Tracking the expansion of the American mink (Neovison vison) range in NW Portugal

Abstract

Invasive in many European countries, the American mink (Neovison vison) was introduced in Portugal in the late 1980’s, presumably escaping from Spanish fur farms close to the border. In spite of the biological richness of the invaded area, no study ever addressed the evolution of the invasion process. We aimed to investigate the current distribution and status of the mink in NW Portugal and discuss some contributing factors to explain the rate of invasion. We detected mink presence using floating rafts as footprint tracking devices, and scats as a molecular tool aiding in species identification. Results demonstrate a clear range expansion southwards, with mink already occupying most of the region’s hydrographic basins. After a first phase of slow expansion (55 km in 20 years), mink seems to have expanded its range quite rapidly in only 2 years (45 km). The initial delay could be due to local thriving otter populations, whereas the recent establishment of red swamp crayfish (Procambarus clarkii) in the area could be a plausible explanation for the acceleration in the mink’s expansion. Being a key food resource, crayfish may be playing an important role as an expansion facilitator. Mink eradication is probably no longer feasible since well established populations near the border continue to function as sources for the Portuguese population. Therefore, a control program should start immediately in the NW region, preferably in conjunction with Spanish authorities.

Full text

1 23
Biological Invasions
ISSN 1387-3547
Biol Invasions
DOI 10.1007/s10530-014-0706-1
Tracking the expansion of the American
mink (Neovison vison) range in NW
Portugal
Diana C.Rodrigues, Luciana Simões,
Jacinta Mullins, Simone Lampa, Raquel
C.Mendes, Carlos Fernandes, Rui
Rebelo & Margarida Santos-Reis

1 23
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INVASION NOTE
Tracking the expansion of the American mink (Neovison
vison) range in NW Portugal
Diana C. Rodrigues
•
Luciana Simo
˜
es
•
Jacinta Mullins
•
Simone Lampa
•
Raquel C. Mendes
•
Carlos Fernandes
•
Rui Rebelo
•
Margarida Santos-Reis
Received: 11 October 2013 / Accepted: 21 April 2014
Ó Springer International Publishing Switzerland 2014
Abstract Invasive in many European countries, the
American mink (Neovison vison) was introduced in
Portugal in the late 1980’s, presumably escaping from
Spanish fur farms close to the border. In spite of the
biological richness of the invaded area, no study ever
addressed the evolution of the invasion process. We
aimed to investigate the current distribution and status
of the mink in NW Portugal and discuss some
contributing factors to explain the rate of invasion.
We detected mink presence using floating rafts as
footprint tracking devices, and scats as a molecular
tool aiding in species identification. Results demon-
strate a clear range expansion southwards, with mink
already occupying most of the region’s hydrographic
basins. After a first phase of slow expansion (55 km in
20 years), mink seems to have expanded its range
quite rapidly in only 2 years (45 km). The initial delay
could be due to local thriving otter populations,
whereas the recent establishment of red swamp
crayfish (Procambarus clarkii) in the area could be a
plausible explanation for the acceleration in the
mink’s expansion. Being a key food resource, crayfish
may be playing an important role as an expansion
facilitator. Mink eradication is probably no longer
feasible since well established populations near the
border continue to function as sources for the
Portuguese population. Therefore, a control program
should start immediately in the NW region, preferably
in conjunction with Spanish authorities.
