Individual study: Floating rafts as population monitors of American mink Mustela vison on the upper Avon and tributaries, Wiltshire, England
Reynolds J.C., Short M.J. & Leigh R.J. (2004) Development of population control strategies for mink Mustela vison, using floating rafts as monitors and trap sites. Biological Conservation, 120, 533-543
American mink Mustela vison were introduced to Europe for fur-farming in the early 1900's. Following accidental escapes and deliberate releases, mink were first recorded breeding in the U.K. during the 1950's, and they are now naturalised. In the UK and elsewhere, introduced mink have had a big detrimental impact on populations of water vole Arvicola terrestris, some river birds, and especially populations of ground-nesting seabirds and waders. For these reasons, and as there are few native mink predators, it is important that their numbers are controlled. It is currently unknown, however, whether present trapping practises are successful in reducing mink populations, and whether traps are suitably placed. During the coypu Myocaster coypus eradication programme in eastern England in the 1970's, live traps were set on rafts and mink were regularly caught incidentally in these.
The study described here assessed whether rafts could be used to monitor mink numbers, and how effective raft surveys are in determining the presence of mink compared to land owner observations and field-sign surveys. Another study looking at the effectiveness of using a mink scent attractant on rafts was also undertaken (see Case 294).
Study sites: A study was undertaken along the Upper Avon and four of its tributaries, the Ebble, Nadder, Wylye and Bourne to assess the efficacy of rafts to monitor mink and determine their presence, compared to land owner observations and field-sign surveys.
The Game Conservancy Trust mink raft: The rafts used in the survey were designed by the Game Conservancy Trust and consisted of three main parts: 1) a buoyant raft base, 2) a removable tracking case (to record animal footprints), and 3) a wooden tunnel.
1) The raft consisted of two 1220 x 610 x 6 mm sheets of exterior plywood sandwiching a 1220 x 610 x 50 mm sheet of expanded polystyrene, held together with six 80 mm M6 roofing bolts and nuts. A 200 x 160 mm hole was cut in the middle to accommodate the tracking case (see 2). Wire mesh (500 x 250 x 15 mm mesh) was stapled over both ends of the raft.
2) The tracking case consisted of a seed tray (external measurements 215 x 175 x 50 mm) with an out-turning lip. The seed tray was filled to 10 mm below the top with absorbent foam (OASIS brand floral foam, Smithers-Oasis, Kent, OH, USA) that had been saturated with water. A 10 mm layer of tracking substrate, mixed from two parts of fine buff earthernware/stoneware clay (supplied moist; Briar Wheels and Suppliers Ltd, Fordingbridge, U.K.) and one part of kiln-dried block-paving sand, was spread using a spatula to the top of the seed tray. The tracking case was sunk in the hole within the raft. The OASIS drew water though the seed tray base to ensure the tracking substrate was kept wet.
3) The wooden tunnel was made from two 500 x 150 x 25 mm tanalised side-boards and a 6 mm thick plywood roof screwed together. It was placed lengthways over the tracking cartridge and fixed to the raft with screws though metal brackets. Metal brackets were made from 100 x 20 x 1 mm lengths of multi-purpose fixing band (Expamet, London, UK) folded to form right angles.
To position the raft amongst emergent vegetation at the rivers edge, it was tethered to riverside shrubs or to an anchor post by passing a rope though 2.5 cm staples driven into the top surface of the raft.
Raft surveys for mink: Each tributary was separated into 2 km sections from its confluence. Thirty-six rafts were placed along alternate 2 km sections between 14 June and 17 July, and thus were separated by an average of 4 km of river. Within a 2 km section, the raft was placed within promising sites for mink. Each raft was checked three times, at two-week intervals from placement. All mink tracks were recorded and, once checked, the tracking substrate was smoothed using a wetted spatula.
Field-sign surveys for mink: Once a raft site was selected, river banks and other features were searched for evidence of mink by searching for tell-tale field signs such as footprints in soft substrate, spraints and prey remains. One person followed the bank and another waded in the river for 500 m looking for signs of mink.
Site owner mink questionnaire: For each site that was to be surveyed, the land owner was questioned about whether they thought mink were present or absent.
Raft surveys: The tracking substrate on the rafts recorded mink footprints unambiguously. The likely hood of mink visiting a raft did not change across the study, eliminating the possibility that mink were initially inquisitive. Rafts recorded mink at 56% (20 of 36) of the survey sites.
Field-sign surveys for mink: At only 33% (12 of 36) of the survey sites were the presence of mink dectected through field-signs found by experinced observers. Mink signs were hovever found at two sites where rafts did not detect mink, but rafts recorded mink at ten sites were natural signs were not found.
Owner perception: Owner perception of mink presence ascertained through questionnaire surveys was low. Only 19% (seven of 36) of the survey sites were believed to harbour mink.
Conclusions: The results from this study suggest that monitoring rafts could play an inportant role in ascertaining American mink presence/absence and also be used to obtain minimum mink population estimates. The raft may also have a purpose for mink trapping, and the wooden tunnel used in the raft design is the correct size to fit a BMI Magnum 116 spring trap (Butera Manufacturing Industries, OH, USA; UK suppliers Magnum Trap Co. Ltd., Southampton).
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