Is eradication of invasive mink now possible?
We are pleased to share this insightful post written by Darren Tansley who is the Chairman of UK Water Vole Steering Group and Wilder Rivers and Protected Species Manager at Essex Wildlife Trust.
The rise of the feral mink
Little did we know in 1929, that as the first North American mink Neovison vison were brought over to the UK for fur farming, this would spell the end of the road for the majority of our native water voles Arvicola terrestris. Often used as a second income for landowners with space for some pens, mink soon started escaping into the wild and by the 1950s a number of feral populations had become established. In 1962 there were 600 fur farms and with fears of mink impacting fisheries and game birds the species was added to the Destructive Imported Animals Act (1932) that had previously been used to control the escape of Muskrat Ondatra zibethicus.
Unfortunately despite regulation and trapping, mink populations continued to spread and by the early 1970s they had been recorded in every county in England, long before the better publicised animal rights releases had begun. Eventually fur farming was banned entirely but the damage had already been done. Water vole surveys conducted by Rob Strachan for the Vincent Wildlife Trust in the late 1980s and through the 1990s revealed a dramatic decline in water vole distribution which could not be accounted for through habitat loss alone. It was clear that mink were systematically wiping out water voles at a completely unsustainable rate.
The unique impact of mink on water vole
But why were mink so deadly to water voles; a prey item for a huge array of native predators looking for a large, easy meal? The answer could be found back in North America where the preferred prey of mink was the muskrat, a rodent similar in size and habits to the water vole. Mink were supremely adapted to hunt water vole, even tracking them into flooded burrow entrances, behaviour unlike any native predator with which water voles had co-evolved. Suddenly the water vole’s main defence of diving underwater and disappearing into a burrow was no longer an effective protection.
While water vole colonies were effectively being used as a summer larder by mink, these resourceful ‘switching predators’ were also depleting other prey when the voles ran out. Small mammals, fish, eggs, kingfishers, crustacea and even water fowl as large as cygnets were all taken to feed litters of typically 6-8 mink. These juveniles then dispersed each autumn and despite initial optimism that otters would suppress numbers (which proved not to be the case) the population expanded to cover most of the UK.
Protecting water voles in the 2000s
In the early 2000s, conservationists realised that without mink control, water voles were rapidly heading for extinction. The level of control required was unknown but it had to be long-term and at scale or mink would just move back in from surrounding areas. In East Anglia, The Eastern Region Mink Control Group was set up; a loose partnership of County Wildlife Trusts, Councils, RSPB, landowners, BASC and the Environment Agency sharing good practice for rolling out landscape scale mink control using the new GWCT Mink Monitoring Rafts. Each county had a few thousand pounds from water companies or the Environment Agency to fund equipment for landowners and volunteers to undertake their own mink trapping. This army of volunteers would visit their rafts every two or three weeks and check for footprints on the clay tracking pads, installing live capture traps when mink signs were found.
Mink capture rates plateaued at a few hundred animals per year over the next decade during which time the UK Water Vole Steering Group succeeded in getting legal protection for water voles. The National Water Vole Database and Mapping Project was set up by the Environment Agency and Wildlife Trusts, funded in part by People’s Trust for Endangered Species, in order to track annual water vole distribution from 2006 onwards. Many water vole conservation projects were now in operation including species reintroductions, mink control and habitat creation. With new water vole mitigation advice published for developers by the Mammal Society everything looked set for a water vole resurgence.
So it was with some shock that the 10 year report from the National Water Vole Database revealed that water vole distribution, far from increasing, had declined a further 30% across England and Wales. A survey of Suffolk identified water voles occupying 46% of their historic range, an improvement on 2003, but only being maintained by 15 years of continuous mink control. The Eastern Region Mink Control Group partners were still trapping, using tiny pots of money to pay for equipment for volunteers, but a weariness had set in and it was becoming harder to keep volunteers motivated.
Moving from mink control to eradication
In 2018 Professor Tony Martin was invited to talk to The Eastern Region Mink Control Group about the potential for mink eradication. New technologies and methods were becoming available and it was clear that mink control at scale was unsustainable in the long term. Two advances were key to the prospect of eradication.
1. Smart Rafts. Checking traps every 24 hours is extremely time-consuming so traps were only being set once footprints were found. However mink might have moved on by then. With new “smart rafts”, a sensor on the trap would send a text when the door closed so traps could be left set all year but only visited when they were triggered. Not only did this allow coordinators to respond to a trapped animal more rapidly, but one person could monitor multiple traps in an area without travelling to every one each day.
2. DNA Analysis. Professor William Amos at Cambridge University agreed to analyse DNA from every captured mink in order to build up a population profile. Over time, as more and more samples were collected, it became possible to identify which mink were related to each other, and a probable location of where they were born.
Both these advances required large funding bids and staff for coordination, something almost impossible to distribute from one county based organisation to another, so the Eastern Region Mink Control Group decided to wind itself up in favour of a single charity – The Waterlife Recovery Trust (WRT) – which would be able to gather funding, employ coordinating staff and target resources wherever they were required. The WRT Steering Group decided It was time to attempt a mink eradication pilot consisting of 6000km2 of Norfolk and Suffolk, surrounded by a 60km wide buffer zone to prevent mink returning. Natural England were also setting up a Water Vole Species Recovery Programme pilot so the Eastern Region fit this scheme perfectly with this new initiative.
Map of all the live smart rafts on the WRT database (May)
Hundreds of Smart Rafts were installed across the core area and a large part of the buffer zone, and hundreds of mink were removed during the first full season. All captures were recorded through a Regional Database, adapted from an existing county based system already used by Norfolk, Suffolk and Essex. Within 18 months, mink captures had virtually dried up across the 6000km2 pilot zone. With databased records of where traps were, how long they had been set and details of every trap visit, successful or false alarm, it was possible to see how long (on average) it took to catch a mink. This Catch Per Unit Effort data was essential in charting successful eradication and could be mapped at any scale; site, catchment, county or region. But even if a mink appeared within the core zone it was now possible to use DNA to show whether this was a local mink or one that had travelled in from another area, vital to establish whether breeding was still occurring. In October 2023, with no mink breeding across the core zone, the pilot was deemed a success. By now the main captures in the traps were mainly water voles with their distribution increasing rapidly in response to the mink free waterways.
Future plans
The Waterlife Recovery Trust area now spans from Lincolnshire to London but is spreading west and south into Kent, Surrey, and Oxfordshire. The speed with which mink were removed from the original pilot area was a surprise even to the WRT Steering Group but the only way to capitalise on this success will be to roll this eradication zone out into other regions. We are now able to accurately track mink capture rates and mapping data through the WRT Database, including the genetic profile of the mink. We have found that 87% of our mink do not reach their second birthday and none reached six. But most importantly we have a roadmap to a mink free GB that secures a future for water voles, and all the other native species along our rivers that have been so badly impacted. This project is only three years old so it is where we go now that will be vitally important.
Written by Darren Tansley
Please note that the opinions and views expressed in this article are those of the author alone and do not necessarily reflect the official stance of the Mammal Society.