Prospects for a mink-free GB

The Scottish Invasive Species Initiative was delighted to have Tony Martin of the Waterlife Recovery Trust attend and present at our recent project conference in Aviemore.  Tony leads work which controls and is close to eradicating the American mink from a sizeable chunk of East Anglia using volunteers and an extensive network of smart traps.  In this blog he tells us about this work and offers his thoughts and ambitions for a “mink free GB”.  The Waterlife Recovery Trust approach to things is in some ways very different to ours in the north of Scotland but there are, of course, many similarities.  We share the same objective of delivering American mink control at landscape scale working with volunteers and communities – and it’s always good to hear of different tactics and to share knowledge and experience to learn and improve as we go along. We look forward to seeing how the project progresses over the coming years.

Despite decades of trapping costing millions of pounds, American mink remain firmly established in Britain’s countryside from the south coast of England to the north of Scotland. It’s hardly surprising, therefore, that few people would give any credence to the idea that mink could be entirely eradicated from this rather large island of ours. It’s a completely barking mad concept, obviously.

I had the privilege of leading the rodent eradication project on the sub-Antarctic island of South Georgia – an equally preposterous concept. Who in their right mind would believe it possible to eliminate every single one of the millions of rodents on this huge, mountainous, glacier-riven island, a thousand miles from anywhere, with no roads or access by air? But we did it, and in so doing I learned an important lesson – that it’s foolish to dismiss even the daftest idea without giving it proper consideration. Might mink eradication in Britain be possible, after all? Would it not be worth carrying out a trial to discover if it might be feasible?

Pest eradications can only work if the area to be ‘treated’ cannot be readily re-invaded. Usually the barrier to re-invasion is the sea, which is why small islands lend themselves to this process. But a meaningful trial of mink eradication would need to cover a very large area (5,000 sq km, say), and there are no islands of a suitable size with mink on them in Britain. Even Lewis is less than 2,000 sq km in extent. The next best thing might be a large peninsula, with effort put in place to stop re-invasion from its landward side. I live in East Anglia, which happens to be a large peninsula, sort of…..

The first part was easy – increasing collaboration and cooperation. A Steering Group of key stakeholders was set up and has continued to meet quarterly. The Steering Group discusses and agrees matters such as methodology, Best Practices, fund raising and strategy, ensuring a consistent, effective, safe and humane way of removing mink across the region. The second part was establishing the scientific basis for the strategy. To be successful, the project would need to be adaptive – continuously learning and improving as evidence dictated. That evidence would come from close examination of the mink caught, and from keeping a close eye on changes in the Catch Per Unit Effort (CPUE), a proxy for mink abundance – number of mink caught per thousand trap nights, to aid targeting of traps and volunteer effort. To that end, every mink caught would need to be measured, weighed, DNA sampled (the tip of an ear is sufficient) and teeth taken to determine age.

The third and fourth factors were to do with the practical process of mink removal. Aerial spreading of toxic bait, as used in many pest eradication campaigns worldwide (including on South Georgia) was not practical for many reasons, and trapping was the next best option. The question was whether we could find ways to improve the efficacy of trapping to the extent that literally every mink could be captured, while at the same time avoiding harming the good guys – the native wildlife we seek to protect. The two elements we focused on were (a) making trapping far less labour intensive than hitherto, and (b) increasing the probability that a mink near a trap would be enticed to enter it.

We tackled the first problem by taking advantage of electronic boxes that alert the trapper when a trap door closes; we call them Remote Monitoring Devices (RMDs). These boxes (one per trap, turning it into a ‘smart’ trap) use the cellular phone system to send alerts by email and text message with 100% reliability, and their use means that a trap visit need only be made when there’s a high probability of an animal being in the trap, rather than daily. This reduces labour (and often fuel) by up to 98%. These huge efficiency gains mean that traps can be active 24/7/365 at every site, other than when temporary retreat is demanded by flash flooding, and that’s crucial. Trapping effort per location is far greater than is normally achieved with a traditional raft fitted with a clay pad to detect mink footprints. We detect mink by catching them, rather than by looking for their footprints.

The second issue was approached by recognising what every mink trapper knows – that a second mink will often follow a first mink into a trap, within days or even hours. They do so because of the scent left by the first animal, and that eye-watering scent mostly comes from glands either side of the anus. We now harvest that eau de mink from every animal captured, carefully expressing it, drop by noxious drop, onto roll-your-own cigarette filters. The filters are then stored in a freezer until used as scent bait in our traps, suspended above the treadle inside a hollow practice golf ball. And they work – there’s now plenty of evidence to show that mink are drawn into traps by following this scent to its source.

