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American mink Mustela vison in the upper Thames catchment: Relationship with selected prey species and den availability
Biological Conservation 76 (1996) 51-56 Copyright © 1996 Elsevier Science Limited Printed in Great Britain. All fights reserved 0006-3207/96/$15.00+.00
0006-3207(95)00072-0
ELSEVIER
A M E R I C A N M I N K Mustela vison IN THE UPPER THAMES CATCHMENT: RELATIONSHIP WITH SELECTED PREY SPECIES A N D D E N AVAILABILITY E. C. Halliwell* & D. W. Macdonald Wildlife Conservation Research Unit, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
(Received 31 December 1994; accepted 15 May 1995)
and across much of Scotland (Birks, 1986; Dunstone, 1993). Mink are associated with a wide range of aquatic habitats, including upland and lowland rivers and streams, canals and coastline, although they can live away from water where there is sufficient prey (Dunstone, 1993). They live in individual territories with little overlap between members of the same sex, but often considerable overlap between males and females (Dunstone, 1993). Those living along rivers concentrate their activity along the water's edge in linear home-ranges 1-6 km long which contain several dens (Birks, 1986; Dunstone, 1993). Mink are generalist predators, feeding on fish, invertebrates, birds, amphibians, small mammals and rabbits Oryctolagus cuniculus (Gerell, 1967; Akande, 1972; Chanin & Linn, 1980; Birks, 1986). Although regarded as a pest by farmers and game-keepers, mink generally have only a negligible and localised commercial impact (Dunstone, 1993). In a study in south-west England, for example, chickens and gamebirds made up less than 1% of prey remains in mink faeces (Birks, 1986). Concern has been voiced over the effects of mink on salmonid fish (Lever, 1985), but there is little evidence of a decline in any commercially important salmonids as a result of mink predation (Dunstone, 1993). During the early 1990s, however, anglers in the Upper Thames region became concerned that mink might affect fish stocks. Mink can have severe effects on bird populations. On the west coast of Scotland, for example, they effectively prevent breeding in colonies of gulls and terns (Craik, 1995). In riverine habitats waterfowl are particularly at risk from mink predation during the nesting season (Dunstone, 1993). Chanin and Linn (1980) found the remains of Rallidae in 15-16% of scats at two of their study sites in southwest Britain, and Chanin (1981) suggested that because coots Fulica atra and moorhens Gallinula chloropus are relatively small and spend much of their time in reed beds where they can be ambushed, they are easier prey for mink than ducks. Jefferies et al. (1989), from a survey of County Mammal Reports and questionnaires, found that mink were considered the most important predator of water voles Arvicola terrestris and concluded that the water vole is in decline. There is now growing evidence that these
Abstract American mink Mustela vison and three prey types of economic and conservation importance were censused along four rivers in the Upper Thames catchment area. There were no significant correlations between any measures o f mink abundance and any measures o f f i s h or moorhen Gallinula chloropus abundance, suggesting that mink were neither limiting nor limited by these prey items. Water voles Arvicola terrestris, however, were negatively correlated with mink, being absent at four sites where mink were most numerous. Sites at which mink were most numerous had a high availability of potential den sites and low emergent vegetation cover, but there were no correlations between water voles and the availability o f their preferred habitat. Keywords." American mink, Upper Thames catchment,
water voles, moorhen, fish, den sites. INTRODUCTION
The American mink Mustela vison is the only widely distributed non-indigenous wild carnivore in the UK. Mink have a popular image as voracious predators, and their establishment and supposed impact on wildlife, game species and domestic stock in the UK has generated a considerable amount of discussion and controversy (Lever, 1978; Linn & Chanin, 1978; Clark, 1991). Elsewhere, introduced mustelids such as stoats Mustela erminea in New Zealand, and mongooses Herpestes auropunctatus in the West Indies (King, 1984; Lever, 1985) have decimated native species, and consideration of the conservation and economic implications of the establishment of mink in the UK is therefore justified. Here we look at the relationships between mink and three prey species of economic or conservation importance in the Upper Thames catchment area. Mink were first imported into the UK to be bred on fur farms in 1929. Feral populations became established during the 1950s and 1960s, and by 1970 the species was present throughout England and Wales, *Present address: Institute of Terrestrial Ecology, Hill of Brathens, Banchory, Glassel, Kincardinshire, AB31 4BY, UK 51
E. C. Halliwell, D. I41. MacdonaM
52
I
I'Vin., ....
10 km
Fig. 1. Map showing location of study areas.
populations are threatened by mink predation as well as by habitat loss and fragmentation (Woodroffe et al., 1990b; Strachan & Jefferies, 1993) In view of their potential impact on vulnerable prey such as water voles, mink may be limiting the densities of certain species, with negative economic or conservation consequences. Similarly, mink density may be limited by that of their prey. Clode et al. (1995) demonstrated that mink from our study areas were significantly smaller and in poorer body condition than mink from coastal habitats in Scotland, possibly because food availability and quality is lower in riverine habitats. Other authors (Birks & Linn, 1982; Dunstone & Birks, 1985, 1987; Ireland, 1990) have also suggested that coastal habitats have richer resources than riverine habitats. If mink in the Upper Thames catchment were limited by overall prey availability this would not preclude them having a limiting effect on some species. The abundance of both mink and water voles may also be affected by habitat. In particular, mink rely on mature willows Salix spp. and pollarded trees for den sites (Mason & Macdonald, 1986), while water voles prefer areas with low tree cover and high emergent vegetation cover (Lawton & Woodroffe, 1991; Strachan & Jefferies, 1993). Here we investigate the potential for mink to limit the density of three prey types - - fish, moorhens and water voles - - chosen for their financial and conservation importance in the Upper Thames region. If mink were limiting certain prey populations we would expect a negative correlation, with fewest of those prey in areas with most mink. Conversely, mink density might
be limited by den availability or by the availability of one of the chosen prey types (which is unlikely given their dietary diversity). In this case we would expect a positive correlation between mink density and prey or den availability, with most mink in areas with highest availability.
