Environmental monitoring in watercourses

Fish populations in running waters serve as clear indicators of the state of the environment. Changes in species richness, density, or recruitment can give early signals of acidification, eutrophication, habitat alterations, or migration barriers. SLU Aqua’s fish surveys help detect such changes in time and provide a basis for measures that strengthen sustainable fish populations in our watercourses.

How we use our results

The data we collect is used to track trends and identify environmental problems in Swedish watercourses. The results form the basis for assessing ecological status according to the EU Water Framework Directive, follow-up of national environmental quality objectives, and evaluation of measures such as liming, habitat restoration, or improved fish migration routes.

The monitoring also supports advisory work on fisheries management, follow-up of threatened species such as the freshwater pearl mussel, and reporting to international collaborations such as ICES, Helcom, and EU environmental agencies.

National monitoring

Electrofishing in watercourses

In 28 small to medium-sized watercourses across the country, SLU Aqua conducts annual wading electrofishing. The method is gentle on the fish and makes it possible to obtain a reliable picture of fish population density, species composition, and recruitment.

The watercourses selected for monitoring are chosen to provide representative time series with as little local impact as possible. The monitoring is designed to be long-term, which is essential for detecting trends over decades. This allows us to follow environmental changes and distinguish natural variations from human influence.

Data from electrofishing are stored and made accessible in the database for fish monitoring in rivers and streams – SERS.

Read more about the sub-programme Trend stations in watercourses, part of the Swedish Agency for Marine and Water Management’s freshwater monitoring programme.

Boat electrofishing

Wading electrofishing and net surveys work well for data collection in smaller watercourses, but are not suitable for larger, slow-flowing rivers. In these cases, boat electrofishing is used, and we are currently developing an index to assess trends based on this method. The hope is that boat electrofishing will become an integral part of Swedish environmental monitoring. Read more about how electrofishing is opening up environmental monitoring in more aquatic environments.

Monitoring salmonid species and eel

In several of Sweden’s most important salmon rivers – from the Torne River in the north to the Mörrum River in the south – SLU Aqua monitors salmon and sea trout. We also lead and carry out Sweden’s eel monitoring programme. 

This monitoring provides vital information on how environmental conditions and human activities affect our migratory fish species, and forms the basis for both national and international fisheries management. The monitoring of salmon, trout, and eel is part of Sweden’s data collection programme for these species and contributes to the EU’s Data Collection Framework (DCF).

Integrated studies of the effects of liming acidified waters, ISELAW

This programme follows up the effects of acidification and liming in lakes, where SLU Aqua is responsible for fish surveys and sampling. More information is available on the Integrated studies of the effects of liming acidified waters (ISELAW) website.

Data collection in practice

All surveys are carried out using standardised methods to ensure that results are comparable over time and between different watercourses.

Electrofishing usually takes place in August–September, when water levels are low and it is easier to catch that year’s fry. Survey areas are the same each year. Fish are temporarily netted, measured, and then released. For each fish caught, species and length are recorded, and in some cases, tissue samples are collected for DNA analysis or health screening.

In larger rivers, electrofishing may be complemented by other methods, such as smolt traps and automatic fish counters.