Management to help water voles

In view of the population dynamics and dispersal behaviour of water voles, the protection of individual colonies in isolation is unlikely to achieve any lasting conservation benefits. Instead, a strategy that considers a number of nearby populations together is, realistically, the only way likely to ensure long-term persistence.

The basic principles for conserving water voles can be summarised thus:-

• Ensuring habitat connectivity exists between individual colonies
• Maintenance of abundant herbaceous riparian vegetation (including the management of trees to avoid excessive shading)
• Minimising the opportunity for mink colonisation

Good upland habitat for water voles

Rob Strachan

Upland habitats

In upland river systems conservation effort should be directed at extensive catchment or multi-catchment scale areas. At any one time such extensive areas will contain suitable but unoccupied water vole habitat. It is essential that such areas be protected as their loss may threaten the persistence of the metapopulation. The objective is to minimise the degree of isolation between adjacent colonies, such that occupied sites are within a 1.5 km radius of one another.

The following management recommendations are considered to be particularly relevant

• Good quality upland water vole habitat comprises sedge-rich areas, including grasses, rushes and heather adjacent to slow flowing, shallow burns with moderately steep banks and penetrable (often peaty) substrate. These areas are also those likely to be favoured by grazing deer and sheep and may be vulnerable to excessive grazing and poaching. Fencing to exclude grazing is unlikely to be a realistic option. The preferred approach is to reduce grazing levels at a catchment or multi-catchment scale through stock reduction where necessary.
• Where they include areas of suitable water vole habitat new native woodland schemes will need to make provision for suitable riparian corridors as an integral part of their design. . Elsewhere, the creation of such corridors where plantation woodland already exists, would benefit water voles.
• Known water vole colonies or potentially suitable habitat should not be subjected to muirburn. There is considerable potential for damage from this form of management. Extensive and uncontrolled burning can expose burrows and destroy food plants and cover leading to an increased risk of predation. An unburnt buffer strip of at least 10 m on each side of any watercourse used by water voles (irrespective of the width of the watercourse), is required to protect the voles’ habitat.
• It may be possible to create habitat refuges away from regularly flooding burns. For example, in otherwise uniform areas of open hill, pool systems could be excavated, and sedges, rushes and grasses encouraged to colonise. Such refuges could also act as ‘stepping-stones’ enhancing overland dispersal at a time when voles could be particularly vulnerable to predation.
• Under certain circumstances, e.g particularly vulnerable and isolated surviving colonies where there is no opportunity for natural recolonisation to occur, consideration should be given to the ‘seeding’ of captive-bred animals at a number of sites within the catchment with a view to establishing colonies as sources of dispersing voles. Due to the genetic differences between Scottish water voles and those from England, only stock of known local origin should be used.
• Constant vigilance will be required to prevent mink colonisation. This could be achieved by targeted and co-ordinated trapping effort concentrated at critical times and at critical sites e.g. during the months of January to April inclusive at the mouths of upland sub-catchments. Trapping during the dispersal period in September and October is also recommended. A co-ordinated system for monitoring trapping effort, trapping results and mink distribution will also be required.
• Where rabbits are known to occur, they may constitute a key alternative food resource for mink. In addition, female mink use rabbit warrens as breeding dens. Rabbits may therefore encourage the spread and establishment of mink by providing potential breeding sites and enabling permanent rather than transitory colonisation. Every effort should be made to prevent the spread of rabbits into upland sub-catchments. In those upland sub-catchments where mink colonisation is considered possible, and where rabbits are already established, colonies should be destroyed, subject to there being no other overriding conservation concerns.

Lowland habitats

In lowland river systems, water vole colonies are best protected from excessive grazing of riparian habitats by the creation of buffer strips either side of the water course. This buffer strip should be fenced off to allow the riparian vegetation to grow tall and lush if the adjacent land is grazed. Such riparian buffer zones should be 6 m wide on both sides of the watercourse. Scrub encroachment should be prevented by occasionally cutting all this vegetation back to around 10-15 cm (during the autumn or winter months only). Cutting should only take place on one bank only in alternate years.