Keywords Range expansion  Tracking rafts 
Fecal genotyping  Lutra lutra  Interspecific
competition  Procambarus clarkii
Introduction
Introduced vertebrate predators are rapidly becoming
a conservation problem worldwide since they can
cause catastrophic changes in ecosystems via their
impacts on native species (Dickman 1996; Genovesi
and Shine 2003) and ecosystem functioning. During
the different stages of an invasion, population growth
and range expansion rates of an introduced species can
vary markedly. Some have speedy invasion rates,
whereas others appear to have long lag times between
initial introduction and subsequent population explo-
sions, when range expansion and species visibility
increase apace; also, an exotic species past history is a
poor predictor of potential population growth and
D. C. Rodrigues (&)  L. Simo
˜
es  J. Mullins 
R. C. Mendes  C. Fernandes  R. Rebelo 
M. Santos-Reis
Centro de Biologia Ambiental, Faculdade de Cie
ˆ
ncias da
Universidade de Lisboa, Ed. C2 - 58 Piso, Campo Grande,
1749-016 Lisbon, Portugal
e-mail: rodriguesdia@gmail.com
S. Lampa
Department of Conservation Biology, UFZ – Helmholtz
Centre for Environmental Research, Permoserstraße 15,
04318 Leipzig, Germany
123
Biol Invasions
DOI 10.1007/s10530-014-0706-1
Author's personal copy

range expansion on new invaded areas (Crooks and
Soule
´
1999). In spite of known impacts on species and
ecosystems, authorities are often slow to respond to
these invasions and frequently management efforts are
neither coordinated nor science-based. To attain
effective control, it is essential to respond rapidly to
the invasion, making informed decisions based on
knowledge about the distribution, demography, and
impact on native biota (Genovesi and Shine 2003).
Originally from North America, the American mink
(Neovison vison) was brought to Europe during the
1920’s for fur farming (Dunstone 1993). Today, this
carnivore is naturalized in many European countries,
as well as in South America and Asia, due to
accidental escape or deliberate release from fur farms
(Bonesi and Palazo
´
n 2007). As a generalist predator,
mink has a large impact on native prey populations. Its
presence is commonly associated with drastic reduc-
tion of population densities, breeding success, and
distribution changes of water birds (Clode and Mac-
donald 2002), small mammals (Jefferies 2003),
amphibians (Ahola et al. 2006), crayfish (Fischer
et al. 2009), and fish (Melero et al. 2012). Moreover,
mink also affects the entire guild of native predators by
competing for similar food and habitat resources. Its
expansion and abundance are associated to population
declines of the European mink (Mustela lutreola,
Sidorovich and Macdonald 2001), European polecat
(Mustela putorius, Sidorovich and Macdonald 2001),
and spotted genet (Genetta genetta, Melero et al.
2012). Yet, where Eurasian otter (Lutra lutra) popu-
lations present high densities, otters may be more
competitive than mink due to their larger body size
and more streamlined shape, preying more efficiently
on fish and crayfish underwater (Dunstone 1983,
Bonesi and Macdonald 2004). According to some
studies, the observed decline of mink in England up to
the beginning of the last decade is concurrent with an
increasing native otter population (Bonesi et al. 2006,
McDonald et al. 2007). On the other hand, other
studies report that coexistence of both species is
possible mainly by mink avoiding otters through
temporal (Harrington et al. 2009) and/or food niche
partitioning (Bonesi et al. 2004). On account of its
serious negative impacts, the Bern Convention rec-
ommended American mink eradication in regions
where it is invasive, and several control/eradication
programs have been carried out so far. Although some
programs have successfully controlled mink popula-
tions on the mainland, the most effective eradication
programs in the long-term were carried out on
relatively small and isolated islands difficult for mink
to re-colonize (Bonesi and Palazo
´
n 2007).
Mink invasion is poorly investigated in the Med-
iterranean region, except for Spain, where control
programs are underway in order to protect the native
European mink (Melero et al. 2010
). In Portugal, the
first reported sighting of this invasive carnivore
occurred in 1985 in the margins of the River Minho,
which constitutes a natural border between northwest
Portugal and Galicia, Spain (Vidal-Figueroa and
Delibes 1987). These escapees possibly originated
from fur farms in Galicia or/and from the single mink
farm in Portugal, located near River Minho. Since
then, there were only sporadic references to mink
presence in Portugal, mainly in the rivers Minho,
Coura, and Lima, in the northwest of the country
(Santos-Reis and Petrucci-Fonseca 1999). Compared
to most European countries, the American mink
arrived to Portugal relatively late and range expansion
seems to be slow, in marked contrast to other countries
where mink became nationally widespread within
30–40 years (Bonesi and Palazo
´
n 2007). Yet, neither
the country-wide distribution is known nor the impact
on the native fauna has been studied so far. Due to the
uniqueness of the Portuguese situation, with minks
constrained to the northwestern corner of the country
(Spain on the north, the Atlantic Ocean on the west,
and a high mountain system on the East), this region
can function as the ideal case-study to explore the
spreading process of the mink and test control
measures.