METHODS Study sites Mink were studied along a 20 km section of each of four lowland rivers in the Upper Thames catchment area in Oxfordshire and Berkshire, southern England (Fig. 1): the Thames (SP 309002-SP 435042), Thame (SP 621056--SP 578932), Coin (SP 109074-SU 205988) and Kennet (SU 288712-SU 456672). The Coin is a game fish river in which trout Salmo spp. predominate; the other rivers largely contain cyprinids such as roach Rutilus rutilus, chub Leuciscus cephalus and bream Abramis brama. The Thames, Thame and Kennet were up to 20 m wide in places, while the Coin was narrower, reaching a maximum width of about 10 m. The Thames and Thame were the deepest rivers (> 1 m), the Coin was intermediate (0.5-1 m), and the Kennet was the shallowest (0.254)-5 m). The rivers were fringed with trees such as willow Salix fragilis, alder Alnus glutinosa and ash Fraxinus excelsior, and vegetation such as nettles Urtica dioica, rank grassland, willowherb Epilobium spp., bramble Rubus fruticosus, blackthorn Prunus spinosa and hawthorn Crataegus monogyna. The band of vegetation emerging from the water consisted of
Mink in the Upper Thames catchment species such as bur-reed Sparganium erectum, common reed Phragmites australis and reedsweet grass Glyceria maxima. Adjacent land was mainly pasture, arable and woodland. Occasional opportunistic killing of mink by water bailiffs and lock-keepers occurred throughout the study area, particularly along the Kennet, but no detailed data were available. Each section was divided into two contiguous 10 km sections designated upper and lower sections. These were nominally divided into 40 c. 250 m samples, which were assessed by eye for the percentage cover of emergent vegetation, the width (m) of the band of emergent vegetation along the river, percentage mature tree cover, and the number of potential mink dens. Percentage emergent cover was estimated to the nearest 5%, and single mature trees were given a value of 2%. Potential mink dens were: willow pollards, tree hollows, holes among tree roots, root plates (the vertical mat of roots and soil formed when a tree falls), stick piles and washed-up debris, and rabbit burrows. Assessment of mink abundance
Mink were live-trapped in commercial cage-traps baited with fish. The upper section of each river was trapped in three sessions, each lasting five nights, between July and November 1991; Lower sections were trapped once for eight nights in November and December 1991. Each session used 30 camouflaged traps set at approximately 300 m intervals. Traps were placed along the river bank in carefully chosen sites where there were field signs of mink or where they were thought likely to be active. The traps were checked every morning, and animals captured for the first time were anaesthetised with Metophane, measured, weighed and eartagged, and any striking markings on their undersides and throats were described to assist individual identification. Mink were classified as juvenile or adult by: body weight, subtle facial characteristics dependent on bone growth, width of the baculum (felt by palpation) in males, and white hairs resulting from mating wounds in females. Following Gerell (1971), adult mink caught on the upper sections of each river were classed as 'transients' if they were caught in one trap session, or 'residents' if they were caught in two or more sessions. All adults caught on the lower sections were considered residents as trapping occurred during a time when there is usually little transient movement, and when the majority of territories have been established (Birks, 1986). Assessment of prey abundance
Data on the population density (number per m 3) and size categories of fish on the rivers Thame, Coln and Kennet were obtained from the National Rivers Authority (NRA) electro-fishing surveys carried out between February 1987 and July 1991. Fish population densities tend to remain constant between years (NRA, pers. comm.), and surveys were done within or very close to study areas. The majority o f fish taken by
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mink are less than 15 cm long, with pike Esox lucius usually less than 20 cm (Gerell, 1968; Birks, 1986). The density (number per m 3) of fish available to mink was therefore calculated as the product of fish population density and the proportion of fish in size categories usually taken by mink. Electro-fishing data were not available for the River Thames, but angler catches, fry surveys and individual observations indicated that the river contained a healthy, mixed fish population similar to that of the River Thame (N.R.A., pers. comm.). Adult and juvenile moorhens were easily counted during each daily trap round, and the maximum daily count in each study section was taken to be the number present. The presence of water voles was determined by a thorough search for field signs (runs, burrows and latrines) within a 100 m radius of each trap. The proportion of traps at which water vole signs were found was used as an index of water vole abundance (Woodroffe et al., 1990a). RESULTS Mink abundance
Highest densities of adult and juvenile mink were found on the Thames (Table 1). Highest densities of resident mink were found along the upper and lower sections of the river Thames (0.6/km and 0.7/km, respectively), followed by the lower Thame (0.5/km) and the lower Coin (0.3/km). The upper Thame and the lower Kennet both had resident mink at d.ensities of 0.2/km, while the upper Coin and upper Kennet had densities of 0.1/km. Trapping success (Table 1) was used as a second measure of mink abundance to counteract differences in trap effort between upper and lower sections; trap success data concurred with the density data. Prey availability
Availability of fish to mink was highest on the upper and lower Thame, and lowest on the Kenneit (Table 2). The Thame and Kennet contained no or very few Table 1. Numbers of resident adult, transient adult and juvenile mink captured, and number of resident mink caught per 100 trap nights (100 tu), along upper and lower sections of four rivers in the Upper Thames catchment area
Site Thames Upper Lower Thame Upper Lower Coln Upper Lower Kennet Upper Lower
Resident Transient Juveniles Total Residents adults adults /100 tn 6 7
0 0
4 1
10 8
1.33 2.92
2 5
2 0
0 2
4 7
0-44 2.08
1 3
3 0
3 2
7 5
0.22 1.25
1 2
3 0
2 0
6 2
0-22 0-83
E. C. Halliweil, D. IV. MacdonaM
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Table 2. Abundance of three mink prey items - - fish, moorhens and water voles - - on upper and lower sections of four rivers in the Upper Thames catchment area
Fish availability was measured as the product of mean fish density and percentage of fish of a suitable size for mink. Moorhen abundance was estimated as the maximum number of individuals (both adult and juvenile) seen within the study site. The percentage of mink traps with signs of water voles within 100 m was used as an index of water vole availability. Site Thames Upper Lower Thame Upper Lower Coln Upper Lower Kennet Upper Lower
Fish Moorhens (no. available/m 3)(no. individuals)
40-
g
Water voles (% traps)
302010-
---
47 57
0 0
0.54 0.11
29 31
0 0
0.01 0.03
72 60
66-7 42.4
0.09 0.00
52 25
39.4 30.3
salmonids respectively, while the Coln contained more than 80%. The upper Thame had by far the highest mean total fish density (0.65/m3), most (83%) of which were available to mink. The lower Thame and the upper Kennet held intermediate total densities of fish (0.17/m 3 and 0-14/m 3, respectively), of which over 60% were available for mink. Total fish densities in the upper and lower Coin and lower Kennet were low (0.09/m 3, 0.14/m 3 and 0-11/m 3, respectively), of which about one-fifth or less were suitably sized for mink. There were no significant correlations between any measure of mink abundance and fish density or availability (calculated as the product of fish density/m 3 and percentage of suitably sized fish). Moorhens were most abundant on the upper and lower Coln and the lower Thames (Table 2). There were no significant correlations between any measure of mink abundance and adult, juvenile or total moorhen abundance. There were no signs of water vole activity along the upper and lower stretches of the Thames or the Thame (Table 2). Traps along the upper Coin had the highest proportion of water vole signs (66.7%), those along the lower Coin and upper Kennet had intermediate proportions of signs, and lowest water vole activity was along the lower Kennet. There was a significant negative correlation between numbers of resident mink and proportions of water vole signs (r = -0-732, d.f. = 6, p < 0-05; Fig. 2). H a b i t a t availability
The Thames and Thame river sections contained the highest mean number of potential den sites, while the Coin and Kennet contained the lowest number (Table 3). There was a significant correlation between mean numbers of potential dens and total numbers o f mink (r = 0-804, d.f. -- 6, p < 0.02), total numbers of adults
0
]
•
2
3
~
•
~__
5
6
Resident M i n k
Fig. 2. Correlation between number of resident mink and percentage of.traps with water vole signs at eight study areas along four rivers in the Upper Thames catchment area.