So far, so good, but back to the eradication trial – how could the test area (termed the ‘Core Area’) be isolated from the rest of Britain, stopping mink immigration from untrapped areas? The answer was to use East Anglia’s natural defence – the sea – to prevent incursion from the north and east, and to establish a 60 km-wide ‘Buffer Zone’ of traps to stop inward movement from the west and south (see Map 1). The Core Area comprises a large number of contiguous river catchments covering the central and eastern portions of Norfolk and Suffolk, totalling 5,853 sq km, while the Buffer Zone covers 10,423 sq km. Together, they amount to 12.5% of England.

Another question was how we would define trial success. Would an absence of mink in the area for a day suffice? Or for a week or a year? And how would we know if that had been achieved? We can’t expect to mobilise East Anglia’s entire population to look for mink on a particular night, let alone every night for a year. Furthermore, perhaps it’s not necessary to remove every mink – eradication would inevitably ensue if mink remained, but all of one gender, or if reproduction stopped. In the end, the decision was made to define trial success as the lack of evidence of reproduction over a 12-month period despite substantial search effort. We could never expect to prove that no reproducton had occurred, no more than we can prove that the Loch Ness Monster is a myth. But we would expect even one mink family to come to our attention, either through a report from a member of the public (as frequently happens from far and wide – a live sighting or a roadkill) or by way of the capture of a very young mink or a lactating female.

With funding from Natural England, water companies, Internal Drainage Boards (IDB) and private donors, and the participation of many hundreds of volunteers, reserve wardens, IDB staff etc, a comprehensive network of electronically-monitored traps was set up across East Anglia during 2020 and 2021, in both the Core Area and Buffer Zone. The average trap density across the Core Area was just one per 18 sq km. Catches were high initially, as was catch per unit effort (CPUE), but both rapidly diminished (see Figures 1 and 2). We found that, on average, CPUE dropped by some 70% each year, which means that fewer than 1% would remain after just 4 years.

So,the big question: is mink eradication feasible on a landscape scale in England? Remarkably, the tentative answer is ‘Yes, it is’. We almost reached the point of zero reproduction in the Core Area in 2022, before belatedly hearing of an unconfirmed but persuasive sighting of a single mink family in eastern Norfolk. The 2023 breeding season has, though, come and gone with no evidence of mink reproduction despite many thousands of hours of sighting effort, and over 106,000 trap nights of catch effort. We’re not quite ready to declare the trial to be over, but that will happen if no evidence of breeding in 2023 comes to light very soon. The occasional mink does turn up in a trap, but the genetic evidence indicates these are probably immigrants that have found their way in to the Core Area along the coast. We are now in little doubt that the eradication of American mink is entirely feasible in England, and almost certainly in Wales, too. What’s more, this could be achieved within no more than five years of setting up a smart trap network, and substantially through the efforts of trained volunteers.

So, what about Scotland? Is mink eradication feasible here? Although there are certainly greater challenges (lower density of people to visit traps, poor to no mobile coverage in some areas, access difficulties in winter, waterways in spate after heavy rains among them), and costs would be greater per unit area in consequence, I suspect that Scottish ingenuity and determination would win through in most mainland regions. The west coast and western isles do pose huge difficulties, though, and I’m unsure whether all mink could be removed from this vast and complex area with existing technology and methodology. There are two new tools on the horizon that may help in even these remote and windswept places, though. Firstly, a satellite-linked RMD is likely to be available within a year, so traps in even the most remote spots can then be electronically monitored. Secondly, a synthetic version of mink lure should soon be available, closely mimicking the real thing, but dispensed from handy containers which every trapper can carry in their kit. Together, these two innovations should bring a mink-free Scotland that bit closer.

I sign off from a train to Oban. I have been invited to Lismore and Mull this week, with a view to seeing if smart traps can enhance the efficiency of trapping mink in western Scotland, as they have in England. Mink are threatening to force the closure of an organic, free-range poultry business on Lismore, so the stakes are high. I very much look forward to what I’m sure will be an educational and enjoyable visit for this resident of the flat Fenlands of Cambridgeshire.

For more information on the Waterlife Recovery Trust you can visit their website at: https://www.waterliferecoverytrust.org.uk/