Clever mink control

Key findings

• Using the GWCT Mink Raft to direct trapping, we were able to achieve a substantial impact on mink in a short time and on a scale of up to 45 kilometres of river.
• During the same period, water vole evidence spread dramatically along the river from nucleus populations.

In 2002 we invented and trialed the GWCT Mink Raft, a low-technology device intended to underpin a new approach to mink control (see our Review of 2002 pages 42-46). The raft proved a sensitive way to detect mink, and therefore promised substantial benefits by directing trapping effort solely at locations with mink, rather than wasting time on locations without them.

The next step, clearly, was to use our raft to guide a trapping campaign. Thanks to support from the Environment Agency, we were able to do this on two different rivers in 2003. We tackled these sequentially, so that lessons learnt on one could be applied to the other.

On the lower River Itchen in Hampshire (between Winchester and Eastleigh), field surveys by the Hampshire Wildlife Trust had shown a 13% decrease between 1996 and 2002 in the number of sample sites with water voles present. This was accompanied by an increase in mink signs from 'absent throughout' in 1996 to 'present throughout' in 2000. Conservation bodies in the area decided that urgent action was needed to safeguard the water voles.

We aimed to eliminate mink from this 12-kilometre section of river for the period April to July, and hoped to demonstrate that by trapping we had removed all detectable mink. Although the raft as the basis of a trapping strategy was on trial, in large measure we were also seeking to develop the best strategy for its use. So we deliberately 'over-gunned' our approach on the Itchen, knowing that from a successful outcome we could define more efficient operating rules for the future.

In total, we deployed 104 rafts in late March/April. This high density (about 10 rafts per kilometre of river corridor) ensured that around six rafts would be available to any female mink, which typically occupy a breeding territory about 1.5 kilometres long. Although pilot work had suggested that raft positioning was not critical, we wanted to leave no doubt that a mink would find at least one raft. Rafts can operate in two modes: as monitors to detect the presence of mink, and as trap sites. In this study, rafts in monitoring mode were checked for footprints at weekly intervals. Once we detected mink on a raft, a cage trap was set within the raft tunnel and we checked it daily until it caught a mink, or for up to four weeks. All of these arrangements were deliberately fail-safe choices. Rafts reverted to monitoring mode after each trapping session.

In conventional trapping, a proportion of trap sites will be redundant because there are no mink active there. For instance, on the upper Avon and its tributaries in 2002, with raft sites placed at a much lower density of one per four square kilometres (400 hectares), 70% showed no mink activity throughout the summer. A second form of redundancy arises where a mink has access to several traps, because it can get caught in only one of them. So after a capture, the dilemma is whether to close down nearby traps, or to keep them running. The dilemma is heightened through not knowing which traps were accessible to the mink already caught, nor whether other mink have access to the same traps. Using rafts, this is resolved by reverting to monitoring mode. If another mink is present it will leave tracks and can be trapped in turn. Thus, mink signs on rafts become both the guiding hand and proof of effectiveness for the trapping campaign.

On the Itchen, the proportion of rafts showing mink signs quickly collapsed to near-zero on the removal of just a few mink (see Figure 1). In fact, the catch at this point was so low that we seriously wondered whether we were missing any. Accordingly, we shifted all rafts on the main river channel to fresh, intermediate positions. Further 'blips' of mink activity were met with the removal of three more mink. Including two mink killed shortly before we started, a total of seven females and three males were killed on the river. A hypothetical breeding territory drawn around each female capture convincingly explains most of the raft evidence and neatly fills up the river corridor, suggesting that we probably did account for most or all breeding females.

During the same period, water vole evidence (also, fortuitously, recorded on rafts) spread dramatically (see Figure 1).

Figure 1. Proportion of rafts showing evidence of mink and water voles for 17 weeks during the spring and summer on the River Itchen

	Mink evidence
	Water vole evidence
	Mink captures

Geographically, water voles spread from isolated nucleus colonies in mid-May, to occupy almost the entire river channel by late July (see Figure 2). An impressive capacity for population growth and spread to take advantage of temporary summer habitats is characteristic of water voles, but sadly it hasn't been evident in recent decades. Its occurrence here encourages our belief that we had a substantial impact on mink numbers. Certainly we prevented the birth of 49 young mink: culled females carried from five to 10 embryos (see Figure 3). 

Figure 2. The spread of water voles on the River Itchen from mid-May to the end of July

	Rafts with evidence of water voles
	Rafts with no evidence of water voles

Importantly, the mink captures were accompanied by only 10 live captures of non-target species (six water voles, grey squirrel, brown rat, moorhen, mallard). Several of these were re-captures. All but one occurred in traps rendered redundant by nearby mink captures, and could therefore have been avoided by reduced trap deployment times. In conservation terms, the occasional detainment of these animals in live-catch traps is trivial compared with the benefits of controlling mink.

Figure 3. Mink breed only once a year, but can be very productive.This female carried 10 foetuses (each denoted by an arrow).The smallest pregnancy we found was five foetuses.

The Itchen experience allowed us to refine our raft- and trap-operating rules for the next stage. This time, we repeated the exercise on a larger scale beginning in autumn. The River Wylye, one of the Avon tributaries, is 45 kilometres long and, in summer 2002, we had found mink throughout its length. Autumn implied greater challenges than spring because mink numbers are at their annual peak in autumn following breeding, and because dispersal could mean that a mink leaving tracks on a raft could have moved elsewhere before a trap was set.

Our refined operating strategy for the Wylye was to use one raft per kilometre of river, to check at two-week intervals during the monitoring phase and to run traps for a maximum of 10 days each before reverting to monitoring mode. The distribution of mink was somewhat changed from that found in 2002, with no tracks at 40% of raft sites. We caught a total of seven mink (six females and one male) and - just as on the Itchen - mink evidence on rafts declined rapidly in response (see Figure 4).

Figure 4. Proportion of rafts showing evidence of mink for 20 weeks during the autumn and winter on the River Wylye.

	Mink evidence
	Mink captures

The Itchen and Wylye projects have shown that, using rafts to direct trapping effort, a substantial impact on mink numbers was possible in a short time, irrespective of time of year. As far as we could tell, we eradicated mink from the target areas for the duration of each project, though given that trap-shyness arises in so many pest control issues, it would be incautious to assume this. Other anticipated benefits of the raft system have been borne out in practice: substantially reduced non-target captures (and hence improved animal welfare), good cost-efficiency, and the ability to monitor water vole presence.

Although demonstrably effective trapping on this kind of geographical scale is unprecedented, we anticipate that recolonisation will be quite rapid. Thus the challenges are no longer so much 'how to do it', but logistic problems of how to manage a progressive control programme that gradually creates a wider and longer-lasting mink-free zone, and how to fund the inescapable cost.

These research projects were jointly funded by The Game & Wildlife Conservtion Trust, the Environment Agency and Hampshire Wildlife Trust.