If it is impossible during the course of a lawful operation to avoid general disturbance to otters or the destruction of their shelters and habitat, then it is essential to undertake appropriate mitigation measures. Where the destruction of a holt has to take place, suitable restitution would be the provision of one or more artificial holts made from logs, boulders or pipes to tried and tested designs. If they are to be successful, advice should be sought on type, location and construction taking into account the general needs of the species.
Provision of Artificial Holts
Artificial holts can be constructed to several different specifications according to the local circumstances. In riverine and other freshwater habitats, both log pile holts and pipe and chamber holts are commonly constructed. In coastal situations, a slightly different design based on the latter is usually more appropriate. In such circumstances, it may be possible to incorporate an artificial holt within the rock armour fabric of, for example, newly constructed coastal defences, provided that the necessary measures are in place to prevent otter access to any nearby roads. (If the prevention of such access cannot be assured, it is wise to discourage the use of rock cavities for this purpose, and infilling the larger ones may, therefore, become necessary).
New roads (and some existing ones) can be a particular problem for otters and may lead to significant mortality amongst the local population. Road mortalities tend to occur at times when rivers are in spate and otters are obliged to leave the watercourse and cross roads, when normally they would safely follow the river channel beneath bridges.
The ideal is to design river crossings that retain a wide strip of accessible riparian habitat on either side which can accommodate spates, thereby providing a safe route under the bridge at all times. Clearly, the local topography is often such that this ideal is not always achievable, in which case mitigation can take a number of forms set out below, some of which may also be appropriate in other situations.
Tunnels and culverts
Both (wet) culverts and dry tunnels enable otters to cross roads safely.
- Dry tunnels: these are often installed as a general wildlife mitigation measure, but should be designed with otters in mind. If the animals are to be persuaded to use them, the longer these structures are, the larger in cross-section they need to be. Thus, for crossings up to 20m in length, a 60cm diameter pipe should be put in place and for crossings between 20 and 50m, a 75cm pipe should be used. After that, pipes should be replaced by box section tunnels at least 100cm high by 200-500cm wide, or low underpasses.
- Culverts: these are designed primarily to allow the passage of water and, because of the current, otters often find it impossible to move upstream especially if the watercourse is in spate. A ledge 45-60cm wide, 15cm above the highest flood level giving a minimum headroom of 60cm, will enable otters to avoid the water and use these culverts successfully. The ledge must be provided with split ramps at each end such that the ledge is accessible both from the water and the bank. Ideally, the surface of these ramps should also be roughened to enhance grip. There are cases where longer culverts (in excess of 200m) have been installed and subsequently used by otters. However, as with dry tunnels, longer structures are more likely to be successful if they are oversized and are square or rectangular in cross section. Large pipe culverts are not recommended as there may not be sufficient air space during high flows. Free air flow is vital, if scent is to be easily carried through the structure and encourage otters to use it.
Both dry tunnels and culverts require suitable fencing to guide otters towards them, if they are to be effective. Fencing may also be appropriate in other areas too, notably as a deterrent to accessing the carriageway. Otters can be very determined if faced by an obstruction of this sort. They are agile and can climb, dig and squeeze through narrow gaps so such fencing is never considered totally otter-proof. The recommended specification is as follows: at least 1.2m high galvanised welded mesh (of at least 2.5mm gauge) above ground level, with a maximum mesh size of 100 x 50mm attached to fence posts and topped with barbed wire. Below ground, the mesh should be dug in to a depth of 300mm, or 100mm with a horizontal lap on the otters’ side of 300-450mm. (This is often necessary in soft, peaty ground). Fencing contractors do not always understand the attention to detail which is required to achieve an effective otter-proof fence. A small unguarded hole may be all that is needed to render the entire fence pointless and so the workmanship needs to be checked with great thoroughness, by an otter specialist or a person experienced in otter fencing. Higher grade and more aesthetically acceptable solutions are possible, while lower grade chicken wire may be suitable on a temporary basis although electric fencing is a much more useful deterrent. In the latter case, electrified rabbit netting or two strands at 100mm and 200mm above ground are ideal, but note that such solutions require daily attention to ensure that they continue to function effectively.
In situations where fencing is required to prevent not just otters, but also badgers (and possibly some other species) from gaining access to a new road, a similar type of fence can be used, except that maximum mesh size should not exceed 25 x 50mm. A single fence to this specification should function as a reasonably effective deterrent for both the above species.
More information is available in Nature Conservation and Roads: advice in relation to otters (2001), by A Grogan, C Philcox and D Macdonald, and the Design Manual for Roads and Bridges(DMRB) - Note that the DMRB guidance is currently being updated.
Roads frequently travel round the more accessible coast, as a result of the mountainous terrain of Scotland. The indented nature of the Scottish coast, in association with the large number of inhabited islands, means that new or improved roads also frequently require new bridges or causeways. Proposals for new roads often raise concerns over the potential for increased otter mortality from traffic, in particular, where a proposed road will interfere with holts or cross established otter paths or areas where otters fish.
In such circumstances, the construction of an otter barrier wall (vertical, with a smooth outer face) at least 1.3m high on the seaward side, will help prevent otter incursion onto the road and encourage access to specially constructed otter tunnels. Walls of this type have the advantage of being extremely durable and weather-resistant. An example of this form of mitigation can be found on the road connection to the Skye Bridge.
A new road may also destroy or damage freshwater pools, necessary as grooming pools for otters which have been feeding in salt water. If this is the case, then new freshwater pools should be constructed to replace those lost. This was done when the Skye Bridge was built.