(r = 0.902, d.f. = 6, p < 0.01) and number of resident adults (r -- 0.781, d.f. = 6, p < 0.05), suggesting that mink densities may have been limited by den availability (Fig. 3). When tested with the variance mean ratio test, potential den sites were significantly clumped along the Thame ( ~ = 318, n = 80, p < 0.01), Coin (g 2 - 200, n = 80, p < 0.01) and Kennet (g 2 = 241, n = 80, p < 0.01), which may have further reduced the number of mink able to live along these rivers. The mean percentage of emergent vegetation cover was similar along the Thame, Kennet and Coin, but was low along the Thames; the mean width of the band of emergent vegetation was similar along all eight sections of the rivers (Table 3). Mean mature tree cover was variable between sites (Table 3), being greatest along the upper Thames and lowest along the lower Thame. There were no correlations between any measure of mink abundance and the mean width of the band of emergent vegetation or mean mature tree cover. Adult resident mink were most abundant where
Table 3. Mean ___SD number of potential mink den sites, mean __ SD emergent vegetation cover, mean _+ SD emergent vegetation band width and mean _+ SD mature tree cover along upper and lower sections of four rivers in the Upper Thames catchment area
Site Thames Upper Lower Thame Upper Lower Coin Upper Lower Kennett Upper Lower
Potential den sites
Emergent cover (%)
Emergent Mature tree width (m) cover (%)
3.8 __4.2 2.3 _+2.0
17.0 _+8-2 26.9_+20.5
1.0 __0.4 1-0 _+0.5
21.7 -- 27.6 16.0 + 15-4
1.1 _+0.4 1.0 _+ 1.2
47.5 _+27.2 46.3 _+27.5
1-1 _+0.9 1.4 _+ 1.0
8.9 -+ 15.3 7-9 _+8.7
0.4 _+0.6 1.0 _+ 1.0
57.3+_21-8 42.1 _+23-3
1.2 _+0.3 0.9 + 0.4
9.3 _+ 14.5 19.0 + 21.5
0-6 + 0.9 0-9 + 0.9
44.1 + 23.8 38-9+ 28.6
0.7 + 0.5 0.9 + 0.6
15.0 + 8.6 13.4 + 9.0
Mink in the Upper Thames catchment
:1 5
0
• "
i
i
"
i
i
Potential Dens
Fig. 3. Correlation between number of resident mink and number of potential den sites at eight study areas along four rivers in the Upper Thames catchment area. mean emergent vegetation cover was low (r = -0.768, d.f. = 6, p < 0.05), but this may have been a result of the negative correlation between mean percentage emergent vegetation cover and the mean number of potential dens available (r = -0.931, d.f. -- 6, p < 0.001). There was no significant correlation between water vole abundance and mean width of emergent vegetation, mean percentage cover by emergent vegetation or mean percentage cover by mature trees. DISCUSSION Mink were clearly more abundant along the Thames than along the Thame, Kennet or Coln. Densities along the Thame, Coln and Kennet, measured as number of residents per kilometre, were lower than in the small number of previous studies in the UK. On the River Teign, Devon, densities range from 0.53/km to 0.26/km, and in Scotland a density of 2/km of coastline has been recorded (Birks & Dunstone, 1991). We believe that all or most of the mink resident in our study areas were captured, with the exception of those along the Kennet. Mink were captured in all areas where there were positive signs of their presence, such as footprints or scats. On the Coln, sections of soft mud revealed prints only in areas where mink were caught. Furthermore, just one new individual was caught in the whole of each third trapping session along the upper Thames and Thame sections. No new mink were caught on the last two trapping days along the lower sections of Thames, and only one new individual was caught during the final three days of trapping along the lower Thame. Numbers may have been underestimated along the Kennet, however, where scats were found in areas where very few or no mink were captured. Mink on the Kennet may have avoided capture because the river divides into several parallel sections, of which only one was trapped. There was no evidence that mink were limiting, or were limited by, fish or moorhen abundance. There were good numbers of moorhens breeding and producing
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young on the Thames, where mink density was highest. Marchant and Hyde (1980) suggested that the moorhen's success is due to its adaptability and territoriality. Nonbreeding birds, described as a 'mobile "floating" stock', are available to occupy new or empty territories as they occur. So long as mink do not kill more than the floating stock they will not affect moorhen productivity. There may have been seasonal variation in the extent to which predator and prey abundance affected each other. Fish can form a large part of mink diet (35-54%), particularly in the winter (Gerell, 1968; Chanin & Linn, 1980) when they may become more available due to being less active in the lower water temperature. Other prey may be less available in winter: Gerell (1968) found that in autumn and winter the percentage occurrence of birds in mink scats decreased. Competition may also affect mink diet: Clode and Macdonald (1995) found, in the Western Isles of Scotland, that mink in allopatric populations preferred fish to other available prey, whereas those sympatric with otters consumed fewer fish. There was evidence that mink in the Upper Thames catchment were limiting water vole abundance, at least along the rivers with the highest densities, the Thames and the Thame. The Thame in particular contained abundant suitable habitat for water voles (a high percentage cover by emergent vegetation in a wide band) and their absence could be explained by the high mink density. Although this evidence is correlational, it does support an increasingly held belief among conservationists that mink are damaging water vole populations throughout the U K (Jefferies et al., 1989; Woodroffe et al., 1990b; Strachan & Jefferies, 1993). Nonetheless mink are not necessarily the only, or the most important, cause of water vole declines, and there are areas, as on the Kennet and Coln, where both species co-exist (Birks, 1986). Habitat destruction and fragmentation, disturbance by humans, agricultural intensification and river pollution may all contribute to water vole losses (Lawton & Woodroffe, 1991) as might flood control. The Thames, for example, has been substantially 'improved', resulting in regular large fluctuations in water level, which may harm water voles by flooding their burrows and making them vulnerable to predation. Furthermore, a negative correlation between mink and water voles could arise from their different habitat requirements. Water voles require slow-flowing waterways with earth banks, little tree cover and luxurious bank-side vegetation, while mink prefer fast-flowing rocky water courses, with a good cover of bankside trees (Strachan & Jefferies, 1993). However, both species inhabit both types of habitat in the UK, and in regions where they are both present there are fewer water voles than expected (Strachan & Jefferies, 1993). The lack of a correlation between water voles and emergent vegetation cover and width at our study sites also points to the possibility that they have been eliminated from their favoured areas.
