Ibis (2008), 150 (Suppl. 1), 114–121 © 2008 The Authors Journal compilation © 2008 British Ornithologists’ Union Blackwell Publishing Ltd Modelling the benefits of American Mink Mustela vison management options for terns in west Scotland NORMAN RATCLIFFE, 1 * CLIVE CRAIK, 2 ALICE HELYAR, 3 † SUGOTO ROY 3 & MARTIN SCOTT 4 1 RSPB Scotland, 10 Albyn Terrace, Aberdeen AB10 1YP, UK 2 Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll PA37 1QA, UK 3 Central Science Laboratories, Sand Hutton, York YO14 1LZ, UK 4 RSPB Scotland, Office 2, Clintons Yard, Rigs Road, Stornoway, Isle of Lewis HS1 2RF, UK American Mink Mustela vison is a semi-aquatic predator that has invaded the west coast of Scotland and many of its associated islands. We developed a GIS model of their potential range based on their dispersal abilities and habitat use, which revealed that most islands in west Scotland are accessible to Mink, and that these host a large proportion of the region’s Common Sterna hirundo and Arctic Terns S. paradisaea. Mink are predators on tern eggs and chicks, and statistical modelling of long-term productivity data demonstrated that unprotected sites within their range have an average productivity of 0.33 chicks per pair, whereas that at sites where Mink were trapped was 253% higher. We assessed the benefits of current Mink control projects for terns in the Western Isles and the remainder of west Scotland using a population modelling approach. This showed that both projects delivered considerable benefits for Common Terns, because a large proportion of their numbers were within the area of the control programmes and in sites that would be accessible to Mink if no control were in operation. For Arctic Terns, the benefits were less clear, as a larger proportion of their numbers were outside the control areas, and many of these were in sites isolated from, or unsuitable for, Mink. We discuss the implications of these findings for future strategic planning of Mink management in west Scotland. Keywords: Predation, American Mink, Tern, strategic conservation planning. American Mink Mustela vison (hereafter Mink) were accidentally introduced to west Scotland when they escaped from fur farms on the mainland, and the islands of Lewis and Arran during the 1950s and 1960s (Cuthbert 1973, Dunstone 1993). Mink are semi-aquatic and able to swim to islets at least 2 km from shore, and to those further offshore if linked by an island chain (Craik 1995, Nordström et al. 2003). By 1994, they had occupied most of the Scottish mainland and inshore islands to the south of the Great Glen, and in the Western Isles had spread throughout Lewis and Harris (Hudson & Cox 1988, Green & Green 1997). Their southward expansion throughout the Uists since 1994 has been well docu- mented (Harrington et al. 1999, Roy 2006), but data on their range expansion on the mainland north of the Great Glen are sparse. However, ad hoc sight- ings confirm they are now present on Skye and along the nearby mainland coast, in the Ullapool area and in northwest Sutherland (Green & Green 2004). Mink prey on eggs and chicks of ground-nesting seabirds, such as Black Guillemot Cepphus grylle, European Shag Phalacrocorax aristotelis, gulls Larus spp. and terns Sterna spp., causing large reductions in breeding success (Folkestad 1982, Andersson 1992, Kilpi 1995, Craik 1998, 2000, Rae 1999). Following several successive years of predation, colonies are often abandoned, with birds moving to sites where Mink are absent (Craik 1997, 1998) or into fewer, larger colonies (Clode & MacDonald 2002). Conse- quently, Mink have had profound effects on both the distribution and the numbers of seabirds in some areas of west Scotland over past decades (Craik 1997, 1998, Craik & Campbell 2000, Clode & MacDonald 2002). *Corresponding author. Email: Norman.Ratcliffe@rspb.org.uk Conflict of interests: The authors declare no conflict of interests. †Present address: Countryside Council for Wales, Cantref Court, Brecon Road, Abergavenny, Monmouthshire, NP7 7AX, UK.