The American Mink Mustela vison in Essex
2002 (Fig. 2). This could represent a genuine increase. However conditions for surveying were
ideal in spring 2002, with much soft, exposed mud beneath bridges and along riverbanks, providing
ample opportunity for locating tracks of the animals.
The results indicate that Mink arc now widespread in the county away from the coastal areas. In
some systems they may also be quite numerous. We have received one report of a gamekeeper
trapping about 60 animals on the River Stour near Great Cornard (per R.D. Brown) in the last five
years, with no apparent affect on numbers. Such trapping is probably counterproductive as it may
remove established dominant males, allowing greater" movement and greater survival of young
individuals.
The Mink is often treated as a scapegoat for changes in the fortunes of British wildlife, generally
without evidence of its adverse impacts (Birks, 1990). As Birks argues, by routinely blaming Mink
we ignore less obvious but very important causes of ecological problems. The Mink is a generalist
predator, tending to concentrate its activities on the most abundant, easily available prey, turning to
less abundant alternatives should favoured species become less common (Birks, 1986). Studies
have shown a relationship between Mink density and the density of Rabbits Oryctolagus cuniculus,
a preferred prey item, especially of dominant males (Birks, 1989). Although Mink take a substantial
number of waterfowl, a detailed study at a coastal wetland in Devon (Slapton Ley) demonstrated
that Moorhens Gallinula chloropus and Coots Fulica afra were able to maintain viable populations
in the presence of Mink (Chanin and Linn, 1980; Birks, 1986). Moorhen populations have also
been shown to withstand predation by Mink in the linear habitat of the River Thames (Halliwell
and Macdonald, 1996). However", evidence suggests that Coots suffer high mortality in those stretches
of the Thames with high densities of Mink, and immigration of birds is required to maintain
populations (Ferreras and Macdonald, 1999). Nevertheless, there is no evidence that Mink, despite
their current broad distribution in the UK, have caused widespread population declines in any bird
species (Birks, 1990).
One species for which there is good evidence of severe predation by Mink is the Water Vole Arvicola
terrestris (Woodroffe, 1994; Barreto et al., 1998). There is an inverse relationship between Mink
activity and Water Vole activity (Halliwell and Macdonald, 1996) and Water Vole features
prominently in the diet of Mink in the early years of colonization (Strachan et al. 1998). However
the relationship is not that simple. Barreto et al. (1998) argue that Water Voles along many of our
waterways live on a "tightrope." Due to the intensive management of river banks and overgrazing
they have been forced to live in a fragmented, narrow ribbon of habitat. The invasion of Mink
causes the extermination of Water Voles living on this tightrope. Where habitat is good, Water
Voles can withstand the predatory impact of Mink.
The survey of Water Voles in Essex conducted in 1998 found that 39.4 % of 429 main river sites
were positive for the species (Thompson, 1999). The Stour and Colne proved to be the best rivers
for Water Voles. Mink were found to be established on these catchments (Thompson, 1999), and
are clearly common in parts of the Stour given the observations on trapping recorded above, while
Fig. 1 shows them also widely distributed on the Colne. Despite the Stour having the widest
distribution of Water Voles in the county, Thompson concluded that Mink were having a serious
affect on the species in the catchment - supposition, we suggest, that if Mink are present, they must
be having an impact.
We recorded signs of Water Voles in our own survey but, because a search was stopped as soon as
Otter signs were found, and because our surveys were largely conducted in late winter and early
Essex Naturalist (New Series) 19 (2002)
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