey became increasingly dominant as the programme progressed. This suggests that inland mink increased their reliance on marine food resources and focused their predatory activity on the coastline (Bodey et al., 2010). Furthermore, a radio-tracking study of mink in coastal habitat reported that mink occur at higher densities and occupy smaller territories in coastal areas compared to inland regions (Helyar, 2005). This is likely due to the increased abundance of food sources in coastal habitats, such as cliff-nesting seabird colonies (which are highly calorific), where species such as razorbill and kittiwake can nest in high densities.

37. Based on the innate dispersal ability of mink, the flexibility they exhibit in their feeding ecology with preference for coastal habitats and previous observations of mink predating kittiwake and other seabirds within Fowlsheugh SPA and Troup Head (X. Lambin, 2024 pers. comm), it is probable that all sections of cliff-nesting seabird colonies within SPAs are vulnerable to mink predation following incursion. Many of the sites within Fowlsheugh SPA and North Caithness Cliffs SPA (for example) that host cliff-nesting seabird colonies contain sections of down-sloping, grassy patches leading from cliff tops into lower sections of the cliff face (Figure 2.3). These access points could feasibility permit incursion from land-based mink directly into seabird colonies.
38. However, even under the scenario in which mink cannot access certain areas of a cliff-nesting seabird colony, there are likely to be indirect effects resulting from the areas that mink can access that negatively affect reproductive success of all species within the colony. A study investigating the response of shags to mink predation at nest sites demonstrated that individuals would change nesting locations to sites of lower quality to avoid predation at a cost to reproductive success (Barros et al., 2016). This shift in nest-site selection in response to mink predation has also been observed in razorbills (Nordström and Korpimäki, 2004). This may have population-level consequences that negatively impact colony size, as nest-sites at lower risk of mink predation can result in increased density-dependent competition for resources and greater risk from avian predators (Forero et al., 1986; Hunt et al., 1986).

2.2.4. Mink Feeding Ecology

39. Mink are generalists and opportunistic predators that feed on fish, reptiles, mammals and ground-nesting birds (Dunstone, 1993; López et al., 2023). They are prolific hunters, with the ability to predate adult birds along with eggs and young. Mink are able to swim across open water for distances up to 6.5 km (Thomas et al., 2017) and are able to access seabird nesting locations and individual nest sites which are usually inaccessible to mammalian predators (such as kittiwake cliff nests) (Mitchell et al., 2004; Figure 2.1   Open ▸ ).
40. Mink can have a considerable impact on the populations of their prey when they specialise. It is likely prey specialisation does account for a large amount of the predation events undertaken by mink (Dunstone, 1993). The fact that evidence from mink impacts on seabird colonies features kittiwake and auks (guillemots, puffins, razorbills) indicates that as prey they are profitable, with mink being unlikely to target prey in great quantities when uneconomic in terms of calorific content. Prey preference is also likely to focus on those prey species which require less energy expenditure. Aquatic prey (such as fish) have a high calorific yield but require a considerably greater level of energy expenditure to provide an equivalent level of calorie intake when compared to terrestrial animals (Stephenson et al., 1988).
41. Mink are single-prey loading, central place foragers which means they collect single prey items during each foraging bout and carry them back to a cache to store resources, particularly while prey is abundant (Houston and McNamara, 1985). During the breeding season, mink will surplus-kill chicks and adults within the colony and cache them in their dens, of which they may have two to ten near their favoured hunting grounds depending on habitat quality (Breault and Cheng, 1988; British Wildlife Centre, 2024). As noted above, one Icelandic example recovered 200 dead guillemot chicks within a single mink den (Clode and Macdonald, 2002. An individual mink has been found to have cached 600 tern chicks in one week in on the west coast of Scotland (Craik, 1995). High levels of predation are well documented once a prey source has been established and has been considered as a cause of considerable population impacts on multiple seabird species (i.e. Mitchell et al. 2004 and Craik, 1997). Although the sex and reproductive phase of the above examples are not known, a female mink weaning kits may have an energy requirement five times that of an individual outside of weaning (Ireland, 1990).
42. Estimates of mink density in coastal habitat vary. Females are territorial and hold territories of 1 km to 3 km along a linear waterway, whereas males can hold territories up to 5 km long, which may overlap with female territories (Invasive Species Scotland, 2024). Other studies have reported greater densities of mink in coastal habitats, ranging from 0.75 to 2.27 mink/km ( Table 2.1   Open ▸ ). Additionally, the mean mink density across the five studies in Table 2.1   Open ▸ is 1.42 mink/km in a coastal habitat (CABI International, 2022).

Table 2.1: Coastal Mink Densities. Table taken from CABI International (2022)

43. Information on the feeding ecology of mink will be used to inform the calculations required to ascertain the scale of compensation required for razorbill and kittiwake. The method to define scale has been progressed by the Applicant in collaboration with mink experts working with SISI and are described in the Compensation Plan (appendix 2).