Cassington Quarry lies adjacent to the A40 in the Thames Valley, approximately 4km north west of Oxford city centre, and stage 11 of Cassington quarry comprises an extension to the previous more traditional open water restoration. The original restoration plan prior to the western extension was for more open water, but due to concerns raised from the local Oxford Airport and Ministry of Defence, a new design was put forward featuring a reedbed restoration for this final stage of the extraction.
During the extraction process and land forming stage of the restoration, the water levels at the site were kept artificially low to allow machinery in to build and sculpture the beds ahead of reedbed planting. During this time, taking advantage of the partially de-watered interim habitat an otter created / moved into a hole within the sheer sand and gravel face on the northern boundary of the void.
Following the completion of earthworks in September 2011, the pumps were turned off to allow the water levels to rebound. As this action would lead to the holt being flooded and destroyed, prior to allowing the water levels to fill the void completely, the operator applied for a European Protected Species (EPS) licence from Natural England. The detail of the licence was to install a one way gate to the holt entrance to prevent otters from entering, thereby ‘closing’ the holt and guaranteeing no harm to any young would be caused through the rising water levels.
The creation of reedbed habitat at the site will in time provide significant opportunities for otter, providing rest sites and a valuable foraging resource, while increased areas of scrub around the margins of the void will also provide shelter for the animals during the day. Although not required under the Licence, in order to provide immediate enhancement for this species at the site, the creation of an artificial log-pile holt was proposed. The otter holt design, location and creation were worked up as a partnership project between Hanson, BSG Ecology and Nature After Minerals.
Otter Holt Design
A number of different designs of otter holts are available, varying in complexity and materials, from a pipe and chamber design involving the use of hard materials (concrete blocks and plastic piping), to a more simple single chamber constructed from logs. BSG Ecology provided invaluable experience in otter holt design and recommended a medium sized holt, comprising 3-4 sub chambers, to be made solely from logs and brash, utilising woody material freely available on site, thereby reducing associated costs. The final design loosely followed a design prepared by Dorset Wildlife Trust.
Otter Holt Location
When deciding on the location for an artificial holt at Cassington quarry, and similarly at any site, a number of factors had to be taken into account, for example:
- Potential disturbance through public access
- Proximity to previous sighting of otter utilising the site and adjacent land
- Potential conflict with other species
- Suitable cover
- Accessibility for building materials
At Cassington we had three options, and the table below sets out the pros and cons of each option. Weighing up all the options, it was decided that option 2 would be the most suitable at this site.
| Option 1 – eastern site boundary, where ditch meets hedgerow | Option 2 – North western corner of the reedbed | Option 3 – Within a treed belt offsite to the north | |
| Movement links | Yes, ditch linking holt to tree belt to the north, adjacent to reedbed | Yes, adjacent to reedbed and within close proximity to known otter movement corridor | Yes, adjacent to known otter movement corridor |
| Proximity to sightings | Opposite side of the site, while distances are not huge it may take otter longer to realise the location of the holt | Within same area of the site as the former holt | Adjacent to known otter movement corridor |
| Suitable cover | Yes, hedgerow and developing scrub | No – scrub would need to be planted | Yes |
| Conflict with other species | Yes – the ditch was incorporated as a specific enhancement for water vole | No | No |
| Disturbance through public access | No | Potentially – depending on access rights to this area, currently used by horse riders | Located just north of a public footpath |
| Accessibility for materials | Access easy | Access easy | Access difficult |
| Other | n/a | n/a | Query over land ownership as outside quarry footprint |
Otter Holt Creation
Subsequently native shrubs scrub was planted the following winter (2014), to provide additional shelter.
The chosen location was initially prepared by grading the bank to create a flat area on which the holt could be built.
Suitably sized logs were then placed, forming the outer walls creating a holt approximately 3m in diameter. The log walls were held in place through the use of willow poles that were hammered into the ground.
Smaller lengths of log were when used to build inner walls, creating three chambers of differing sizes.
An entrance was created at the eastern side of the holt, adjacent to the water, while a further entrance was also included on the western side.
The roof of the holt was constructed by laying long, narrow tree branches over the structure and then using thinner branches, brash and clay to fill in any gaps, and to create a waterproof layer.
The information set out within this advisory sheet in no way constitutes legal or regulatory advice and is based on circumstances and facts as they existed at the time Nature After Minerals compiled this document. Should there be a change in circumstances or facts, then this may adversely affect any recommendations, opinions or findings contained within this document