The dynamics of water vole populations are such that just because a section of otherwise suitable water course is unoccupied in one year, it does not mean that the site will remain so. In view of this, unoccupied, but potentially suitable, habitat that is within about 1.5 km of an occupied site should also be managed with water voles in mind.

As mink have colonised more and more tributaries within river systems, so water voles have retreated to small headwater burns and associated drainage ditches. In intensively-farmed lowland river catchments, overgrown drainage ditches often form field boundaries. These require occasional vegetation clearance and dredging to ensure that they continue to function.

Reed beds

Although water voles and mink do not generally coexist, ree bedds may represent an exception to this rule. In some extensive systems, water voles appear to be able to survive in close proximity to mink, provided they can take refuge in the middle of the reed bed away from the margins where mink concentrate their hunting effort. Isolated areas of higher ground (islands) within a reed bed may act as long-term refuges for water voles.

At some sites, there may be opportunities for creating artificial islands for water voles, although such features would require ongoing management, notably the removal of scrub which will become attractive to mink through the provision of den sites, if allowed to become established. These artificial islands need to remain dry at all times and should be located as far from the edge of the reed bed and main channels as possible (at least 50m). The islands should be 5m in width and in lengths of 25-50m per hectare. They should also be planted with preferred water vole winter food sources such as the rhizones of yellow flag iris and kept clear of potential mink denning sites.

The canal at Firhill

Rob Strachan

Urban sites

Some large urban areas, notably Glasgow, still support viable water vole populations in increasingly isolated patches of undeveloped land. The future of these individual colonies depends on the successful maintenance of the overall metapopulation structure. In built-up areas the opportunities for overland dispersal of voles are restricted and so watercourses can assume even greater importance. Where a proposed development threatens the integrity of a water vole site, developers and planners need to look beyond the immediate boundary of the site and consider the possible effects of such proposals on a larger scale. For instance, even if the habitat at a given site can be protected, how will the proposal influence the natural movements of water voles within a 5 km radius? What other sites within this zone are likely to be used by water voles? If long sections of watercourse will be culverted, either as a result of this scheme or a separate one nearby, this can present a serious barrier to vole movement.

One approach that may be beneficial to water voles is the establishment of suitably located and carefully designed Sustainable Urban Drainage Systems (SUDS) to increase the availability of suitable wetland habitat for a variety of wildlife, including water voles, in urban areas.

Canals

Although these do not constitute a major resource for water voles in Scotland there are sections of canal in the Central Belt where the species still occurs. These tend to be where the banks are made of a relatively soft substrate such as clay or loam earth. Old banks in poor repair often have gaps in stonework or rotten wooden piles that allow water voles access to the earth bank behind. Well maintained sections of canal are less likely to be occupied because the banks are often reinforced with stone or piling. Other factors such as the presence of mink, the degree of shading by bankside trees or the presence of heavy cattle poaching also restrict the areas where voles can survive.

The following general management recommendations apply to both canals and the few slow-flowing lowland rivers where water voles still occur:-

• Use appropriate natural materials for erosion control: willow spiling, hazel hurdles and coir fibre rolls should be over stone, brick and metal/wood piling.
• Encourage the development of water margins dominated by reeds, sedges and stands of emergent plants together with tall grasses and herbs on the banks. Management of the margin vegetation should be achieved through a late summer cut of the bankside vegetation. Mid-channel weed clearance should seek to maintain a minimum of 1m reed margin on each bank.
• Fence the banks to protect against the trampling effects of livestock.

Cattle grazing where banks have not been adequately fenced

Rob Strachan

As elsewhere, targeted mink control may also be beneficial in some situations, particularly in the early stages of habitat regeneration.