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E. C. Halliwell, D. W. Macdonald
Differences in breeding biology, territoriality and dispersal capacity may at least partly explain differences in the potential for mink to affect water vole and moorhen populations. Water voles have a low dispersal capacity and breeding colonies may consist of as few as 10 adults, dropping to as little as two or three in the winter (Lawton & Woodroffe, 1991). By eliminating populations of water voles, mink make those that remain increasingly isolated (Woodroffe et al., 1990b; Lawton & Woodroffe, 1991) and more vulnerable to the effects of predation and habitat destruction. Our work has shown that mink appear to be limited by den availability, not only as the number of den sites, but also the pattern of den dispersion. Highest densities of mink were present along the Thames, which had a high number of evenly spaced potential den sites along river stretches. However, since mink sometimes den away from watercourses the frequency of adjacent blocks of woodland and mature hedgerows that support rabbits is also likely to play a role in determining a river's carrying capacity for mink. It is possible that the severity of the mink's influence upon vulnerable species such as the water vole is directly related to the distribution of suitable den sites, especially those that may provide secure breeding dens. However, the effects of mink must be weighed against other, possibly more fundamental problems facing the water vole, such as habitat loss and pollution, which may increase the vole's vulnerability. ACKNOWLEDGEMENTS We gratefully acknowledge the funding of the National Rivers Authority, and their collaboration in this study. In particular we thank Alistair Driver, Graham Scoley and Vaughan Lewis for their support. Rob Strachan and Fran Tattersall contributed greatly to the preparation of this paper. We thank Danielle Clode for her helpful comments. REFERENCES Akande, M. (1972). The food of feral mink (Mustela vison) in Scotland. J. Zool. Lond., 167, 475-9 Birks, J. D. S. (1986). Mink. Antony Nelson, Oswestry, Shropshire. Birks, J. D. S. & Dunstone, N. (1991). Mink. In The handbook of British mammals, ed. G. B. Corbet & S. Harris. Blackwell Scientific Publications, Oxford, pp. 212-18. Birks, J. D. S. & Linn, I. J. (1982). Studies on the home range of feral mink (Mustela vison). Syrup. Zool. Soc., Lond., 49, 231-57. Chanin, P. R. F. (1981). Diet of the otter (Lutra lutra) in relation to feral mink (Mustela vison) in two areas in south west England. Acta. Theriol, 26, 83-95.
Chanin, P. R. F. & Linn, I. J. (1980). The diet of the feral mink (Mustela vison) in southwest Britain. J. Zool., Lond., 192, 205-23. Clark, H. (1991). Mink hunting on the Nene. International Fieldsports and Conservation, Sept-Oct, 76-8. Clode, D., Halliwell, E. C. & Macdonald, D. W. (1995). A comparison of body condition in riverine and coastal mink (Mustela vison). J. Zool., Lond., 237, 686-9. Clode, D. & Macdonald, D. W. (in press). Evidence for food competition between mink and otter on the Scottish Islands. J. Zool., Lond., 237, 435~4. Craik, J. C. A. (1995). Effects of North American mink on the breeding success of terns and smaller gulls in west Scotland. Seabird, 17, 3-11. Dunstone, N. (1993) The mink. T. & A. D. Poyser, London. Dunstone, N. & Birks, J. D. S. (1985). The comparative ecology of coastal, riverine and lacusterine mink Mustela vison, in Britain. Z. angew. Zool., 72, 52-70. Dunstone, N. & Birks, J. D. S. (1987). The feeding ecology of mink (Mustela vision) in a coastal habitat. J. Zool. Lond., 212, 69-83. Gerell, R. (1967). Food selection in relation to habitat in mink (Mustela vision Schreber) in Sweden. Oikos, 18 233-46. Gerell, R (1968). Food habits of the mink (Mustela vison Schreber) in Sweden. Viltrevy 5, 119-211. Gerell, R. (1971). Population studies on mink (Mustela vison Schreber) in southern Sweden. Viltrevy, 8, 83-114. Ireland, M. C. (1990), The behaviour and ecology of the American mink (Mustela vison Schreber) in a coastal habitat. PhD thesis, Durham University. Jefferies, D. J., Morris, P. A., Mullineux, J. E. (1989). An enquiry into the changing status of the water vole Arvicola terrestris in Britain. Mammal Rev., 19, 111-31. King, C. M. (1984). Immigrant killers, introduced predators and the conservation of birds in New Zealand. Oxford University Press, London. Lawton, J. H. & Woodroffe, G. L. (1991). Habitat and the distribution of water voles: why are there gaps in a species' range? J. Anita. EcoL, 60, 79-91. Lever, C. (1978). The not so innocuous mink? New Scient., 78, 812-14. Lever, C. (1985). Naturalised animals of the worM. London, Hutchinson. Linn, I. & Chanin, P. (1978). Are mink really pests in Britain? New Scient., 77, 560-2. Marchant, J. H. & Hyde, P. A. (1980). Aspects of the distribution of riparian birds on waterways in Britain and Ireland. Bird Study, 27, 183-202. Mason, C. F. & Macdonald, S. M. (1986). Otters - - ecology and conservation. CUP, Cambridge. Strachan, R. & Jefferies, D. J. (1993). The water vole Arvicola terrestris in Britain 1989-1990: its distribution and changing status. Vincent Wildlife Trust, London. Woodroffe, G. L., Lawton, J. H. & Davidson, W. L. (1990a). Patterns in the production of latrines by water voles (Arvicola terrestris) and their use as indices of abundance in population surveys. J. Zool. Lond., 220, 439-45. Woodroffe, G. L., Lawton, J. H. & Davidson, W. L. (1990b). The impact of feral mink Mustela vison on water voles Arvicola terrestris in the North Yorkshire Moors National Park. Biol. Conserv., 51, 49~i2.
