Evaluation of sign surveys and trappability of American mink: management consequences

1. What they did

The American mink is an invasive species in Europe, and where they have spread they overlap with the endangered European mink. Several projects aim to remove American minks, so managers need to know how many there are and whether trapping works. The authors asked two questions: are sign surveys and trapping reliable ways to estimate how abundant American minks are, and how well does trapping work as a way of removing them?

The study ran from November 2004 to April 2005 along 20 km of the Butron river system in northern Spain, where sandy riverbanks hold clear tracks.

• Sign surveys. The main river was divided into separate 500 m sections. Two trained surveyors walked each section, one upstream and one downstream, looking for mink tracks on the sand within one or two metres of the water. Sections were surveyed in November, January and April. Because the tracks of American and European minks cannot be told apart, any track was recorded as “mink presence”.
• Trapping. In each season, 37 to 40 live-traps baited with sardines were set every 100 m in every surveyed section, whatever the survey had found. Traps stayed open for four days and were checked each morning. The team also read the sand around each trap to see whether minks had visited without being caught.
• Radio-tracking. Trapped minks were fitted with radio collars and located twice a week, mostly by day, to see how males and females used the river and its side streams. The team also searched the areas used by tagged minks for tracks and scats of other minks.

2. What they found

Tracks versus trapping

• In autumn, mink tracks were found in 17 of 24 stretches. Minks were caught in 6 of the stretches with tracks and in 1 without.
• In January, tracks were found in 11 of 18 stretches. Minks were caught in 1 stretch with tracks and in 3 without.
• Over all three seasons, minks were caught in a similar share of stretches with tracks and without tracks. There was no significant link between finding tracks and catching a mink in autumn, in January, or over all seasons together.

Trapping success

• Autumn: 7 American and 2 European minks (2.3 and 0.66 per 100 trap-nights). Tracks around the cages showed minks had visited traps without being caught at 9 other sites.
• January: 5 more American minks (3.38 per 100 trap-nights), and 8 more sites where minks visited traps without entering.
• April: the team tried to catch 14 known minks (5 tagged and 9 known only from their tracks) and caught just 1.
• Over the whole study, 13 minks triggered traps without being caught. One tagged mink visited 11 traps and set off 6 of them without being caught.

How males and females used space

• Male American minks used stretches of stream averaging 7.09 km and females 4.92 km (leaving out animals followed only briefly).
• Females used small streams much more than males: small streams made up about 60% of a female’s range, against about 17% for males. Males were on main streams for 82.7% of their locations, and females for 40.22%.
• Streams used by females were narrower and shallower, and females stayed farther from the main stream (523 m on average, against 149 m for males).
• Females often stayed for more than three weeks in small side streams under dense brambles, often less than 1 m wide and 15 cm deep. Tracks there were very hard to search for, so these minks went unnoticed in sign surveys.

3. What they concluded

• Neither sign surveys nor trapping is a reliable way to estimate the relative abundance of American minks, because both have strong biases. Minks were present in places where the sign surveys found no tracks.
• Trapping is likely to underestimate numbers, even at high density, because some individuals avoid traps, and the chance of catching a mink changes with the season.
• The catch rate (one capture for every 29.6 to 43.5 trap-nights) was close to that of a high-density area in the United Kingdom, so the Butron catchment may have reached its carrying capacity for American minks.
• The sex ratio was 1:1, but males and females were not equally likely to be trapped. Because females live largely in small side streams, a large part of the population, mainly females, can go unnoticed.
• Removal projects that trap only along main streams are bound to fail. They must also deal with minks that never enter traps, females in very small streams, and a floating population away from the rivers, which could quickly recolonize.

4. How tracking was or was not used

Tracking was central to the study in three ways:

• Sign surveys: two surveyors walked river sections and recorded mink tracks on sandy banks, as an index of mink presence.
• Reading trap sites: tracks in the sand around each cage showed which traps minks had visited or set off without being caught. This is how the authors detected trap-shy minks.
• Finding other minks: in areas used by radio-tagged minks, the team searched for tracks and scats of other minks. In April they tried to trap nine untagged minks known only from their tracks.

The authors note that mink tracks are easy to tell from those of genets, feral cats and foxes, and that with a little experience they can be told from stone marten tracks. American and European mink tracks cannot be told apart. Tracks were hard to find in small streams under dense vegetation.

5. Tracker or observer certification

Reported but not validated. All sign surveys were done by the same two “trained surveyors”, and trapping by a single team “with experience in mink trapping and handling”. No certification or test of their skill is reported.

6. Other technologies, skills or innovations

• Radio collars (TW-4 and TW-5, Biotrack Ltd) weighing about 15 g, under 3% of the body weight of the lightest adult female (520 g).
• Animals located by homing in on the signal, with an accuracy checked by watching active minks. Locations were plotted on high-resolution aerial photographs in a Geographic Information System (GIS).
• Live-traps baited with sardines in vegetable oil.

7. Implications for conservation (from the paper)

• The American mink now occupies almost all of the European mink’s range, with only some free catchments left in southwest Europe, and complete eradication is likely to be impossible.
• The authors recommend: (1) special conservation measures in the best-preserved areas with healthy European mink populations; (2) a big, continuous effort to keep the neighbouring areas free of American minks; and (3) regular trapping in areas occupied by American minks, to reduce their density and slow their spread.
• Surveys and trapping that cover only main streams will miss many minks, especially females, and need extra effort where small streams join the main stream.