Mink control

Increasing the level of mink control has been widely promoted as a means of reversing the water vole’s decline, but it does require careful consideration. The available evidence suggests that the targeted control of mink in key areas within a river catchment is likely to be the single most effective management tool for retaining surviving vole populations. However, little is known about the level of trapping effort that would be needed to reduce mink to a level compatible with subsequent recolonisation by water voles. There is a need to quantify the relationship between mink control and the response of the water vole colonies it is designed to protect. At present very little information about this relationship exists, hence the need to keep good records of trapping effort and success rate, while concurrently monitoring population changes in relevant water vole colonies.

Similarly, the extent to which control would be needed in the long-term is unknown and raises important issues about sustainability. There would be little point in a major project to remove mink from an area only for them to recolonise a year or two after the trapping programme ceases.

In view of the on-going nature of mink control, a sustainable control strategy is likely to be one that makes best use of existing trapping effort, both in terms of the protection of water voles and the game species, at which control is currently largely focussed. Mink are routinely controlled on many estates to protect sport fisheries. However, tapping is not usually co-ordinated in any way between adjacent estates and this may be the most practical means of maximising the benefits to water voles, especially in the uplands. The optimum time for controlling mink is January-April, at which time the pre-breeding females can be removed from the population. This makes sense not just in terms of mink population dynamics (i.e by removing all potential offspring for the year at the same time), but also because it is the breeding females that are thought to have the greatest impact on water vole colonies. By setting traps during the early spring at strategic points in target sub-catchments, the chances of preventing the ingress of mink into sensitive areas can be maximised. For maximum effect this should be followed up by further trapping during the dispersal period in September and October.

Recently the Game Conservancy Trust has developed a novel system for both detecting and catching mink, based on tethered rafts. This system has the advantage of being highly effective at both detecting mink presence and subsequently trapping animals. Details of their design are available here.

While mink control in isolation cannot be regarded as a sustainable long-term solution to the water vole’s decline, it may prove to be an effective interim measure in some areas while other longer-term recovery measures such as habitat improvements come into play.

Summary of SNH’s position on mink control

SNH maintains an overview of mink predation and control as it relates to the natural heritage. In view of the extensive range which the species now occupies in Britain, total eradication is unrealistic. SNH seeks to apply a strategic approach to action on mink, focusing resources on geographical areas and situations where the greatest natural heritage benifits can be gained. SNH's current priorities for action on mink can be summarised as follows:

• Funding eradication of mink in some island locations where complete removal of the species is considered to be possible;
• Identifying and implementing targeted control in a range of mainland locations where conservation objectives for priority species require urgent action;
• Promoting a broader 'ownership' of the problems associated with feral mink amongst a range of environmental and agricultural organisations.

Rat control

There are cases where water voles have become the victim of incidental poisoning resulting from legitimate rat control operations near watercourses. Water voles have survived in many urban areas and ditches around farms where the habitat is less suitable for mink. These are the very areas where poisoning for rat control is often carried out. Although rat control activities are not thought to be amongst the major threats faced by water voles, pest control workers can minimise the risk of inadvertently killing water voles by taking the following simple precautions:-

• Always check for the presence of water voles before controlling rats along watercourses
• If identification is in doubt seek the advice of a specialist before taking further action
• Where water voles are present, the use of live traps is preferable to lethal methods of control. These should be checked twice a day to release non-target captures. Traps should be located in the open (or near buildings) rather than in dense vegetation. Avoid setting traps at the water’s edge
• If lethal traps must be used, these should be placed at least 5 m from the watercourse
• Avoid the use of poison where water voles occur. Do not place poison inside burrows
• Where the use of poison is the only feasible option, the rodenticide should be covered or enclosed in a bait box. This should be placed at least 5 m from the watercourse. Avoid the use of poisoned grain or pellets. Instead, use wax or soap blocks instead. If possible, site the bait off the ground, as rats are more likely to climb than water voles
• Regularly inspect and monitor the control site, clearing away poisoned corpses. If any dead water voles are found, review the control method

Further details are available here