0006-3207(95)00072-0
ELSEVIER
A M E R I C A N M I N K Mustela vison IN THE UPPER THAMES CATCHMENT: RELATIONSHIP WITH SELECTED PREY SPECIES A N D D E N AVAILABILITY E. C. Halliwell* & D. W. Macdonald Wildlife Conservation Research Unit, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
(Received 31 December 1994; accepted 15 May 1995)
and across much of Scotland (Birks, 1986; Dunstone, 1993). Mink are associated with a wide range of aquatic habitats, including upland and lowland rivers and streams, canals and coastline, although they can live away from water where there is sufficient prey (Dunstone, 1993). They live in individual territories with little overlap between members of the same sex, but often considerable overlap between males and females (Dunstone, 1993). Those living along rivers concentrate their activity along the water's edge in linear home-ranges 1-6 km long which contain several dens (Birks, 1986; Dunstone, 1993). Mink are generalist predators, feeding on fish, invertebrates, birds, amphibians, small mammals and rabbits Oryctolagus cuniculus (Gerell, 1967; Akande, 1972; Chanin & Linn, 1980; Birks, 1986). Although regarded as a pest by farmers and game-keepers, mink generally have only a negligible and localised commercial impact (Dunstone, 1993). In a study in south-west England, for example, chickens and gamebirds made up less than 1% of prey remains in mink faeces (Birks, 1986). Concern has been voiced over the effects of mink on salmonid fish (Lever, 1985), but there is little evidence of a decline in any commercially important salmonids as a result of mink predation (Dunstone, 1993). During the early 1990s, however, anglers in the Upper Thames region became concerned that mink might affect fish stocks. Mink can have severe effects on bird populations. On the west coast of Scotland, for example, they effectively prevent breeding in colonies of gulls and terns (Craik, 1995). In riverine habitats waterfowl are particularly at risk from mink predation during the nesting season (Dunstone, 1993). Chanin and Linn (1980) found the remains of Rallidae in 15-16% of scats at two of their study sites in southwest Britain, and Chanin (1981) suggested that because coots Fulica atra and moorhens Gallinula chloropus are relatively small and spend much of their time in reed beds where they can be ambushed, they are easier prey for mink than ducks. Jefferies et al. (1989), from a survey of County Mammal Reports and questionnaires, found that mink were considered the most important predator of water voles Arvicola terrestris and concluded that the water vole is in decline. There is now growing evidence that these
Abstract American mink Mustela vison and three prey types of economic and conservation importance were censused along four rivers in the Upper Thames catchment area. There were no significant correlations between any measures o f mink abundance and any measures o f f i s h or moorhen Gallinula chloropus abundance, suggesting that mink were neither limiting nor limited by these prey items. Water voles Arvicola terrestris, however, were negatively correlated with mink, being absent at four sites where mink were most numerous. Sites at which mink were most numerous had a high availability of potential den sites and low emergent vegetation cover, but there were no correlations between water voles and the availability o f their preferred habitat. Keywords." American mink, Upper Thames catchment,
water voles, moorhen, fish, den sites. INTRODUCTION
The American mink Mustela vison is the only widely distributed non-indigenous wild carnivore in the UK. Mink have a popular image as voracious predators, and their establishment and supposed impact on wildlife, game species and domestic stock in the UK has generated a considerable amount of discussion and controversy (Lever, 1978; Linn & Chanin, 1978; Clark, 1991). Elsewhere, introduced mustelids such as stoats Mustela erminea in New Zealand, and mongooses Herpestes auropunctatus in the West Indies (King, 1984; Lever, 1985) have decimated native species, and consideration of the conservation and economic implications of the establishment of mink in the UK is therefore justified. Here we look at the relationships between mink and three prey species of economic or conservation importance in the Upper Thames catchment area. Mink were first imported into the UK to be bred on fur farms in 1929. Feral populations became established during the 1950s and 1960s, and by 1970 the species was present throughout England and Wales, *Present address: Institute of Terrestrial Ecology, Hill of Brathens, Banchory, Glassel, Kincardinshire, AB31 4BY, UK 51
E. C. Halliwell, D. I41. MacdonaM
52
I
I'Vin., ....
10 km
Fig. 1. Map showing location of study areas.
populations are threatened by mink predation as well as by habitat loss and fragmentation (Woodroffe et al., 1990b; Strachan & Jefferies, 1993) In view of their potential impact on vulnerable prey such as water voles, mink may be limiting the densities of certain species, with negative economic or conservation consequences. Similarly, mink density may be limited by that of their prey. Clode et al. (1995) demonstrated that mink from our study areas were significantly smaller and in poorer body condition than mink from coastal habitats in Scotland, possibly because food availability and quality is lower in riverine habitats. Other authors (Birks & Linn, 1982; Dunstone & Birks, 1985, 1987; Ireland, 1990) have also suggested that coastal habitats have richer resources than riverine habitats. If mink in the Upper Thames catchment were limited by overall prey availability this would not preclude them having a limiting effect on some species. The abundance of both mink and water voles may also be affected by habitat. In particular, mink rely on mature willows Salix spp. and pollarded trees for den sites (Mason & Macdonald, 1986), while water voles prefer areas with low tree cover and high emergent vegetation cover (Lawton & Woodroffe, 1991; Strachan & Jefferies, 1993). Here we investigate the potential for mink to limit the density of three prey types - - fish, moorhens and water voles - - chosen for their financial and conservation importance in the Upper Thames region. If mink were limiting certain prey populations we would expect a negative correlation, with fewest of those prey in areas with most mink. Conversely, mink density might
be limited by den availability or by the availability of one of the chosen prey types (which is unlikely given their dietary diversity). In this case we would expect a positive correlation between mink density and prey or den availability, with most mink in areas with highest availability.
