Freshwater pearl mussel: surveys and mitigation for development projects

Standing advice for local planning authorities to assess the affect of development on freshwater pearl mussels.

You need survey reports and mitigation plans for development projects that could affect protected species as part of getting planning permission.

Local planning authorities should use this advice to decide what is needed for surveys and plan mitigation measures to protect freshwater pearl mussels (Margaritifera margaritifera).

If a local planning authority consults Natural England and the Environment Agency on the same planning application, Natural England will use this standing advice and the Environment Agency will lead on providing additional advice.

You need to decide which survey and mitigation methods are right for the project you’re working on. If you don’t follow this advice, include a statement with your application explaining why.

What you must not do

The freshwater pearl mussel and its habitat are fully protected by law, under Schedule 5 of the Wildlife and Countryside Act 1981 (as amended) and Annex 2 of the Habitats Directive.

You’re breaking the law if you:

  • capture, kill, disturb or injure them (on purpose or by not taking enough care)
  • damage or destroy their breeding or resting places (even accidentally)
  • obstruct access to their resting or sheltering places (on purpose or by not taking enough care)
  • possess or sell freshwater mussel pearls

If you’re found guilty of an offence you could get an unlimited fine and up to 6 months in prison.

Decide if you need to survey

Freshwater pearl mussels live in clean, fast-flowing rivers across northern England, Shropshire and Devon. They bury or lodge themselves in stable riverbeds of sand, gravel and cobbles, and in shallow ‘riffle areas’ that are well-oxygenated and silt-free.

Survey for freshwater pearl mussels if distribution and historical records suggest they may be present by checking:

Find further information on the ecology of freshwater pearl mussels.

Survey methods

You’ll need a licence to survey for freshwater pearl mussels. Contact a Natural England Site Officer for additional permission if you’re surveying in or near SACs or sites of special scientific interest (SSSIs).

You should get a suitably trained and competent person to survey the river bed for freshwater pearl mussels:

  • by hand using a glass-bottomed viewing bucket
  • between April and September
  • under favourable conditions, eg when the water is clear, in bright light (between 10am and 4pm) and when the river flow is low

How much of the river you should survey depends on the kind of work you’re planning to do. For example, for river engineering you should survey the entire river bed that’s directly affected, extending the search area by at least 0.1 km upstream and 0.5 km downstream. You should also survey any length of river where your work may reduce water flow.

Assess the impact

Assess the potential impacts of your plans without any mitigation in place.

Freshwater pearl mussels are very sensitive to any kind of pollution and their breeding cycle depends on a host relationship with salmon and trout.

Harmful activities include:

  • disturbing the mussel beds, eg pearl fishing
  • intensive land-use
  • river engineering or works, eg dredging river shingle, sand and gravel
  • storing materials in the water
  • using machinery and vehicles in the water
  • removing (abstracting) water and changing river flow

Activities may also be harmful if they:

  • reduce water quality, eg discharged chemicals or nutrients that lower oxygen levels suitable for
  • lead to a decline in populations of salmon or trout (mussels’ host fish)
  • lead to a decline in habitat conditions, eg losing tree cover that provided shade for mussels
  • pollute the water, including silt and sediment deposits (siltation)

You must follow the laws protecting freshwater pearl mussels even if you don’t need planning permission for the work you’re doing. Contact the Environment Agency for conservation advice before carrying out any work on riverbanks or channels.

You must aim to avoid negative effects, eg by redesigning the scheme. If this isn’t possible, use mitigation measures to reduce the impacts and use compensation measures if there are still negative impacts for freshwater pearl mussels.

You may need to get advice from Natural England and the Environment Agency on the survey results and proposed mitigation measures.

Mitigation and compensation methods

Address the potential impacts identified on freshwater pearl mussels by creating a mitigation plan.

Your mitigation plan should include how you will:

  • protect the species population and distribution
  • protect the river bed and the habitat immediately around it
  • consider how other species could be affected by the mitigation

Mitigation measures can include:

  • doing work outside of sensitive periods
  • doing work in small sections
  • reducing disturbance to the river bank
  • reducing the area affected
  • avoiding disturbing the river bed
  • preventing any sediment from being released
  • preventing pollution including silt

If you expect the work to have a significant impact on mussel habitat, you should plan compensation measures to improve its size or quality. This includes fixing any loss of connections between sections of the habitat.

Compensation measures can include:

  • reducing the effect of sediment upstream of the mussel beds, eg by fencing off river banks (‘riparian habitat’), creating buffer zones or shoring up banks by managing or planting new trees
  • stopping nutrients and sediment from entering the river from sources of pollution such as ditches, cattle drinking points, roads and farms
  • controlling non-native and invasive plant species
  • in-channel works to maintain water flow and improve river bed composition, eg by narrowing the channel width to speed up flow or by removing weirs
  • improving the survival of fry and juvenile mussels by cleaning and adding river gravel or boulders
  • installing large woody debris to improve habitat conditions for host fish

You may need to monitor the site for several years to check that any mitigation or compensation measures are working.

Get a licence to disturb freshwater pearl mussels

In most cases you should be able to avoid disturbing or harming freshwater mussels or damaging their habitat by adjusting your planned work. If you can’t avoid it, you may be able to get a licence to disturb them for scientific or conservation purposes.

Applying for a licence should be your last resort.

Decide if you need a licence

It’s up to you to decide if your activity will affect freshwater pearl mussels or their habitats, and whether you’ll need a licence. You can get expert advice from an ecologist to help you decide, but this isn’t a legal requirement.

You can get a licence to take or handle freshwater pearl mussels for:

  • scientific or educational purposes eg to do a survey or for research
  • conservation, when there’s no option but to remove them in order to prevent them being harmed or killed

You can’t get a licence for development, maintenance or land management activities. If there are no alternatives and mussels need to be moved, Natural England may issue a conservation licence. These will only be given if you enhance the species’ habitats and help to conserve them.

Published 19 June 2015