Fen Management Handbook - Scottish Natural Heritage

Sensitive periods
Harvest mice breed from May to October, and possibly through to December in mild winters, so time
cutting of vegetation appropriately. See Harris & Yalden (2008) for more details.
Otter
Recommended management
Wetlands should be managed in such a way as to retain features of primary importance for otters,
notably food supply (fish), high water levels and sufficient vegetation cover. Reedbeds and willow
scrub benefit from rotational cutting (Environment Agency, 1999). See Chapter 6 on vegetation
management] for further details. Note also that woodland and scrub are favoured habitat for otters,
so maintenance of areas of climax and wet woodland should also be considered. Fish are the main
prey item for otters, so maintenance of fish stocks is important, and actions such as restocking after
pollution incidents may be required. On sites where there is a lack of suitable refuges for otters in a
particular watercourse, consider the construction of artificial otter holts (Roper, 2008).
Sensitive periods
Otters reproduce aseasonally, so there are no specific seasonal constraints on works. Work near an
otter holt may require a licence (see legislation section below) if disturbance to the otters is likely to
occur.
Birds
Birds associated with the margins of open water bodies
Habitat features
The features included here are those areas of tall monocotyledon plants including common reed,
sedges, bulrush and rushes that grow as both emergent plants and on waterlogged ground adjacent
to an open water body within, or associated with, a fen. Such vegetation frequently occurs as a
margin to the water body. It is the open water that attracts the suite of birds listed as associated
with this habitat in Table 2. If open water is not present other than as drains or ditches then it is
included in the “reedbed and mixed fen swamp” habitat feature below.
Recommended management
Most of the birds identified as associated with this habitat feature favour the margins of waterbodies
where they find food, shelter and a place to nest. The few species that feed out in open water e.g.
diving duck and grebes, still requiring as minimum, marginal vegetation for securing a nest. Most
wildfowl seek open water as a refuge against the risk of predation, with grazers and dabblers
remaining in or close by to open water when foraging.
Merritt (1994) describes the creation and management of a ‘duck marsh’ for wildfowl species.
Water management of this habitat requires deliberate intervention and manipulation and the creation
of such a habitat is unlikely to be compatible with the other conservation features and objectives of
a natural fen. In winter a ‘duck marsh’ should be managed to have a maximum winter water depth of
0.3-0.4 m, have undulations of +/-100 mm across the majority of the marsh and with a network of
deeper channels. Such duck marshes are generally most suitable when greater than 2 ha. Water
management of ‘duck marsh’ continues to remain critical following the departure of most wintering
wildfowl. In spring (April – May), water levels should be dropped to and fluctuated around 0-50 mm
to provide feeding conditions for passage waders and some wildfowl e.g. garganey. Water levels
should then be allowed to continue to drop throughout May and June to expose damp mud. This
allows rapid colonisation by annual plant species which set seed throughout the remainder of the
summer months. Water levels need then to be raised in September to around 50-100mm, which
kills the ‘terrestrial’ plants whilst liberating the associated seeds and invertebrates. These conditions
can result in an abundance of food for passage waders and returning wintering wildfowl. Water
levels are then gradually raised to the winter maximum. Where management is directed towards
passage waders, a gradual reduction in water levels of a water body throughout the migration
periods (April-May and July-September) to continually provide wet mud for foraging is necessary.
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