The aim of this paper is to report the current
distribution and status of the American mink in NW
Portugal, contribute to the identification of potential
factors promoting the spread and rate of invasion, and
call the attention of the competent authorities to the
need for an action plan.
Materials and methods
Study area
The area under investigation corresponds to the
northwest corner of Portugal (districts of Viana do
D. C. Rodrigues et al.
123
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Castelo, Braga, and Porto) bounded in the north by the
River Minho (representing the frontier with Galicia,
Spain), in the south by the River Douro, in the west by
the Atlantic ocean, and in the east by the mountain
systems of Peneda-Soajo, Gere
ˆ
s, and Alva
˜
o-Mara
˜
o,
totalling an area of 7,323 km
2
. It includes 8 roughly
parallel main rivers running over varying extensions.
From north to south: Minho (79 km), Lima (67 km),
Neiva (45 km), Ca
´
vado (122 km), Ave (91 km), Lec¸a
(47 km), Ferreira (43 km), and Sousa (65 km).
As part of the Atlantic biogeographical region, the
area is characterized by a temperate Atlantic climate
with Mediterranean influences in the southeast. Riv-
erbanks are mainly covered by mixed deciduous
forests composed by English oaks (Quercus robur),
common alders (Alnus glutinosa), willows (Salix sp.),
poplars (Populus sp.), and narrow-leafed ashes (Frax-
inus angustifolia); plantations of maritime pine (Pinus
pinaster) and blue gum (Eucalyptus globulus). Hea-
thers (Erica sp.), gorses (Ulex sp.), royal ferns
(Osmunda regalis), brambles (Rubus sp.), or the
invasive acacia (Acacia sp.) compose the understory.
Otter, polecat, spotted genet, common weasel (Mus-
tela nivalis), stone marten (Martes foina), pine marten
(Martes martes), stoat (Mustela erminea), Eurasian
badger (Meles meles), and red fox (Vulpes vulpes)
share the riparian habitats with American mink
(Santos-Reis and Petrucci-Fonseca 1999). The impor-
tance of the flora and fauna of this region is reflected
in the allocation of protection status to some areas,
namely Special Areas of Conservation (e.g. mountain
ranges of Peneda-Gere
ˆ
s and River Lima) and Special
Protection Areas (e.g. estuaries of rivers Minho and
Coura).
Methods
Starting at River Minho, where mink was first spotted
in Portugal, we used the GCT mink raft as tracking
device (Reynolds et al. 2004) to sample all the eight
main rivers southwards (Minho, Lima, Neiva, Ca
´
vado,
Ave, Lec¸a, Ferreira, and Sousa) from November 2008
to April 2009. As no mink were detected in the last
four rivers, we resampled them from November 2010
to April 2011 to verify the expansion of the species
range. Late spring and summer were not sampled to
avoid the kit-rearing and juvenile dispersal seasons
(Dunstone 1993), since female mink reduce their
movements during the kit-rearing season (Birks 1981),
having lower chances to be discovered, and juvenile
mink tracks are more difficult to identify and distin-
guish from similar species (e.g., polecats). The GCT
mink raft is a floating track-recording device that
employs a wet tracking medium to record footprints
and is also used by mink as a marking spot. They are
simple, inexpensive, and proved efficient in several
studies, detecting mink presence at significantly more
sites than other established mammal survey tech-
niques like trapping, local knowledge, or systematic
searches for field signs (Reynolds et al. 2004,
Harrington et al. 2008). Rafts were placed every
5 km along main river courses, the same distance used
for site intervals in established sign surveys for the
detection of riparian mustelids (Chapman and Chap-
man 1982). Exceptions were highly rugged terrains or
urban areas. Rafts were checked weekly for footprints
for a period of at least 5 weeks (each check corre-
sponding to a trap-week). We calculated raft visiting
frequency as the percentage of trap-weeks positive for
mink visits within the total number of trap-weeks the
rafts stayed assembled in the field.
