FAQs

We currently recommend one raft per kilometre of river. Our research shows that this allows each mink access to at least one raft, and usually two to three rafts. In open water, our best guess would be to follow the same spacing along the water’s edge, including islands. For land intersected by many ditches and channels, we would suggest one raft per square kilometre as a starting point (not including rafts on the main river channel), but more may be necessary depending on the density of ditches.

It’s tempting to leave traps ‘just a little longer’, in the hope of catching the mink that left its footprints. However, experience tells us that a mink will usually be caught within a few days, if at all. If it isn’t caught in that time frame, don’t jump to the conclusion that you have a ‘trap-shy’ individual. It is much more likely that it has been caught elsewhere, died through some other means or that it was a ‘transient’ individual simply passing through. If you keep the trap set you will probably only increase the number of non-targets you catch. Return the rafts to monitoring mode after each capture, or after a maximum of ten days without a capture.

In trapping mode we use a physical excluder modified after Short & Reynolds (2001), to ensure that otters cannot force their way into the trap. This has a central space of 60mm between two upright dowel bars, leaving smaller spaces to the sides (see raft description and diagram).

In a field experiment using paired rafts, with and without excluders (autumn 2002), visits by mink to rafts were shown to be unaffected by this excluder. Otters do visit mink rafts, leaving spraints on the ends and on the tunnel roof to prove it. When excluders are not present (we remove them when the raft is in monitoring mode) some otters do enter the tunnel – which is a tight squeeze, to judge by the deep footprints! But after seven years of research, in all seasons, in places with otters present, we haven’t had any otters pass through tunnels that were protected by this excluder.

Other designs of otter excluder are offered by trap manufacturers, for instance by restricting the trap entrance to a three-inch-square aperture. We now use this kind of trap without the dowel excluders. These too have allowed efficient capture of mink including large males, of which the largest encountered was 1.7kg.

Polecats (a species with Schedule 6 protection under the Wildlife & Countryside Act 1981) are similar in size to mink, so clearly they can enter tunnels even with excluders fitted, as do other non-target species such as moorhens and water voles.

It is illegal to return either mink or grey squirrels to the wild after capture, so these species must be dispatched.

There is no statutory restriction on when to trap mink. Arguably the most important time to monitor and trap is in the spring. The idea is to prevent on-site production of young mink and to safeguard prey species, which are breeding themselves and are therefore vulnerable. In spring mated female mink will be choosing den sites in which to pup, and until their pups are weaned the family will be relatively sedentary. Female mink we have trapped in spring time were carrying up to ten foetuses. A family of this size develops a huge need for food as the summer progresses and the implications for prey species are obvious. If trapping commences as early as mid-February or early March, you are also likely to catch adult males as they roam around searching for females to mate.

As summer progresses you start to run into a welfare dilemma. If you kill adult females that have dependent young, those young will die of starvation and cold (mink dens are difficult to locate). If you want to avoid this scenario, you must suspend trapping until the young are free-ranging, have already eaten a lot of prey and have to be trapped one at a time. There is no easy solution to this dilemma. It arises in most other vertebrate pest control issues, e.g. mice, rat, rabbit, woodpigeon, fox etc. Clearly the ideal is to trap adult females before their young are born. Rafts help to achieve this quickly, but they will also show if any mink are present during the summer, bringing the dilemma to uncomfortable prominence. You need to decide whether you are prepared to compromise effectiveness in the interests of welfare. If you are using rafts on a fairly wide scale, a ruthless campaign in year one that quickly reduces the population to low levels is arguably more humane than a campaign that is drawn out because it has a close season.

Having stressed spring-time control because it has the greatest impact on the mink population, we don’t actually know when mink have their greatest impact on prey species like water vole, crayfish or amphibians. It may be that autumn or winter predation by mink is critical for some of these species and the advice above may need to be revised in due course.

