How to build an Otter Holt

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Carefully sited and constructed artificial holts may encourage otters to recolonise and breed along stretches of waterways from which they are at present absent. 

Where to build them: Careful site selection is required to avoid unnecessary wastage of effort and money. However, the criteria for selecting sites are, as yet, poorly investigated. Good places for the establishment of artificial holts are where a shortage of good breeding places seems the major limiting factor. Otters must be within dispersal distance and perhaps be known to pass occasionally through the area. However, other environmental factors must also be favourable. If river and bank management, for example, has been responsible for the removal of natural features, disturbance may be a reason for the absence of otters. They may never return, unless this is rectified. Other factors such as an adequate food supply, and lack of pollution must also be taken into account.

All sites should be above highest winter flood level, as a dry dark bed area seems important.   The holts constructed in Victorian times for which we have descriptions were sited across a meander, bend or between two streams. Some simply had an access tunnel at 90 degrees to the river.  Islands are good sites provided access is difficult or prohibited, a strong water current and dense vegetation reduces accessibility. 

Description: Artificial holts must have two or three entrances below, at or above water level.   No above ground entrance should be too high above the water level, they should be about 20cm across, and flush with the bank. Some cover should be present; willows can be planted if necessary. These grow quickly and eventually their roots will act as a ladder if the bank becomes worn away round the entrance. 

The tunnel should slope upwards from the entrance and may be curved to help exclude light.  It is essential that the chamber is dry and draught free, a raised bed area could be provided. Another description is a horse-shoe shaped holt in East Anglia; “sleeping chamber about a 50cm in cross-section, with two entrances at the heels of the horse-shoe”. 

Materials: A simple artificial holt can be made using log piles.  In the wild otters will use piles of various sizes from structures as large as 40m long, 10m wide and 4m high to small features of 5m x 3m x 4m.  Large logs should be used for the base, which will form a series of chambers, thinner logs can be used to roof the chambers and debris piled on top to make the structure waterproof. 

A holt constructed from rough stones (see diagrams) is described in the Badminton library, and the River Tanat  holt is also of this type. In others, pipes have been used for the tunnels, such as glazed tile or clay field drainage tiles usually of 24cm. 

Breezeblocks or unmortared bricks and. paving stones can be used for the walls and roof of the chamber, and wood. is a. suitable alternative, providing the site is on dry soil.

Corrugated tin or polythene of bigger dimensions than the roof is often placed over the chamber to ensure that it keeps dry in the wettest weather. The chamber roof is .usually covered with soil - a depth of 15cm is all that is required although there is no limit. The top may be turfed to ensure the exclusion of light from above; a ventilation pipe may be installed. 

Monitoring: It is important that an artificial holt is not disturbed after installation, but discrete and infrequent checks should be made to determine whether otters are using it.  Fine sand can be put down near the entrance so that you can observe prints.  Holts may take 3 months to 2 years to be used.  It is worth putting spraint in the holt to encourage otters to use it.