METHODS Study sites Mink were studied along a 20 km section of each of four lowland rivers in the Upper Thames catchment area in Oxfordshire and Berkshire, southern England (Fig. 1): the Thames (SP 309002-SP 435042), Thame (SP 621056--SP 578932), Coin (SP 109074-SU 205988) and Kennet (SU 288712-SU 456672). The Coin is a game fish river in which trout Salmo spp. predominate; the other rivers largely contain cyprinids such as roach Rutilus rutilus, chub Leuciscus cephalus and bream Abramis brama. The Thames, Thame and Kennet were up to 20 m wide in places, while the Coin was narrower, reaching a maximum width of about 10 m. The Thames and Thame were the deepest rivers (> 1 m), the Coin was intermediate (0.5-1 m), and the Kennet was the shallowest (0.254)-5 m). The rivers were fringed with trees such as willow Salix fragilis, alder Alnus glutinosa and ash Fraxinus excelsior, and vegetation such as nettles Urtica dioica, rank grassland, willowherb Epilobium spp., bramble Rubus fruticosus, blackthorn Prunus spinosa and hawthorn Crataegus monogyna. The band of vegetation emerging from the water consisted of
Mink in the Upper Thames catchment species such as bur-reed Sparganium erectum, common reed Phragmites australis and reedsweet grass Glyceria maxima. Adjacent land was mainly pasture, arable and woodland. Occasional opportunistic killing of mink by water bailiffs and lock-keepers occurred throughout the study area, particularly along the Kennet, but no detailed data were available. Each section was divided into two contiguous 10 km sections designated upper and lower sections. These were nominally divided into 40 c. 250 m samples, which were assessed by eye for the percentage cover of emergent vegetation, the width (m) of the band of emergent vegetation along the river, percentage mature tree cover, and the number of potential mink dens. Percentage emergent cover was estimated to the nearest 5%, and single mature trees were given a value of 2%. Potential mink dens were: willow pollards, tree hollows, holes among tree roots, root plates (the vertical mat of roots and soil formed when a tree falls), stick piles and washed-up debris, and rabbit burrows. Assessment of mink abundance
Mink were live-trapped in commercial cage-traps baited with fish. The upper section of each river was trapped in three sessions, each lasting five nights, between July and November 1991; Lower sections were trapped once for eight nights in November and December 1991. Each session used 30 camouflaged traps set at approximately 300 m intervals. Traps were placed along the river bank in carefully chosen sites where there were field signs of mink or where they were thought likely to be active. The traps were checked every morning, and animals captured for the first time were anaesthetised with Metophane, measured, weighed and eartagged, and any striking markings on their undersides and throats were described to assist individual identification. Mink were classified as juvenile or adult by: body weight, subtle facial characteristics dependent on bone growth, width of the baculum (felt by palpation) in males, and white hairs resulting from mating wounds in females. Following Gerell (1971), adult mink caught on the upper sections of each river were classed as 'transients' if they were caught in one trap session, or 'residents' if they were caught in two or more sessions. All adults caught on the lower sections were considered residents as trapping occurred during a time when there is usually little transient movement, and when the majority of territories have been established (Birks, 1986). Assessment of prey abundance
Data on the population density (number per m 3) and size categories of fish on the rivers Thame, Coln and Kennet were obtained from the National Rivers Authority (NRA) electro-fishing surveys carried out between February 1987 and July 1991. Fish population densities tend to remain constant between years (NRA, pers. comm.), and surveys were done within or very close to study areas. The majority o f fish taken by
53
mink are less than 15 cm long, with pike Esox lucius usually less than 20 cm (Gerell, 1968; Birks, 1986). The density (number per m 3) of fish available to mink was therefore calculated as the product of fish population density and the proportion of fish in size categories usually taken by mink. Electro-fishing data were not available for the River Thames, but angler catches, fry surveys and individual observations indicated that the river contained a healthy, mixed fish population similar to that of the River Thame (N.R.A., pers. comm.). Adult and juvenile moorhens were easily counted during each daily trap round, and the maximum daily count in each study section was taken to be the number present. The presence of water voles was determined by a thorough search for field signs (runs, burrows and latrines) within a 100 m radius of each trap. The proportion of traps at which water vole signs were found was used as an index of water vole abundance (Woodroffe et al., 1990a). RESULTS Mink abundance
Highest densities of adult and juvenile mink were found on the Thames (Table 1). Highest densities of resident mink were found along the upper and lower sections of the river Thames (0.6/km and 0.7/km, respectively), followed by the lower Thame (0.5/km) and the lower Coin (0.3/km). The upper Thame and the lower Kennet both had resident mink at d.ensities of 0.2/km, while the upper Coin and upper Kennet had densities of 0.1/km. Trapping success (Table 1) was used as a second measure of mink abundance to counteract differences in trap effort between upper and lower sections; trap success data concurred with the density data. Prey availability
Availability of fish to mink was highest on the upper and lower Thame, and lowest on the Kenneit (Table 2). The Thame and Kennet contained no or very few Table 1. Numbers of resident adult, transient adult and juvenile mink captured, and number of resident mink caught per 100 trap nights (100 tu), along upper and lower sections of four rivers in the Upper Thames catchment area
Site Thames Upper Lower Thame Upper Lower Coln Upper Lower Kennet Upper Lower
Resident Transient Juveniles Total Residents adults adults /100 tn 6 7
0 0
4 1
10 8
1.33 2.92
2 5
2 0
0 2
4 7
0-44 2.08
1 3
3 0
3 2
7 5
0.22 1.25
1 2
3 0
2 0
6 2
0-22 0-83
E. C. Halliweil, D. IV. MacdonaM
54
Table 2. Abundance of three mink prey items - - fish, moorhens and water voles - - on upper and lower sections of four rivers in the Upper Thames catchment area
Fish availability was measured as the product of mean fish density and percentage of fish of a suitable size for mink. Moorhen abundance was estimated as the maximum number of individuals (both adult and juvenile) seen within the study site. The percentage of mink traps with signs of water voles within 100 m was used as an index of water vole availability. Site Thames Upper Lower Thame Upper Lower Coln Upper Lower Kennet Upper Lower
Fish Moorhens (no. available/m 3)(no. individuals)
40-
g
Water voles (% traps)
302010-
---
47 57
0 0
0.54 0.11
29 31
0 0
0.01 0.03
72 60
66-7 42.4
0.09 0.00
52 25
39.4 30.3
salmonids respectively, while the Coln contained more than 80%. The upper Thame had by far the highest mean total fish density (0.65/m3), most (83%) of which were available to mink. The lower Thame and the upper Kennet held intermediate total densities of fish (0.17/m 3 and 0-14/m 3, respectively), of which over 60% were available for mink. Total fish densities in the upper and lower Coin and lower Kennet were low (0.09/m 3, 0.14/m 3 and 0-11/m 3, respectively), of which about one-fifth or less were suitably sized for mink. There were no significant correlations between any measure of mink abundance and fish density or availability (calculated as the product of fish density/m 3 and percentage of suitably sized fish). Moorhens were most abundant on the upper and lower Coln and the lower Thames (Table 2). There were no significant correlations between any measure of mink abundance and adult, juvenile or total moorhen abundance. There were no signs of water vole activity along the upper and lower stretches of the Thames or the Thame (Table 2). Traps along the upper Coin had the highest proportion of water vole signs (66.7%), those along the lower Coin and upper Kennet had intermediate proportions of signs, and lowest water vole activity was along the lower Kennet. There was a significant negative correlation between numbers of resident mink and proportions of water vole signs (r = -0-732, d.f. = 6, p < 0-05; Fig. 2). H a b i t a t availability
The Thames and Thame river sections contained the highest mean number of potential den sites, while the Coin and Kennet contained the lowest number (Table 3). There was a significant correlation between mean numbers of potential dens and total numbers o f mink (r = 0-804, d.f. -- 6, p < 0.02), total numbers of adults
0
]
•
2
3
~
•
~__
5
6
Resident M i n k
Fig. 2. Correlation between number of resident mink and percentage of.traps with water vole signs at eight study areas along four rivers in the Upper Thames catchment area.
