Environmental monitoring in lakes

Fish are important indicators in lake ecosystems. Different species react in various ways to changes in their habitat, such as temperature, oxygen levels, pH, and food availability. By monitoring fish populations’ composition, size, age structure, and growth, we gain insight into the environmental status of the lakes and can detect changes and problems in time.

We also monitor crayfish, both the native noble crayfish and the invasive signal crayfish.

How our results are used

Data from SLU Aqua’s fish surveys are used to assess the ecological status of lakes, follow up on environmental goals, and evaluate the effects of measures such as liming and fisheries management. The results form the basis for managing fish and crayfish stocks and support decision-making for both national authorities and local stakeholders.

Monitoring data are also used in research and international reporting, providing important knowledge on how climate and environmental impacts are altering lake ecosystems over time.

National monitoring

Many of SLU Aqua’s lake investigations are conducted as part of national and regional environmental monitoring programmes. Others are carried out in protected areas, heavily impacted waters, or to follow species that are threatened, non-native, or of particular management interest.

SLU Aqua’s work in lakes is largely funded through environmental monitoring programmes coordinated by the Swedish Agency for Marine and Water Management (HaV), and through special commissions from county administrative boards, water conservation associations, and other authorities.

Sweden's largest lakes

In Sweden’s largest lakes – Vänern, Vättern, Mälaren, Hjälmaren, and Storsjön – SLU Aqua regularly collects data to monitor changes in fish and crayfish populations, understand ecosystem development, and provide a basis for sustainable management.

The surveys include standardised gillnet sampling, standardised hydroacoustic surveys with trawling, crayfish trapping, and sampling in collaboration with commercial fishers. The data reflect both predator and prey fish and are complemented with information from both commercial and recreational fisheries. Together, the results provide a basis for assessing the biological status of the stocks. Results are published on Fiskbarometern.se.

The differing conditions of the lakes are reflected in species presence, species composition, and ecology:

• Lake Vänern is Sweden’s largest lake, with deep areas and a large catchment area. The most common species is vendace, but pike-perch, whitefish, salmon, and brown trout also occur. Pike-perch and vendace are important for commercial fishing, while trout, salmon, and pike are key for recreational fishing.
• Lake Vättern is deep and clear, with cold waters that host smelt, vendace, whitefish, Arctic char, trout, and signal crayfish. Recreational fishing mainly targets salmonid species, while the signal crayfish dominates commercial fishing.
  
  
• Lake Mälaren consists of several distinct basins with varying depth and nutrient levels. Common species include smelt, pike-perch, perch, roach, and white bream. Pike-perch is the most economically important species for commercial fishing and, along with pike and perch, is also significant for recreational fishing.
  
  
• Lake Hjälmaren is shallow and nutrient-rich, with high fish productivity. Dominant species include pike-perch, perch, bream, and vendace. Pike-perch and signal crayfish are the most important species for both commercial and recreational fishing.
  
  
• Lake Storsjön is deep and clear, with a diverse fish community. Species such as Arctic char, trout, whitefish, and perch are found here. Char and whitefish are particularly important for commercial fishing, while trout and perch attract many recreational fishers.

Data collection and analysis are conducted by SLU Aqua in close collaboration with county administrative boards, water conservation associations, and other regional actors. Parts of the data collection are included in the national environmental monitoring sub-programme – "Stora sjöarna" – within the Swedish Agency for Marine and Water Management’s freshwater programme.

Trend- and reference lakes 

This involves long-term, recurring fish surveys in relatively unaffected smaller lakes across the country. The programme tracks long-term trends in environmental status. Read more about the sub-programme Trend lakes, part of the freshwater programme within national environmental monitoring.

Integrated studies of the effects of liming acidified waters, ISELAW

The programme tracks the effects of acidification and liming in lakes, where SLU Aqua is responsible for fish surveys and sampling. More information about the effects of liming acidified waters, ISELAW, is available via the Swedish Environmental Protection Agency.

Data stewardship

SLU Aqua is the data steward on behalf of the Swedish Agency for Marine and Water Management, and raw data from gillnet surveys in lakes are stored and made accessible in the lake fish survey database – NORS.

Crayfish data (including survey results and plague-declared waters) are stored in the crayfish database.

Lake fish surveys in practice

To monitor changes over time and compare different areas, we always use standardised methods in our surveys. Gillnet fishing, which catches fish of various sizes at different depths, is the most common method. The fish are identified, weighed, and measured – sometimes also analysed for age, sex, and presence of disease.

Hydroacoustic methods (echosounding and trawling) are used in large lakes to estimate the abundance and size distribution of pelagic fish, such as vendace and smelt.

Crayfish surveys are conducted using baited traps.