However, because it is not a nationwide eradication campaign, the mink you remove will eventually be replaced through immigration. If you are addressing only a small section of river, and/or you are in a region with a high mink density, this may happen very quickly. The peak time for dispersal is August/September and this is the second key period of the year after spring. Only experience in your location will tell you how much immigration typically takes place, but using the raft system you can clear the river again quickly.

The GWCT Mink Raft was developed on southern England chalk streams, but it has been used successfully on rain-fed rivers in (for example) Cumbria, Aberdeenshire and the Welsh borders. Of course, you must be aware of the dangers of rafts being swept away by sudden spates or flood water, and it may be that in rainy seasons you simply cannot use them. But we have made the system work despite these practical difficulties. It is advisable to rope only to one corner of the raft, allowing it to find quieter water, but also to fix a second ‘back-up’ rope in case the first one fails.

Knowledge of how your river behaves in spate is vital. Experience will show you where the slack water areas are. Try to position the raft so that it is protected by a large tree or other feature, so when the river floods there will often be slack water directly behind (downstream of) the tree and the raft will float quite safely. This will also protect it from heavy debris (tree trunks, etc) floating down stream. The rope should be anchored quite high up allowing maximum rise and fall – too low and the raft may be dragged under water and lost or damaged. When water levels recede it will be necessary to re-float any rafts that have become beached. Experimentation is the key.

We use no bait or scent lure on our rafts. In general, we have no problem in catching a mink within a few days, once we have detected it using a raft (see below for exceptions). Both bait and scent lures introduce complications which we feel are unnecessary. Bait needs to be replaced regularly to ensure it is fresh, and it may increase the interest from non-target species. Scent lures may have different effects on different mink depending on their sex and age class. Please don’t use bait and then ask us why you aren’t catching.

Don’t jump to the conclusion that the mink is ‘trap-shy’. There are several more likely reasons for this situation. Mink tracks found during during late winterearly spring and in autumn, may have been left by ‘transient’ animals moving through the catchment, either to find mates or to disperse. Setting traps in response to these will waste time if the animal responsible for leaving the track has already moved away from the vicinity. The only options here are to shorten the raft-checking interval to give a faster response time and to set traps, not only on the raft where you have found tracks, but also on adjacent ones. Obviously in taking these steps you sacrifice some of the labour-saving benefits of mink rafts.

Alternatively, the mink you have detected may live predominantly in terrestrial habitats, making only rare visits to the river. This applies particularly to male mink, which are more inclined to live on terrestrial resources such as rabbits. The problem may be increased in the presence of otters, which are thought to displace mink aggressively from the main river channels. Catching such individuals becomes more of a lottery. It may be necessary to set some traps on land nearby, to catch mink known to be present. But think: if the aim is to protect the river channel, then a mink that visits the river only occasionally is not such a big problem. If it’s a male, then it is largely irrelevant either to water vole conservation, or to mink population control. Also the tracks you recorded could be one of the occasional visits to a raft by a polecat.

If the tracks were large, it’s possible that the animal entered your trap but failed to trigger it. A very large male mink standing in front of the treadle plate will have its nose up against the end mesh, and may not take that final step before turning round and leaving the trap. In these circumstances the use of a spring trap may be justifiable.

If you are using an otter excluder different from the one described here, it may have the effect of reducing mink captures. Intuitively one would expect any such effect to be on the larger (male) mink.

One thing that isn’t a problem is the use of single entry traps, as mink have no trouble finding the way in.

Where there is a lot of ‘traffic’, mink tracks may become overlaid by those of other species, which can make identification difficult. Ironically, water voles have proved to be the main culprit. Territory-holding voles scent-mark in raft tunnels, then drum their feet over the latrine, which can quickly obliterate any other tracks present on the clay mix. The problem can be overcome by more frequent raft checks. Moving the rafts a short distance (50-100m) along the river may avoid the middle of a water vole colony.

Trapping can still proceed if mink tracks are identified, but it should be remembered that water voles will not be excluded from traps and will enter them, rendering them unavailable to mink.

Arguably, if this is an issue, then water vole populations are healthy, suggesting that mink presence is minimal.