To increase the probability of mink detection, we also
conducted scat surveys along the riverbanks at each raft
site (100 m), focusing on mink and its main native
competitors: otter and polecat. Additionally, with the
exception of the Minho River, where the species
presence was well known upfront, we conducted scat
surveys at the banks of the tributaries along transects
with varying lengths (100–600 m) depending on the
vegetation density (Chapman and Chapman 1982). The
distribution of these additional transects coincides with
the sampling sites of an American red swamp crayfish
trapping survey (Moreira et al. (in press)). These surveys
were done between July 2010 and April 2012.
We also mapped mink sightings made either by
team members or others, and confirmed through the
analysis of photographic proof by team members. We
included Vidal-Figueroa and Delibes’ (1987) first
reported mink sighting in our final mink distribution
map.
Given that polecats are known to also enter rafts,
and mink and polecat tracks are very similar in shape
and size, we used Harrington’s classification algo-
rithm to identify the tracks (Harrington et al. 2008).
This is a discriminant function based on three mea-
surements of a track with a classification success
greater than 90 %. Each track was photographed
Tracking the expansion of the American mink
123
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several times and the best-quality picture of each track
was chosen for measurement using the software
Animal Track (Gruber et al. 2007). To calibrate the
algorithm for individuals of our study area, we
measured tracks from 12 farmed mink and 10 polecats
held at a local small nature reserve, the Gaia Biolog-
ical Park. Because a few mink tracks were classified as
polecats according to the suggested identification
threshold of 0 (polecat \ 0 [ mink), we decided to
be more conservative and to shift the threshold
(polecat \-1; 1 [ mink); if the function resulted
between -1 and 1, track identification was considered
inconclusive. This more conservative threshold hold
for 86 % of the captive animals’ tracks.
As the study region is carnivore-rich, some species
being similar to mink in terms of morphology and
body size (e.g. polecat, stoat) and/or using the same
resources such as riparian refuges and prey (e.g. otters,
genets), genetic identification of scats was mandatory
for species assignment instead of solely relying on scat
morphology and scent (Hansen and Jacobsen 1999;
Harrington et al. 2010). Field-collected samples were
immediately stored in 98 % ethanol and frozen
at -20 °C. DNA was extracted with PSP Spin Stool
Kit (Invitek, Berlin, Germany), following the manu-
facturer’s instructions. We used a novel set of species-
specific mitochondrial DNA markers suitable for
Iberian carnivores that can be used for low-quantity
and degraded DNA samples identification (Fernandes
et al. 2008a). We started by screening all DNA extracts
via Polymerase Chain Reaction (PCR) amplifications
with otter (LlutraF2/Llutra R2) and American mink
(MvisonF1/Mvison R1) specific primers, as we
expected the majority of samples to be from one of
these two species given the initial identifications made
in the field by team members. Amplifications were
prepared in 10 ll reactions with 1 9 HOT FIREPol
Buffer B1, 2 mM MgCl2, 0.2 mM of each dNTP,
0.25 lM of each primer, 0.6 U of HOT FIREPol DNA
Polymerase (Solis BioDyne, Tartu, Estonia), and 4 ll
of DNA extract as template. Thermocycling parame-
ters consisted of an initial denaturation at 95 °C for
15 min, followed by 50 cycles of 30 s at 94 °C, 45 s at
54 °C (mink primers) or 55 °C (otter primers), and
45 s at 72 °C. The final extension was 7 min at 72 °C.
