Freshwater surveys

Freshwater ecosystems cover less than 1% of the Earth’s surface but are known to contain 41% of the worlds fish species and 6% of all recorded species. Therefore, it’s unsurprising that regulatory authorities are beginning to insist on thorough assessments of aquatic ecosystems before granting planning and operational consents to businesses.

The protection of our freshwater ecosystems is perhaps one of the more difficult conservation challenges because it is influenced by the upstream drainage network, the surrounding land, the riparian zone, and in the case of migrating fish, downstream reaches.

Fortunately, Thomson’s Aquatic Team provide our clients with a comprehensive range of surveys that give a robust understanding of the impacts of their operations on aquatic ecosystems.

Freshwater survey services

Fish population surveys and fish rescues

Freshwater survey - fish removal
Consultant Ecologist, Joseph, holding a wells catfish captured as part of a fish removal © Joseph Baker

Fish are a vital part of the freshwater ecosystems, transferring energy up and down the food chain and fulfilling many important roles. They also provide ecosystems services (most notably recreational angling), that is of significant value to the UK economy. Due to their size and the abundance of fish, they are easily sampled. That provides crucial information which helps to manage and understand freshwater ecosystems.

Thomson’s aquatic consultants carry out fish population surveys and fish rescues using a variety of methods and industry standard equipment. The methods we use include electric fishing, Seine netting and fyke netting. Which method is most appropriate will depend on a number of factors including the species that are likely to be present, depth of water, flow regimes and the end use of survey data. All fish sampled during our surveys are identified to species level, measured and weighed.

Our fish survey methods

Electric fishing

Electric fishing is an active fishing method that can be used in a wide range of habitats where safe wading or boating is possible. Electric fishing uses electricity to temporarily stun and capture fish that come within the electrical fields produced by two electrodes. Our electric fishing survey staff use generator and backpack systems which are fully compliant with UK/EU legislation. Thomson personnel are certified by the Institute of Fisheries Management as being safe and competent in this field, and carry out fish rescues, presence/absence surveys and quantitative catch depletion surveys using this method.

Seine netting

A Seine is a long sheet of net with a float line at the top and a lead line on the bottom with a central bag in which the catch is collected. Seine nets are typically set in a broadly circular shape and hauled. Seine netting is a simple method of sampling a large area in a relatively short time. It generally cannot be used among dense and robust aquatic vegetation, in habitats with abundant stumps or logs, in fast currents, or in deep water. Efficiency varies widely among habitats and species. Benthic species are less catchable than mid-water species. Smaller individuals are more susceptible than large individuals. Very small individuals and fragile species may suffer significant mortality, but for robust species survival is high. Seine netting gives a reasonable estimate of fish abundance and the ratios of species present.

Fyke netting

A fyke net is simply a hoop net to which wings are attached. Wings are short lengths of mesh with float and lead lines that are attached to the first hoop and extended at ∀45º to the trap hoop net to create a funnel effect. Fyke nets can be used to capture fish with little chance of injury in a variety of habitats but are difficult to use where currents are strong and/or carry a lot of debris. The sizes normally used for survey purposes can be set and lifted by two people. These nets are passive gear, and therefore catch fish that are moving. This method is particularly well suited to intercepting fish moving along known migration routes, such as during spawning migrations. They are also highly effective for sampling European eel that are resident in freshwater environments.

Aquatic macroinvertebrate surveys

Freshwater bodyAquatic macroinvertebrates are an essential component of freshwater ecosystems and are important biological indicators of water quality. Whilst chemical testing of water quality provides an accurate, single snapshot of the quality of the water at the time of sampling, it does little to indicate the quality of the water in the previous weeks and months.

Different groups of macroinvertebrates tolerate different environmental conditions and levels of pollution. Their presence or absence can therefore be used to assess water quality over longer time periods. For example, most larvae of caddisflies, mayflies, and stoneflies cannot survive in polluted water so waterbodies in which they are present are assumed to have consistently good water quality.

Thomson’s aquatic ecologists deliver macroinvertebrate surveys that accurately gauge water quality and ecosystem health. We carry out surveys using the methods developed for the River Invertebrate Classification Tool (RICT) and identify macroinvertebrates to family level or lower. Invertebrate identification allows us to calculate various biotic indices that can be used to measure water quality including the Biological Monitoring Working Party (BMWP), Average Score Per Taxon (ASPT) and Walley Hawkes Paisley Trigg (WHPT) indices.

Aquatic Invasive Non-Native Species (INNS) surveys

Invasive Non-Native Species (INNS) can have a devastating impact on waterbodies in terms of ecosystem damage and financial loss. Whilst the risk of introduction can never be eliminated it is possible to greatly reduce the likelihood of an outbreak occurring through assessing the level of risk. Should an outbreak occur then it is critical to quickly identify the invasive species so that it can be eradicated before it disperses from the point of introduction.

Thomson’s aquatic scientists regularly undertake large scale reviews and surveys of INNS in waterbodies throughout the UK. Our consultants are trained in the identification of all UK aquatic invasive species and are routinely prepare and implement management plans for their control and eradication.

When can aquatic surveys take place?

The optimal time for undertaking aquatic invertebrate surveys is between March and May and between September and November.


Download our ‘Species Survey and Mitigation Planner’ for a month by month guide to the best and most challenging times to survey various species in the UK.

Download a PDF

Badger surveys 3

To discuss your requirements or to find out more about the freshwater ecological surveys that we offer, please contact us.

Our Covid-19
Business Response

Find out more