The American Mink Mustela vison in Essex
spring when Water Voles are less active, we considered that we must have missed many. Nevertheless
we recorded Water Voles at 33% of main river sites, not too dissimilar to that of Thompson (1999).
Some 38% of sites with signs of Mink also had signs of Water Vole. Quite long stretches of Essex
rivers have a dense riparian vegetation, dominated by species such as Nettle Urtica dioica, because
they arc backed by arabic land. By contrast, pasture, with grazing to the edge of the river was the
dominant land-use in studies definitively finding a relationship between Mink colonization and
Water Vole decline. This dense vegetation, which incidentally makes Water Vole surveying in spring
and summer rather difficult, may provide the structured habitat in which the two species can coexist
and an abundant food resource for the Water Vole. On the River Thames Water Voles were strongly
positively associated with Urtica, which is an important food item for them (Woodall, 1993). An
alternative view is that Mink are not yet sufficiently established in Essex to have significantly
impacted Water Vole numbers, something that may happen in the near future. The distribution and
relationships of the two species in Essex requires urgent detailed research.
Strachan and Jefferies (1996) provide evidence that Mink populations have declined with the
expansion of Otter populations, there having been a 50% reduction in Mink occupation in south-
west England since 1984. Otters are currently thinly distributed in Essex and their long-term future
is not secure (Macdonald and Mason, 2002). Should Otters become well established in the county,
it is possible that the spread of Mink may be reversed.
There have been calls to begin extensive trapping programmes for Mink in the county, primarily to
protect Water Voles but also the important breeding and wintering waterfowl populations. As outlined
above there is no evidence yet that Water Voles in Essex are being severely impacted by Mink, and
very little scientific evidence that any mainland bird population has declined due to predation by
Mink despite the many years of concern by conservationists. Birks (1986) describes how intensive
trapping has failed to eradicate Mink even on a local scale. The control of Mink on a landscape
scale is considered costly and difficult to achieve (Rushton et al, 2000), the species being highly
mobile and rapidly recolonizing areas from which they have been removed (Dunstone, 1993). It
would seem sensible that scarce resources would be better directed towards restoring riparian
habitats and river floodplains. This would not only be beneficial to biodiversity in the river valley
as a whole, but it would also lead to improvements in water quality, groundwater recharge and
flood management, to the economic benefit of us all (Mason, 2002, p321).
Acknowledgements
We gratefully acknowledge support by WWF UK as part of the Essex Otter survey.
References
BARRETO, GR., MACDONALD, D.W. and STRACHAN, R. 1998. The Tightrope Hypothesis: An
explanation for the plummetting water vole numbers in the Thames catchment. In: Bailey, R.G., Jose,
P.V. and Sherwood. R.R. (eds), United Kingdom Floodplains: Westbury Publishing, Otley. Pp. 311-327.
BIRKS, J. 1986. Mink. The Mammal Society, London.
BIRKS, J.D.S. 1989. What regulates the numbers of feral mink? Nature in Devon 10: 45-61.
BIRKS, J. 1990. Feral Mink and nature conservation. British Wildlife 1: 313-323.
CIIANIN, P.R.F. and LINN, I. 1980. The diet of the feral mink (Mustela vison) in southwest Britain.
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Essex Naturalist (New Series) 19 (2002)