Amplification products were run on 2 % agarose/TBE
gels stained with RedSafe Nucleic Acid staining
solution (iNtRon Biotechnology). Scats were assigned
to a species whenever amplification products showed a
single band of the expected size (Fernandes et al.
2008a). Scats that remained non-identified after this
analysis were tested with the following most probable
species (based on expert knowledge) until species
identification was achieved. We used species-specific
mitochondrial DNA markers of Fernandes et al.
(2008a) for polecat (M. putorius F2/M. putorius R2),
fox (V. vulpes F2/
V. vulpes R2), genet (G. genetta
F2/G. genetta R2), weasel (M. nivalis F1/M. nivalis
R1), and stone marten (M. foina F2b/M. foina R2b) to
amplify the remaining samples.
Results
In the first sampling season (November 2008–April
2009), mink footprints were found in 29 (55 %) of the
installed tracking rafts (N = 53). Detection occurred
in 84 out of 323 trap-weeks (26 %). Mink was present
in the four most northern rivers (Minho, Lima, Neiva,
and Ca
´
vado) but could not be detected in the
southernmost ones (Ave, Lec¸a, Ferreira, and Sousa).
In the second sampling season (November 2010–April
2011), 22 rafts were installed in these four southern
rivers for 165 trap-weeks, and we were able to find
footprints in 5 rafts (23 %) during 11 trap-weeks
(6.7 %; Fig. 1). In 82 % of the confirmed presence
sites, the species was detected in the first 3 weeks of
sampling and 94 % within four sampling weeks.
The averaged raft visiting frequency was higher in
the northern rivers (Minho—51 %; Lima—31 %;
Neiva—27 %; Ca
´
vado—40 %), whereas rafts in riv-
ers Ave (10 %) and Sousa (2 %) were scarcely visited
(Fig. 2). Some sites in the northern rivers were visited
more than 75 % of the monitored weeks (three sites at
Minho and two sites at Ca
´
vado; Fig. 2).
A total of 434 carnivore scats were collected on
transects established along the banks of the main rivers
(N transects = 55) and those of several tributaries
(N transects = 100), between July 2010 and April
2012. Out of these scats, 325 were genetically tested
and 271 (83 %) were successfully assigned to species:
239 (88.2 %) as otter, 20 (7.4 %) as mink, and 12
(4.4 %) as other carnivore species, namely fox, genet,
weasel, and stone marten. No polecat scats were found
during surveys. Of the remaining scats, 54 (17 %)
failed to amplify with any of the species-specific
primers. South of River Minho, mink scats were found
in the main rivers Lima, Ca
´
vado, Ave, and Sousa and
D. C. Rodrigues et al.
123
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tributaries of River Lima (streams Vade and Estora
˜
os;
lagoons of Sa
˜
o Pedro d’Arcos and Mimoso), Ca
´
vado
(streams Febros, Lamas, and Tojal), and Ave (stream
Po
´
voa) basins (Fig. 1).
Tracing back to 1985 (Vidal-Figueroa and Delibes
1987), we further assembled 22 mink observation
events in the study area but most (86 %) occurred
since 2007. The majority concern single observations
(68 %) but in seven occasions more than one individ-
ual were seen together. Sightings were concentrated
on the northern basins (Fig. 1): Minho and tributaries
(River Mouro and streams Cerdal and Ara
˜
o; 23 %),
Lima and tributaries (rivers Lourente and Laboreiro;
lagoons of Sa
˜
o Pedro d’Arcos, Mimoso, and Vila
Franca; 45 %), and Ca
´
vado and tributaries (River
Homem; 27 %). Only one sighting was registered
further south, on a tributary of River Ave in 2013.
