Freshwater Surveys

Otter Survey

Why do I need to consider freshwater species?

There are a number of species protected under UK legislation which utilise freshwater habitats.  These include:

  • Otter
  • Water vole
  • White clawed crayfish
  • Freshwater pearl mussels

All of these species are protected under the Wildlife and Countryside Act to differing degrees. Otter, water vole and freshwater pearl mussels are fully protected under this legislation, however white clawed crayfish are only protected from taking under this Act.  This legislation makes it illegal to:

  • intentionally capture, kill or injure water voles, otters or freshwater pearl mussels or take white clawed crayfish
  • damage, destroy or block access to the places of rest or shelter used by otter, water vole or freshwater pearl mussels
  • disturb otter, water vole or freshwater pearl mussels whilst they are using a place of shelter or protection

These species are found in the UK in a variety of riverine and standing water habitats.  Otters can range across long distances and have large territories, whereas water vole will often have relatively small territories in comparison.  Freshwater pearl mussel are only found within a small number of river systems in England, and the white clawed crayfish is also declining significantly due to the prevalence of the Signal Crayfish, which is an American, invasive species. 

When do I need to consider freshwater species?

When you are considering any development or activity that could harm freshwater habitats directly though affecting the waterbody, its banks or source, such as hydro-electrical generation, road schemes where the road crosses a watercourse, or development which results in the loss of a pond or larger body of water.  In addition, watercourses can be affected by a range of developments by being an outlet for surface water drainage systems.  Otter can be disturbed up to around 50m from their holt sites and, as such, development not directly affecting the watercourse itself but close to watercourses should also take the potential presence of this species into account.

Furthermore, development in close proximity to watercourses should also consider protected species as there is a risk of polluting adjacent watercourses should appropriate measures not be put in place to protect these watercourses. 

Any development which can affect a watercourse either directly or indirectly should consider impacts on freshwater species.  Our ecologists have provided advice for a large number different sites with varying levels of impact on freshwater habitats advising on levels of survey work required, avoidance measures to ensure works can proceed with no impact on local populations of these protected species, or developing a suitable licensing strategy along with mitigation and compensation measures ensuring projects can proceed with minimal impact and delay.

What does a survey consist of?

Initially a walkover survey should be carried out to confirm whether there is any potential for protected species such as otter, water vole, white clawed crayfish or freshwater pearl mussel.  This is based on a desktop study, the nature of the habitats present and the geographic location of the watercourse.  If this survey finds that there is a risk of the presence of any of these protected species then a species-specific survey may be required.


If the initial walkover survey finds that there is the potential for otter to use the watercourse or adjacent land, then a more detailed survey may be required.  This involves looking in more detail for evidence that otter have used the watercourse or land.  Such evidence can include spraint, anal jelly, footprints and feeding remains.  The survey would also look for suitable resting places that otter could use such as holes under roots, to areas of ground that have been pushed down as a result of otter use.  If further information is required on a resting place such as a holt or above ground location, wildlife cameras might be deployed to gather additional information whilst causing minimal disturbance. 

Water Vole

If the initial walkover survey finds that there is the potential for water vole to use the watercourse or adjacent land, then a more detailed survey may be required.  A more detailed survey for water vole involves walking the watercourse looking for burrows, droppings, footprints and feeding remains in order to determine the area that water vole might be using.  Locations used by water vole would be marked on a GPS handset and then mapped to determine the area being used and density of signs present.  Two water vole surveys should be completed in order to make a robust assessment.

Freshwater Pearl Mussel

If the habitats and location of the site are suitable for freshwater pearl mussel, then a more detailed assessment for the species may be needed.  This involves entering the water using bathyscopes or glass bottomed buckets in order to view the river bed and search for mussels themselves.  Where mussels are found their position would be marked on a hand-held GPS receiver and then mapped.  In addition, areas of clean gravel may also be searched evidence of juveniles. 

White-clawed Crayfish

If habitats are suitable for the species, then a more detailed hand search or trapping exercise may be conducted based on the nature of the watercourse involved.  For a hand search, surveyors walk along the river bed turning larger rocks and cobbles with a small hand net downstream in order to catch crayfish, and then confirm their species, sex and age class.  Their locations would then also be mapped.  For trapping exercises, crayfish traps would be deployed at intervals along the banks which would be baited and then left overnight.  They would then be checked for animals and again any captures would be speciated, sexed and assigned an age class. 

What OSE can provide:

OS Ecology Ltd have extensive knowledge of freshwater surveys, impact assessments, mitigation and compensation schemes and licensing.  Our directors hold licences for white clawed crayfish and freshwater pearl mussel and have completed a large number of otter and water vole surveys for a range of schemes.   Our directors have also held conservation licences for otter and white clawed crayfish and hold a class licence for maintenance and management works involving white clawed crayfish.

OS Ecology