The original version of these guidelines included details of how to build the GWCT Mink Raft, but rafts are now available commercially. Modern materials such as recycled plastic board in place of plywood mean that raft life extends well beyond our DIY original which was always meant to be retired after three years. We therefore feel that there are unlikely to be any long term savings in home construction. When the raft was invented there was little understanding of microplastics as pollutants, so erosion of the expanded polystyrene used in the centre of the raft was not seen as a big problem. Today we take a very different view and would only support home construction if damp proof course or similar is used to seal in the flotation layer. For anyone wishing to construct their own raft, details are available separately.
The tracking cartridge and clay
The tracking cartridge consists of a plastic basket with a solid bottom and vented sides. It is packed to within 10mm of the rim with highly absorbent florists foam (originally Oasis, but now replaced with Fibre Floral brick, a biodegradable alternative). Because the basket has tapered sides, the rectangular block of foam must be cut to fit, and the offcuts are used as outer wedges to secure the foam in place. After the foam is saturated with water, a 10mm-thick layer of tracking substrate is smeared over the top surface with a broad spatula. The tracking substrate is a mixture of fine buff earthenware/stoneware clay and kiln-dried block-paving sand.
The point of using kiln-dried sand is to be able to control how much water is in the mix. The ingredients are worked together by hand in a bucket, gradually incorporating enough water (approximately 100ml for the above quantities) to make a stiff but workable paste. By smoothing with a wetted spatula (eg. decorator’s trowel), and finally with a rubber blade (grouting squeegee), the surface is left perfectly smooth and level with the rim of the tray. The foam acts as a wick, drawing water from the river to maintain a permanently-moist tracking surface.
The tunnel
The tunnel has four intended functions: to provide a dark cavity which mink might find attractive to investigate; to protect the clay surface from rain and debris; to house a trap when required; and to provide a closed-off area whose entrances can be regulated using physical barriers to exclude non-target species larger than mink.
Anchoring the raft
The raft is tethered to riverside shrubs or trees, or to a post by a rope passing through the eyebolts in the raft corners. You must choose whether to fasten the rope to one ring on the raft or to two. Two provides more security, but one attachment allows the raft to find a position where it catches less current and may make it less vulnerable to swamping in spate conditions. We haven’t found any satisfactory solution that allows the tethering point to rise and fall with the water level. If water levels are liable to rise considerably, fasten your rope high and allow enough slack – you will probably have to make adjustments in wet periods.