Discussion
This paper reports the results of the first research
project focusing on American mink in Portugal. Our
findings indicate a clear spread of this invasive species
towards the south with the invader already occupying
the majority of the hydrographic basins of NW
Portugal, a vast extension in a biodiversity-rich region.
Whereas mink spread is confirmed on the basis of all
the information gathered (sightings, genetically iden-
tified scats and footprints), doubts still subsist about
how fast the spreading is occurring and where the
southern limit of the species range lies today. Absences
registered may not correspond to true absences but to
an extremely low population density that limits species
detectability. Based on the result that 82 % of the
mink’s track records could be detected after 3 sampling
Fig. 1 American mink current distribution in NW Portugal (the
study area within the Iberian Peninsula) according to different
types of information: sightings (black squares), genetically
identified scats (black rhombus), and footprints in track-
recording rafts (black circles). White circles represent rafts
without mink footprints. Bold grey lines are main rivers,
whereas soft lines are tributary rivers. Grey areas are under
environmental protection
Tracking the expansion of the American mink
123
Author's personal copy

weeks and 94 % after 4 sampling weeks, we assumed
that 5 sampling weeks are long enough to indicate if a
sampled area was inhabited by the mink. Furthermore,
since almost all rafts placed in rivers with mink
presence were visited by mink (except those close to
the estuaries), we assume that the four southern rivers
without any mink signs in the first sampling season
(2008–2009) were either not inhabited by mink or that
the species was present in low densities. Consequently,
we assume that the species density either increased
between our first and second (2010–2011) sampling
seasons or we indeed observed the expansion of the
mink southwards. The latter would imply that since its
presumed introduction in 1985, the mink required
20 years to expand its range from River Minho to River
Ca
´
vado (circa 55 km; mean linear expansion rate of
2.75 km per year).
Compared to other European countries like Belarus,
Finland, and the UK, where mink became nationally
widespread within 30–40 years (Kauhala 1996; Sid-
orovich 1997a; Jefferies 2003), the invasion progress
in Portugal seems to be rather slow. Possible contrib-
uting factors for this might be the fast flow and low fish
productivity of the NW rivers (Giller and Malmqvist
1999), the high number of competitors in the area
(Sidorovich 1997b), as well as the high abundance of
local otter populations. Also, this could correspond to
a lag phase, frequent in the establishment of invasive
species after introduction, when the species remains
localized and difficult to detect (Crooks and Soule
´
1999). A comparable slow mink expansion rate was
only recently reported by Fraser et al. (in press) in
north-eastern Scotland, with a mean linear expansion
rate of 2.6 km per year.
Fig. 2 Visiting frequency of track-recording rafts by American
mink in NW Portugal. White circles represent rafts without mink
footprints. Positive raft sites are coloured in a scale of grey to
black according to mink visiting frequency to rafts (4 classes:
1–25, 26–50, 51–75, 76–100 %). Bold grey lines are main
rivers, whereas soft lines are tributary rivers. Grey areas are
under environmental protection
D. C. Rodrigues et al.
123
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Portugal is known to be an otter stronghold (Conroy
and Chanin 2003). When mink first arrived, otters
were widespread and presumed to be abundant all over
the country (Santos-Reis and Petrucci-Fonseca 1999,
Conroy and Chanin 2003). Currently, the local otter
population continues to thrive in the study area (88 %
of scats genetically identified were from otter). High
otter densities could have acted as a delaying factor in
the progression of the invader. This situation differs
from what was observed in most European countries,
where mink invasion occurred in a time period when
otter populations were declining and becoming endan-
gered such as in Belarus, Finland, Poland, Spain,
Sweden, and UK (Conroy and Chanin 2003). A similar
situation may occur in Ireland, another otter strong-
hold (Chapman and Chapman 1982), where mink is
considered widespread but not abundant due to the
healthy local otter population (Bonesi and Palazo
´
n
2007).
