5. Mink Control In Scotland
5.1. Introduction
37. 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 without human intervention, are introduced into these habitats, the consequences for bird populations globally can be severe (e.g. Courchamp et al., 2003; Jones et al., 2008; Towns et al., 2011).
38. 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).
39. The American mink Neovison vison (hereafter referred to as “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).
40. As a result of the substantial impacts associated with the introduction of mink and native Scottish wildlife (summarised within section 5.2, and with further detail provided within the Ecological Evidence Report (appendix 1), mink have been controlled by various mechanisms in Scotland.
41. The concept of this compensation measure is to continue, enhance and intensify the current Scottish Mink Control Project (MCP) in partnership with SISI which is managed by NatureScot. The MCP operates across large areas of Scotland, protecting native Scottish wildlife, including razorbill and kittiwake and other seabirds, from mink. The Applicant would provide funding to facilitate continuation of the MCP once current funding stops in 2026, which would maintain and enhance control of mink to prevent the recolonisation of mink at seabird breeding colonies (where predation is documented) in northeast Scotland. Furthermore, the Applicant also intends to provide resources to increase and intensify the coverage of the control project across other areas of Scotland not currently covered by the MCP, which are important for razorbill and kittiwake.
42. The following sections of this document detail how the measure would be implemented, along with information on scale, location, design, monitoring and adaptive management. NIRAS has worked in consultation with the SISI project and Professor Xavier Lambin who is a leading mink expert and the academic advisor to the project, and NatureScot, to develop this compensation measure for the Applicant. Furthermore, a letter of intent between the Applicant and SISI (which is project managed by NatureScot) is an annex to this report (annex B).
5.2. Summary of Evidence
43. Detailed evidence in relation to this measure is presented within the Ecological Evidence Report (appendix 1). A summary of key evidence of mink impacts and successes from control projects are presented in sections 5.2.1 and 5.2.3.
5.2.1. Impact of Mink
44. Mink have been documented as a serious threat to seabird colonies in every part of their invasive range (Spatz et al., 2023; 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, and they have since had unprecedented negative impacts on seabird populations. 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).
45. 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).
46. 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).
47. 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).
48. Mink are agile, 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). This often leads to high levels of predation once a prey source has been established and has been documented as a cause of considerable population impact on multiple seabird species (i.e. Mitchell et al., 2004 and Craik, 1997). This is especially relevant to kittiwake, which 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 (Furness et al., 2013). Mink are excellent swimmers and climbers, able to access nesting locations along sheer cliffs to access nesting seabirds (see Ecological Evidence Report (appendix 1) for examples and images). For example, Furness et al., (2013) notes two counts of mink predation at British kittiwake colonies, one of which was at St. Abbs head 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 a dozen partially eaten carcasses reported to be floating in the waters below the colony (X. Lambin, 2024 pers. comm).
49. Furthermore, the authors of the Seabird Populations of Britain and Ireland (JNCC) (Mitchell et al., 2004) suggest it is likely to be more than just a coincidence that razorbill (and black guillemot Cepphus grylle) 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) whereas during the same time period, guillemots (which nest in less accessible habitat and are therefore less vulnerable) have increased. Further examples are described in section 5.2.3 and within the Ecological Evidence Report (appendix 1) providing clear evidence that when the breeding habitat of seabirds, and particularly razorbill or kittiwake, is within the territory of mink there is likely to be considerable population level impacts.
Figure 5.1: Model Predictions for Probability of Occurrence of Mink in Scotland. Green Cells Indicate a Very High Probability of Mink Occurrence, White Cells Indicate an Extremely Low Probability of Mink Occurrence. Figure Taken from Fraser et al. (2015)
5.2.2. Mink Dispersal and Colony Access
50. 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 > 80km from their natal patch (Melero et al., 2018). Furthermore, 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). The highly mobile nature of mink and the predicted probability of mink occurrence in Scotland ( Figure 5.1 Open ▸ ) based on habitat suitability modelling and innate dispersal ability therefore imply a substantial threat to seabird colonies outside of the range of current SISI coverage in the absence of programme continuation.
51. Notwithstanding the difficulty of predicting mink incursion due to the confounding influence of current control programmes (Lieury et al., 2015; Oliver et al., 2016), 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 30 km based on average dispersal distances (31 km for females and 38 km for males), which range from 4 to 100 km (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).
52. 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.
53. 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.
54. 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 Troup Head (X. Lambin, 2024 pers. comm), it is highly 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 5.2. These access points could feasibly permit incursion from land-based mink directly into seabird colonies.
55. 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 impact 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).
5.2.3. Control Success
56. A global review of mink control strategies found 51 studies on mink control that have been carried out in 28 locations in Europe and South America since 1992 (López et al., 2023). Trapping experiments in Patagonia have been effective in removing at least 70% of the mink population using the latest trapping techniques (Bonesi and Palazon, 2007). Additionally, a mink control programme in the Baltic Sea removed the species from several small islands and found significant increases in the breeding densities of seabirds. Razorbill and guillemot were both extinct from the islands, but recolonised following the mink eradication (Nordström et al., 2003). Despite the presence of invasive mink in 28 European countries, several local control projects appear to be effective in reducing invasive populations and protecting native biodiversity (Bonesi and Palazon, 2007).
57. Control efforts in Scotland have been successful in reducing mink populations through successive joint projects despite short-term funding (Lambin et al., 2019). A notable example is the ‘Hebridean Mink Project’. Initiated in 2001, the project aimed to reduce the mortality rate of ground-nesting birds in the Outer Hebrides, which was considerably impacted by mink (NatureScot, 2023). Over the course of the Hebridean Mink Project, a total of 2,198 individual mink were successfully captured. Following the completion of the trapping campaign, monitoring efforts have continued. These efforts have recorded the presence of mink at substantially reduced levels, indicating the success and effectiveness of the control measures implemented. Other programmes include the Scottish Mink Initiative which focused on removing mink from north Scotland between 2011 and 2015 (MacLeod, 2023; McMullen, 2015). Currently, the control of mink in the north of Scotland is managed through NatureScot via SISI and the MCP.
58. The longest gap between control efforts in Scotland was the transition between the Scottish Mink Initiative and SISI MCP. During this period data showed a temporary recovery of mink populations (see section 5.3).
59. The SISI MCP has had considerable success in reducing the overlap in mink and breeding seabirds. In 2018, the MCP superseded the Mink control Initiative and initiated a strategic shift towards targeting bird cliffs from Aberdeen to Spey Bay, resulting in the capture of approximately 30 mink (X. Lambin, 2024 pers. comm).
60. The MCP run by SISI is the largest active project, and between 2018 and 2023 caught 673 mink in 305 different locations ( Figure 5.3 Open ▸ ). The project found that just 78 trapping locations accounted for 75% of total captures between 2018 and 2021 (Invasive Species Scotland, 2024). Note that Figure 5.3 Open ▸ does not include any metric of mink control effort, which has been inconsistent due to control being done through a network of volunteers, recruited by a small number of staff to act locally. This highlights the benefit of long term support in preventing short term mink bounce back and recovery that could lead to catastrophic seabird predation and mortality, especially if a more substantial and targeted volunteer network were to be established.
61. The following sections provide detail on how a compensation measure can be implemented to support the success reported by mink control schemes in Scotland.