Estimating the relative abundance of American mink Mustela vison on lowland rivers: evaluation and comparison of two techniques

1. What they did

American mink are an introduced species in the UK, linked to the decline of native animals such as water voles. To check how mink are spreading, or whether control is working, managers need reliable ways to track their numbers. Mink are scarce, active at night and hard to see, so surveys usually rely on their signs.

The authors compared two ways of measuring how many mink are present (relative abundance) on six 20 km stretches of river in the Upper Thames Valley, England:

• Scat surveys. Observers walked both banks of each river, searching up to 5 m from the water’s edge at about 1 km an hour, and recorded mink scats by size, shape and smell, with a Global Positioning System (GPS) location for each one. They counted scats only, not footprints, because footprints on riverbanks depend on the mud and sand available. The rivers were then divided into 500 m sections, and the index was the percentage of sections with at least one mink scat.
• Tracking rafts. Floating wooden rafts with a tunnel over a moist clay plate were placed about every 1 km. The clay was smoothed at the start of each survey and checked for mink footprints two weeks later. The index was the percentage of rafts with mink tracks.

Surveys took place in March, July, September and November, matching the mink’s mating season, kit-rearing season, young mink dispersing, and the stable winter period. In total there were 37 surveys between September 2003 and March 2006.

To check scat identification, 61 scats chosen at random were tested with DNA. To compare both indices with actual mink numbers, they live-trapped mink on the rafts on two rivers over four survey periods, marking each with a microchip, and used the numbers of mink removed by control trapping on three other stretches. They also compared the costs of the two methods.

2. What they found

Identifying scats

• All 61 scats tested with DNA were American mink.
• A separate diet study of 198 of the same scats found that about 6% were probably not mink: nine contained polecat hair groomed off the animal, and three held mostly fruit.

Scats versus rafts

• They recorded 512 mink scats over 37 surveys. One survey (River Cherwell, July 2004) had an unusually high number of scats, when otters were very scarce there, and was left out of the analyses.
• Rafts almost always detected mink in a higher share of places than scats did: a median of 73.7% of rafts had tracks, against 18.9% of river sections with scats.
• The two indices were linked (correlation r = 0.373), but 81% of the variation between them was unexplained. Season improved the fit a little; observer, year, recent weather and site did not.
• Neither index showed a clear seasonal pattern, and seasonal differences were not statistically significant.

Comparison with mink numbers

• Live trapping caught 19 mink (10 males, 9 females) on the Thames and 22 (12 males, 10 females) on the Cherwell. About 60% were caught in more than one session.
• Males used up to five rafts; females usually one or two, at most three.
• Estimated density was about 0.26 to 0.52 mink per km on the Thames and 0.5 to 0.83 per km on the Cherwell. Removal trapping took 12, one and one mink from the three control stretches.
• The percentage of rafts with tracks was closely related to the number of mink caught (r = 0.858). The percentage of sections with scats was only weakly related, and not significantly (r = 0.463).
• At two control sites with very few mink, no rafts had tracks, but scats were found in one and five sections. These may have come from mink that had passed through earlier and left.
• Polecats seemed to use the rafts little: only two polecats and one feral ferret were caught, all in one session on the Thames.

Costs

• Each scat survey of a 20 km stretch took five days. A raft survey took two days (one to smooth the clay plates, one to check them), once the rafts were in place.
• Setting up 20 rafts cost about £856, against about £100 for a handheld GPS for scat surveys. After that, a raft survey cost about £106 and a scat survey about £250.
• By six surveys the totals were similar (£1,492 for rafts and £1,600 for scats), and rafts become cheaper the longer they are used, allowing about £200 a year for repairs.

3. What they concluded

• Mink scats can be identified reliably in the field without costly DNA tests, though this may depend on how many polecats live in the area.
• Rafts consistently performed better than scat surveys. The number of rafts with tracks matched the number of mink using them over a range of densities and seasons.
• Raft surveys cost about the same as scat surveys and may be cheaper in the long term.
• Unlike earlier studies, they found no clear seasonal pattern in either index.
• Because the rafts were also used for live trapping, the raft index and the trapping numbers were not fully independent, so a close link between them is expected. What the study shows is that the link held across densities and seasons.
• Rafts carry a risk of confusing the tracks of mink with those of polecats. The authors recommend further work on telling the two apart, especially as polecats are spreading in the UK.

4. How tracking was or was not used

The study compared two tracking methods. In scat surveys, observers searched riverbanks and identified mink scats by size, shape and smell; the accuracy of this identification was checked with DNA and a diet study. On the tracking rafts, mink footprints left in clay were recorded as present or absent. The paper notes that mink and polecat tracks are similar in size, that qualitative differences have been described, but that there is no quantitative way yet to tell them apart reliably. Footprints found naturally on riverbanks were deliberately left out of the scat surveys, although the authors note many were found in March.

5. Tracker or observer certification

Not reported. Several fieldworkers, including volunteers, carried out the scat surveys, and nothing is said about their training or experience. Their identification of mink scats was, however, checked: all 61 scats tested by DNA were mink, and a diet study suggested an error rate of about 6%. The effect of observer on the scat index was not statistically significant. Nothing is said about who identified the tracks on the rafts.

6. Other technologies, skills or innovations

• Floating tracking rafts with a clay plate kept moist by a wicking layer drawing up river water, also used as trap sites.
• DNA analysis of scats (mitochondrial cytochrome b gene) to confirm species.
• Handheld GPS for scat locations, and ArcGIS to divide rivers into 500 m sections.
• Live trapping with otter guards, anaesthesia, and microchip (PIT tag) marking.

7. Implications for conservation (from the paper)

• Reliable ways to detect mink and track their numbers are needed to follow their spread and judge whether control works.
• Tracking rafts give a consistent index of mink numbers and cost about the same as, or less than, scat surveys.
• Raft trapping seemed to catch females well, so it may give a steadier picture of the resident population than trapping methods biased towards wandering males in the mating season.
• As polecats spread in the UK, confusing their tracks and scats with mink will become more important.