After a first phase of slow expansion, our observa-
tions indicate that the mink expanded its range quite
rapidly from River Ca
´
vado to River Sousa, the right-
bank tributary of Douro (circa 45 km), in only 2 years
(from the first to the second sampling season) or at
least increased its density remarkably. Numerous
factors can act as triggers to increase population
growth rates and range expansion, notably natural or
human-induced disturbances (Crooks and Soule
´
1999). The relatively recent expansion and establish-
ment of the American red swamp crayfish (Procamb-
arus clarkii) in the area is the most plausible
explanation for the acceleration in the mink’s expan-
sion. Red swamp crayfish has only been recorded in
the region after the year 2000 (Moreira et al. (in press))
and is spreading northwards, in an opposite direction
to mink. In the meantime, it became the mink’s main
prey (Gonc¸alves 2012). Being a key food resource
([50 % occurrences; Gonc¸alves 2012), crayfish may
now act as an expansion facilitator by increasing
habitat carrying capacity for mink (as well as for otters
and other predators), as was observed in many other
Mediterranean areas (Tablado et al. 2010; Melero et al.
2014). Its facilitator role may be expected to further
increase under the current climate change scenario
given that crayfish distribution is primarily driven by
propagule pressure and climatic suitability (Capinha
et al. 2013).
Range expansion and potentially increasing mink
abundance might also affect the abundance or
distribution of polecats, a species which is scarce in
Western Europe and shows a decreasing population
trend (Fernandes et al. 2008b). In Portugal, the polecat
is considered a Data Deficient species but it is believed
to be closely connected to humid habitats and dense
vegetation cover (Mestre et al. 2007) and declining at
an unknown rate (Cabral et al. 2005). Our results
support the presumed fragile situation of this native
predator considering that no polecat signs were found
during the entire 3-year study period. In contrast to
otters, polecats and mink have more similar sizes with
the latter showing a high body robustness that will be
beneficial in direct confrontation (Sidorovich et al.
1999). Moreover, whereas otters and mink share
common food resources, they also exhibit niche
partitioning in terms of the main prey (fish for otters
but small mammals and waterfowl for mink—Bonesi
et al. 2004); this also applies to our study area where in
spite of crayfish being the dominant prey ([50 % of
occurrences for both species), small mammals and
waterfowl corresponded to 26 % of mink’s diet and
fish to 37 % of otter’s diet (Gonc¸alves 2012). The
mink’s dependence upon the terrestrial environment
allows for a terrestrial prey shift and avoidance of
otters, putting them into greater contact and poten-
tially conflict with polecats that prefer more terrestrial
habitats (Harrington and Macdonald 2008). Even if
studies in the UK (Harrington and Macdonald 2008)
reported only a partial niche overlap for mink and
polecats, other authors (Sidorovich and Macdonald
2001; Melero et al. 2012) reported significant negative
effects of mink on polecat abundance and/or distribu-
tion. Since Melero et al. (2012) studied both species on
the Iberian Peninsula, we believe that this study is
likely to be more comparable with our case than
studies in the UK or Belarus. Hence, polecats in our
study area might be at risk to further decline due to
mink.
American mink sightings in rivers (Mouro and
Laboreiro) inside the largest and most important
protected area in the region, Peneda-Gere
ˆ
s National
Park, are especially worrying. Besides presenting
considerable reptile and amphibian diversity, the park
is one of the strongholds of the endemic Iberian
desman (Galemys pyrenaicus), suggested as vulnera-
ble to mink predation, although scientific evidences
are still lacking (Nores et al. 2007). Mink has also been
detected in wetlands of local or international impor-
tance for water birds, such as the Litoral Norte
Tracking the expansion of the American mink
123
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Protected Park, Lagoons of Bertiandos and S. Pedro
d’Arcos Protected Area, and the Special Protection
Area of Minho and Coura estuaries, with the possible
negative consequences being documented elsewhere
(Clode and Macdonald 2002).
