What is ‘special’ about Llyn Hafodol and Cors Clegyrog SSSI?

Llyn Hafodol and Cors Clegyrog has two special features:

Two wetland basins situated very close to one another in north Anglesey; both of biological interest, principally as examples of lowland wetland habitat.

As well as the features listed above, Llyn Hafodol and Cors Clegyrog have other habitats that contribute to the special interest. These include marshy grassland, standing water, and scrub. This mixture of habitats is important for much of the wildlife including, three-lobed crowfoot, water horsetail, greater spearwort, eight stamened waterwort, great crested newt, birds and some (locally and nationally) scarce beetles and flies including the rare cranefly Limonia magnicauda. Unless specified below, management of this site should aim to look after these habitats and species as well as the listed features of interest.

What do we want Llyn Hafodol and Cors Clegyrog to look like?

Cors Clegyrog should have extensive carpets of Sphagnum bog mosses in the centre of the basin with cottongrass, cross-leaved heath, bottlesedge, pennywort,  bog bean, and areas of marshy grassland around the perimeter. .

Fen on Llyn Hafodol should be dominated by plants such as water horsetail, bog-bean, marsh cinquefoil and bottle sedge and the site should support a range of associated native species including the uncommon greater spearwort, three-lobed crowfoot and eight-stamened waterwort. Areas of marshy grassland, dominated by soft rush, should be widespread especially around the perimeter..

Willow should cover no more than 10% by area at both Llyn Hafodol and at Cors Clegyrog and should occur individually or in small clumps only. Water levels should be at or near the surface throughout the year, with much of the surface floating. Water supply to the site should be unpolluted.

Rhododendron, Japanese knotweed and other invasive non-native species of plants should be absent. Bullrush (reedmace) should not cover more than 5% of Cors Clegyrog and there should be no large areas of scrub (including bramble or gorse).

Great crested newts should occur on the site.

What management is needed on Llyn Hafodol and Cors Clegyrog SSSI and why?

Although Llyn Hafodol and Cors Clegyrog are excellent places for wildlife they will only remain so if the necessary management continues. CCW’s aim is to work with you to ensure that this management is carried out.

What does this mean in practice?

There are many factors that could damage the special features at Llyn Hafodol and Cors Clegyrog if they are not properly managed. These are the ones we regard as most important:

Grazing: light grazing helps maintain this site in good condition and to prevent scrub from spreading. Grazing at appropriate levels will prevent the build up of dense mats of leaf litter, which smothers smaller species. Cattle or ponies are the best stock to achieve this. No supplementary stock-feeding should take place within the SSSI or adjacent to the boundary of the site. The rare three lobed water crowfoot occurs in muddy areas near Llyn Hafodol, prefering slightly poached ground, so it is particularly important that cattle grazing should be maintained in this area.

Water quality: Plants such as bulrush and floating sweet-grass respond to increased nutrients and may out-compete less vigorous species of greater conservation value. Artificial inputs of nutrients and pollutants should be prevented since an increase in nutrient levels will damage the site. Runoff of farmyard manure and slurry or sewage discharge threatens the site and efforts should be made to ensure that no polluted drainage reaches the SSSI. There should be no applications of herbicides, pesticides, lime or fertilisers of any kind (including artificial fertilisers, slurry, manure and abattoir or creamery waste) on or within a 20m buffer zone of the SSSI. There should also be no dumping, spreading or storage of materials such as manure or silage on the SSSI .

Water levels: Water levels are controlled by a sluice at Llyn Hafodol and by the outfall drain at Cors Clegyrog. There is a need to ensure these are not lowered. It may be necessary to carry out maintenance of these structures but without deepening or lowering long term water levels. There should be no creation or deepening of ditches or channels within either part of the SSSI.

Scrub and bulrush: Willow scrub is widespread at Llyn Hafodol. Whilst some willow is desirable (it is very good for insects and some birds), we wish to maintain the site as predominantly open habitat for the foreseeable future. There is evidence that recurrent (natural) attacks by chrysomelid beetles limits regeneration and spread, despite current low grazing pressure. If beetle outbreaks fall, willow scrub is likely to increase. Willow should be maintained at less than 10% of each section of the SSSI. Small quantities of bramble are also present in the middle of the bog and should be removed. Areas of bulrush are increasing at a rapid rate in Cors Clegyrog due to nutrient pollution from the surrounding areas.

Invasive non-native plants: plants such as water fern, Japanese knotweed and Indian balsam do not occur here at present, but could damage the special interest if they arrive. Care should be taken not to introduce these species deliberately or accidentally (for instance, on vehicle tyres). Should they be discovered on site action may be required to control them.


Our knowledge and understanding of wildlife is continually improving. It is possible that new issues may arise in the future, whilst other issues may disappear. This statement is written with the best information we have now, but may have to change in the future as our understanding improves, in particular, of the possible/probable impact of climate change. Any information you can provide on the wildlife of your site, its management and its conservation would be much appreciated.

If you would like to discuss any aspect of your SSSI, or have any concerns about your SSSI, please contact your local CCW office.

Your local office is:
Countryside Council for Wales
North Wales Region
Llys y Bont
Ffordd y Parc
Parc Menai
LL57 4BN

Telephone: 01248 672500

Fax: 01248 679259