Oxfordshire Gravel Pit: Otter Survey and Enhancement Works

Background

The discovery of an otter spraint near a hole in a working face of a gravel pit in Oxford, led us to conclude that the hole was potentially in active use as a holt.  The gravel pit was in the process of being restored to reed bed.  As part of this process, the pumps that controlled water levels needed to be turned off.  The worst-case scenario would have been the loss of an otter breeding site (and potentially the death of dependent cubs). It was therefore important to understand the level of use of the holt and whether it was used for breeding, to ensure that appropriate mitigation for its loss was put in place.

Monitoring by remote camera (deployed over 8 weeks) allowed us to establish that otters were not using the holt for breeding, although they were recorded infrequently inspecting the hole. The infra-red film evidence was an important part of the information used to secure a Conservation Licence to legally close the holt. It demonstrated that closure could be done in accordance with the law and in such a way that appropriate mitigation for the loss of the holt could be put in place. Without this technology we would have had to carry out many site visits in which to look for evidence of use of the holt, and would probably not have achieved the level of certainty that the camera evidence helped supply. This also resulted in significantly reduced costs for the client.

BSG Ecology’s Role in the ProjectOtter - Final Holt Structure_small

Although not required as part of the mitigation for the Conservation Licence, it was agreed with the landowner that an otter holt could be created by BSG Ecology working in conjunction with the Hanson Aggregates Ltd, the Principal Landscape Manager, and the RSPB Nature After Minerals Project Officer. We were keen to further enhance the site, as it is very close to our Oxford office, and the site provides a focal point for ongoing monitoring of the local population.

A suitable location was chosen above the high water mark well away from publically accessible areas of the pit. Loosely following designs prepared by the Dorset Wildlife Trust a structure made entirely of timber was created by:

  • Grading the bank to create a flat area to house chambers;
  • Using logs up to 1m long and 25-40cm in diameter to form a holt roughly 3m in diameter with two chambers of 1m2 and a smaller chamber of around 50cm2;
  • Pegging the logs down with pointed willow poles to keep them in place;
  • Creating an entrance gap facing the water’s edge with a second entrance at the rear;
  • Laying long poles over the structure to form a roof over the chambers and thinner poles, brash and clay on top to create a dark and dry space.

Saplings of blackthorn and/or willow will be planted around the holt to enable scrub to establish. This will conceal the holt and ensure there is minimum disturbance.

Large stones have been placed at the water’s edge near the entrance in the hope that otter will take the opportunity to use them as sprainting sites (spots where they mark their territory using faecal deposits). This will give us an indication as to whether the holt is being used by otter.

Otter - Holt entranceOutcome

The site has been chosen by the RSPB as a Nature After Minerals case study site. The success of the holt creation will be determined through site monitoring survey visits. We intend to re-deploy the motion activated infra-red camera to monitor whether otter are present when it is not in use on other projects.

This is one of a number of recent projects where BSG Ecology staff have designed otter holts, either as part of conservation initiatives or to mitigate potential effects of development on the species.

Oxfordshire Gravel Pit: Otter Survey and Enhancement Works – Project Profile

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