Besides the probable Galicia-originated founders of
the American mink population in NW Portugal (Vidal-
Figueroa and Delibes 1987), there are other close-by
Spanish regions that may contribute to further intro-
duction events. The Douro basin, which holds the most
recent mink signs in Portugal, flows from its source in
the Spanish province of Soria across the northern-
central Iberian Peninsula until its river mouth in the
Atlantic Ocean, near the Portuguese city of Oporto.
This river basin is considered the natural boundary
between northern and central Portugal. American mink
are known to be well established in the Douro basin in
the Spanish provinces of Valladolid, Segovia, A
´
vila,
and Salamanca, the latter bordering northeastern
Portugal, with one of the largest populations of Spanish
feral mink, comparable to Galicia and Catalonia
(Bravo 2007). To support this hypothesis of multi-
invasion events, three mink observations were recently
reported outside of our study area, in Tra
´
s-os-Montes
(NE Portugal): one in the Douro River, in the
northeastern border with the Spanish province Salam-
anca, and the other two in different Douro tributaries,
Rivers Tua and Ta
ˆ
mega (Fig. 3). This new data
suggests another invasion route, with individuals most
probably originating from central Spanish farms.
Hence, monitoring at a larger geographic scale is
urgently needed to better understand the origin and
direction of new routes of invasion.
This novel information gathered in the frame of this
study represents a major step forward for a potential
control/eradication plan of the invasive species in NW
Portugal. In view of the reported findings, if no action is
taken, it is plausible to assume that the American mink
Fig. 3 American mink sightings in northern Portugal (black squares). Bold grey lines are main rivers, whereas soft lines are tributary
rivers. Grey areas represent the region’s most important areas under environmental protection
D. C. Rodrigues et al.
123
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will continue to expand its range south and eastwards in
Portugal. Eradication is probably no longer feasible.
Although the number of mink farms in Spain was
dramatically reduced at the end of the 1980’s, due to
saturation and decline of the fur market, well estab-
lished populations near the border probably continue to
function as sources for the Portuguese mink popula-
tion. Control appears therefore as the most appropriate
management measure, since the best opportunity to
control an invasive is typically during the lag phase or
early spread before it occupies a large area or achieves
high densities (Crooks and Soule
´
1999). As we report
that the mink is already expanding its range and/or
increasing its density, a control program should start
immediately in the NW region, preferably in conjunc-
tion with Spanish authorities as this neighboring
country seems to be acting as a mink source.
Acknowledgments We would like to thank the Portuguese
‘‘ F u n da c¸a
˜
oparaaCie
ˆ
ncia e Tecnologia’’ for funding Project
DILEMA—‘‘Alien species and conservation dilemmas: the effects
of native competitors and alien prey species on the spread of the
American mink in Portugal’’ (PTDC/BIA-BEC/102433/2008).
Furthermore, we thank the German Academic Exchange Service
(DAAD; D/07/49405) and the Helmholtz Interdisciplinary
Graduate School for Environmental Research (HIGRADE) for
funding Simone Lampa. We are very grateful to Francisco Moreira,
Julien Goebel, Rita Duarte, Sofia Gonc¸alves, Carlo Rusponi, Laura
Kuipers, Mafalda Basto, Teresa Sales-Luı
´
s, Ana Catarina Silva and
Ce
´
line Madeira for assisting in the field and laboratory work. We
also thank Nuno Pedroso, Helena Rio Maior, Renato Fernandes,
Joa
˜
o Branco, Associac¸a
˜
o Guarda-Rios do Lima, Carlos Rio, Ana
Carvalho, Patrı
´
cia and Jose
´
Luı
´
s Sequeira for providing data on
American mink sightings. We are grateful to the staff of the Gaia
Biological Park for allowing us to test the tracking device with
captive mink and polecats and that of the Lagoons of Bertiandos and
S. Pedro d’Arcos Protected Area for logistic support during
fieldwork. Finally, we are most grateful to Lauren Harrington and
another anonymous reviewer for providing valuable inputs that
have greatly contributed to improve this paper.
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