(r = 0.902, d.f. = 6, p < 0.01) and number of resident adults (r -- 0.781, d.f. = 6, p < 0.05), suggesting that mink densities may have been limited by den availability (Fig. 3). When tested with the variance mean ratio test, potential den sites were significantly clumped along the Thame ( ~ = 318, n = 80, p < 0.01), Coin (g 2 - 200, n = 80, p < 0.01) and Kennet (g 2 = 241, n = 80, p < 0.01), which may have further reduced the number of mink able to live along these rivers. The mean percentage of emergent vegetation cover was similar along the Thame, Kennet and Coin, but was low along the Thames; the mean width of the band of emergent vegetation was similar along all eight sections of the rivers (Table 3). Mean mature tree cover was variable between sites (Table 3), being greatest along the upper Thames and lowest along the lower Thame. There were no correlations between any measure of mink abundance and the mean width of the band of emergent vegetation or mean mature tree cover. Adult resident mink were most abundant where
Table 3. Mean ___SD number of potential mink den sites, mean __ SD emergent vegetation cover, mean _+ SD emergent vegetation band width and mean _+ SD mature tree cover along upper and lower sections of four rivers in the Upper Thames catchment area
Site Thames Upper Lower Thame Upper Lower Coin Upper Lower Kennett Upper Lower
Potential den sites
Emergent cover (%)
Emergent Mature tree width (m) cover (%)
3.8 __4.2 2.3 _+2.0
17.0 _+8-2 26.9_+20.5
1.0 __0.4 1-0 _+0.5
21.7 -- 27.6 16.0 + 15-4
1.1 _+0.4 1.0 _+ 1.2
47.5 _+27.2 46.3 _+27.5
1-1 _+0.9 1.4 _+ 1.0
8.9 -+ 15.3 7-9 _+8.7
0.4 _+0.6 1.0 _+ 1.0
57.3+_21-8 42.1 _+23-3
1.2 _+0.3 0.9 + 0.4
9.3 _+ 14.5 19.0 + 21.5
0-6 + 0.9 0-9 + 0.9
44.1 + 23.8 38-9+ 28.6
0.7 + 0.5 0.9 + 0.6
15.0 + 8.6 13.4 + 9.0
Mink in the Upper Thames catchment
:1 5
0
• "
i
i
"
i
i
Potential Dens
Fig. 3. Correlation between number of resident mink and number of potential den sites at eight study areas along four rivers in the Upper Thames catchment area. mean emergent vegetation cover was low (r = -0.768, d.f. = 6, p < 0.05), but this may have been a result of the negative correlation between mean percentage emergent vegetation cover and the mean number of potential dens available (r = -0.931, d.f. -- 6, p < 0.001). There was no significant correlation between water vole abundance and mean width of emergent vegetation, mean percentage cover by emergent vegetation or mean percentage cover by mature trees. DISCUSSION Mink were clearly more abundant along the Thames than along the Thame, Kennet or Coln. Densities along the Thame, Coln and Kennet, measured as number of residents per kilometre, were lower than in the small number of previous studies in the UK. On the River Teign, Devon, densities range from 0.53/km to 0.26/km, and in Scotland a density of 2/km of coastline has been recorded (Birks & Dunstone, 1991). We believe that all or most of the mink resident in our study areas were captured, with the exception of those along the Kennet. Mink were captured in all areas where there were positive signs of their presence, such as footprints or scats. On the Coln, sections of soft mud revealed prints only in areas where mink were caught. Furthermore, just one new individual was caught in the whole of each third trapping session along the upper Thames and Thame sections. No new mink were caught on the last two trapping days along the lower sections of Thames, and only one new individual was caught during the final three days of trapping along the lower Thame. Numbers may have been underestimated along the Kennet, however, where scats were found in areas where very few or no mink were captured. Mink on the Kennet may have avoided capture because the river divides into several parallel sections, of which only one was trapped. There was no evidence that mink were limiting, or were limited by, fish or moorhen abundance. There were good numbers of moorhens breeding and producing
55
young on the Thames, where mink density was highest. Marchant and Hyde (1980) suggested that the moorhen's success is due to its adaptability and territoriality. Nonbreeding birds, described as a 'mobile "floating" stock', are available to occupy new or empty territories as they occur. So long as mink do not kill more than the floating stock they will not affect moorhen productivity. There may have been seasonal variation in the extent to which predator and prey abundance affected each other. Fish can form a large part of mink diet (35-54%), particularly in the winter (Gerell, 1968; Chanin & Linn, 1980) when they may become more available due to being less active in the lower water temperature. Other prey may be less available in winter: Gerell (1968) found that in autumn and winter the percentage occurrence of birds in mink scats decreased. Competition may also affect mink diet: Clode and Macdonald (1995) found, in the Western Isles of Scotland, that mink in allopatric populations preferred fish to other available prey, whereas those sympatric with otters consumed fewer fish. There was evidence that mink in the Upper Thames catchment were limiting water vole abundance, at least along the rivers with the highest densities, the Thames and the Thame. The Thame in particular contained abundant suitable habitat for water voles (a high percentage cover by emergent vegetation in a wide band) and their absence could be explained by the high mink density. Although this evidence is correlational, it does support an increasingly held belief among conservationists that mink are damaging water vole populations throughout the U K (Jefferies et al., 1989; Woodroffe et al., 1990b; Strachan & Jefferies, 1993). Nonetheless mink are not necessarily the only, or the most important, cause of water vole declines, and there are areas, as on the Kennet and Coln, where both species co-exist (Birks, 1986). Habitat destruction and fragmentation, disturbance by humans, agricultural intensification and river pollution may all contribute to water vole losses (Lawton & Woodroffe, 1991) as might flood control. The Thames, for example, has been substantially 'improved', resulting in regular large fluctuations in water level, which may harm water voles by flooding their burrows and making them vulnerable to predation. Furthermore, a negative correlation between mink and water voles could arise from their different habitat requirements. Water voles require slow-flowing waterways with earth banks, little tree cover and luxurious bank-side vegetation, while mink prefer fast-flowing rocky water courses, with a good cover of bankside trees (Strachan & Jefferies, 1993). However, both species inhabit both types of habitat in the UK, and in regions where they are both present there are fewer water voles than expected (Strachan & Jefferies, 1993). The lack of a correlation between water voles and emergent vegetation cover and width at our study sites also points to the possibility that they have been eliminated from their favoured areas.
56
E. C. Halliwell, D. W. Macdonald
Differences in breeding biology, territoriality and dispersal capacity may at least partly explain differences in the potential for mink to affect water vole and moorhen populations. Water voles have a low dispersal capacity and breeding colonies may consist of as few as 10 adults, dropping to as little as two or three in the winter (Lawton & Woodroffe, 1991). By eliminating populations of water voles, mink make those that remain increasingly isolated (Woodroffe et al., 1990b; Lawton & Woodroffe, 1991) and more vulnerable to the effects of predation and habitat destruction. Our work has shown that mink appear to be limited by den availability, not only as the number of den sites, but also the pattern of den dispersion. Highest densities of mink were present along the Thames, which had a high number of evenly spaced potential den sites along river stretches. However, since mink sometimes den away from watercourses the frequency of adjacent blocks of woodland and mature hedgerows that support rabbits is also likely to play a role in determining a river's carrying capacity for mink. It is possible that the severity of the mink's influence upon vulnerable species such as the water vole is directly related to the distribution of suitable den sites, especially those that may provide secure breeding dens. However, the effects of mink must be weighed against other, possibly more fundamental problems facing the water vole, such as habitat loss and pollution, which may increase the vole's vulnerability. ACKNOWLEDGEMENTS We gratefully acknowledge the funding of the National Rivers Authority, and their collaboration in this study. In particular we thank Alistair Driver, Graham Scoley and Vaughan Lewis for their support. Rob Strachan and Fran Tattersall contributed greatly to the preparation of this paper. We thank Danielle Clode for her helpful comments. REFERENCES Akande, M. (1972). The food of feral mink (Mustela vison) in Scotland. J. Zool. Lond., 167, 475-9 Birks, J. D. S. (1986). Mink. Antony Nelson, Oswestry, Shropshire. Birks, J. D. S. & Dunstone, N. (1991). Mink. In The handbook of British mammals, ed. G. B. Corbet & S. Harris. Blackwell Scientific Publications, Oxford, pp. 212-18. Birks, J. D. S. & Linn, I. J. (1982). Studies on the home range of feral mink (Mustela vison). Syrup. Zool. Soc., Lond., 49, 231-57. Chanin, P. R. F. (1981). Diet of the otter (Lutra lutra) in relation to feral mink (Mustela vison) in two areas in south west England. Acta. Theriol, 26, 83-95.
Chanin, P. R. F. & Linn, I. J. (1980). The diet of the feral mink (Mustela vison) in southwest Britain. J. Zool., Lond., 192, 205-23. Clark, H. (1991). Mink hunting on the Nene. International Fieldsports and Conservation, Sept-Oct, 76-8. Clode, D., Halliwell, E. C. & Macdonald, D. W. (1995). A comparison of body condition in riverine and coastal mink (Mustela vison). J. Zool., Lond., 237, 686-9. Clode, D. & Macdonald, D. W. (in press). Evidence for food competition between mink and otter on the Scottish Islands. J. Zool., Lond., 237, 435~4. Craik, J. C. A. (1995). Effects of North American mink on the breeding success of terns and smaller gulls in west Scotland. Seabird, 17, 3-11. Dunstone, N. (1993) The mink. T. & A. D. Poyser, London. Dunstone, N. & Birks, J. D. S. (1985). The comparative ecology of coastal, riverine and lacusterine mink Mustela vison, in Britain. Z. angew. Zool., 72, 52-70. Dunstone, N. & Birks, J. D. S. (1987). The feeding ecology of mink (Mustela vision) in a coastal habitat. J. Zool. Lond., 212, 69-83. Gerell, R. (1967). Food selection in relation to habitat in mink (Mustela vision Schreber) in Sweden. Oikos, 18 233-46. Gerell, R (1968). Food habits of the mink (Mustela vison Schreber) in Sweden. Viltrevy 5, 119-211. Gerell, R. (1971). Population studies on mink (Mustela vison Schreber) in southern Sweden. Viltrevy, 8, 83-114. Ireland, M. C. (1990), The behaviour and ecology of the American mink (Mustela vison Schreber) in a coastal habitat. PhD thesis, Durham University. Jefferies, D. J., Morris, P. A., Mullineux, J. E. (1989). An enquiry into the changing status of the water vole Arvicola terrestris in Britain. Mammal Rev., 19, 111-31. King, C. M. (1984). Immigrant killers, introduced predators and the conservation of birds in New Zealand. Oxford University Press, London. Lawton, J. H. & Woodroffe, G. L. (1991). Habitat and the distribution of water voles: why are there gaps in a species' range? J. Anita. EcoL, 60, 79-91. Lever, C. (1978). The not so innocuous mink? New Scient., 78, 812-14. Lever, C. (1985). Naturalised animals of the worM. London, Hutchinson. Linn, I. & Chanin, P. (1978). Are mink really pests in Britain? New Scient., 77, 560-2. Marchant, J. H. & Hyde, P. A. (1980). Aspects of the distribution of riparian birds on waterways in Britain and Ireland. Bird Study, 27, 183-202. Mason, C. F. & Macdonald, S. M. (1986). Otters - - ecology and conservation. CUP, Cambridge. Strachan, R. & Jefferies, D. J. (1993). The water vole Arvicola terrestris in Britain 1989-1990: its distribution and changing status. Vincent Wildlife Trust, London. Woodroffe, G. L., Lawton, J. H. & Davidson, W. L. (1990a). Patterns in the production of latrines by water voles (Arvicola terrestris) and their use as indices of abundance in population surveys. J. Zool. Lond., 220, 439-45. Woodroffe, G. L., Lawton, J. H. & Davidson, W. L. (1990b). The impact of feral mink Mustela vison on water voles Arvicola terrestris in the North Yorkshire Moors National Park. Biol. Conserv., 51, 49~i2.