Date: 10.08.2023

How many fish are in the lake? Extensive study by hydrobiologists provides answers for European waters

For fifteen years, hydrobiologists from the Biology Centre of the CAS monitored fourteen reservoirs to refine models for calculating fish abundance in reservoirs and lakes. The results of this large-scale study will help interpret data from thousands of European sites.

The question of how many fish are in the lake comes to mind every time when looking at any of the water bodies. All the more it puzzles fishermen, ecologists and quantitative scientists who need to quantify the productivity of waters and understand the role of fish in aquatic ecosystems. However, the answer to a seemingly simple question is not simple in larger waters. Difficulties arise in the waters that cannot be fished completely. Typical examples are dam reservoirs and lakes. Currently, the main method of studying the quantity and composition of fish communities is the use of multi-mesh gill ets according to the European standard EN 14 757. They are used in almost all European countries from Portugal to Turkey and from Italy to Norway.

Gillnets are a typical example of a so-called "passive" fishing device. The nets are set and passively wait for the fish, which must be actively hit in order to be caught. The main advantages of this approach, which led to its mass use, are relative ease, robustness, comparability and the possibility of use in practically all types of lake habitats. The main disadvantages are destructiveness (most fish do not survive sampling) and uncertainty regarding the quantification of the area fished. So far, gillnets have thus provided relative fish abundance and biomass that were thought to reflect actual fish abundance. However, the scientists did not yet have available a calculation how to derive the actual abundance or biomass of fish in the sampled locality from the gillnet catches.

Inspection of gillnet catch. Photo D. Procházková


In addition to gillnets, reservoirs and lakes can also be sampled with so-called active sampling means, i.e. those that sample fish independently of their will, by actively fishing or viewing known volumes and areas. These means thus provide the necessary data for calculating the amount of fish in a given body of water, but their use is in many ways more demanding and has greater limitations than in the case of gill nets.

The Institute of Hydrobiology of the Academy of Sciences of the Czech Republic has been dedicated to improving active and passive sampling methods for many years (For the most recent review, see Kubečka et al. 2022, ). Aware of the limitations of gill nets and their great importance, we carried out combined monitoring in most of the studied locations, where catches with gill nets were accompanied by active sampling with trawl nets and scientific echo sounders. The experiments were carried out so that nets and active means always sampled the same type of habitat – the coastal part (littoral) in the case of gillnets v/s seine nets and open water (pelagial) in the case of pelagic gillnets and scientific echo sounders. Comparative monitoring took place for 15 years at 14 Czech reservoirs. The comparison between fishing gear was carried out using modern statistical procedures in cooperation with the Institute of Informatics of the Academy of Sciences of the Czech Republic. These procedures made it possible to take into account the great variability of the biological data and to clearly demonstrate that there is a direct relationship between the amount of fish in gillnets and the amount estimated by the active sampling approaches.

Active fishing with seine nets. Photo by S. Miranda


The calculated nature of the derived dependence between nets and active fishing gear makes it possible to solve the age-old problem of how to convert, for example, the biomass of fish caught by nets to the actual biomass of fish, which is most often given in kilograms per hectare of water surface. Very encouragingly, this conversion relationship does not differ between open water and littoral. This agreement indicates that we have established a universally valid conversion factor that can find application in a wide variety of inland waters. The found conversion factor confirms the high catching efficiency of gill nets. If we use gill nets with twelve mesh sizes of 5.5-55 mm according to the original European standard, 1 m2 of gill net will catch fish biomass from 8 m2 of the investigated tank in a single night. If we use the original construction with the so-called big-mesh gillnets (Šmejkal et al. 2015 ), we catch a wider size spectrum, including large fish, which, however, are much less abundant in the waters (most fish are naturally small), then 1 m2 of netting will catch fish from 5 m2 of the investigated reservoir in one night. By analyzing a huge amount of complex data, simple conversion coefficients were created that will allow us to determine the amount of fish in many European waters.

A research vessel with scientific sonar transducers. Photo: J. Kubečka


The study was published in open access mode in the prestigious journal Ecological Indicators:  

Říha M., Prchalová M., Brabec M., Draštík V., Muška M., Tušer M., Bartoň D., Blabolil P., Čech M., Frouzová J., Holubová M., Jůza T., Ribeiro de Moraes K., Rabaneda Bueno R., Sajdlová Z., Souza A., Šmejkal M., Vašek M., Vejřík L., Vejříková I., Peterka J., Kubečka J. (2023) Calibration of fish biomass estimates from gillnets: Step towards broader application of gillnet data. Ecological Indicators 153, 110425.  




Biology Centre CAS
Institute of Hydrobiology
Na Sádkách 702/7
370 05 České Budějovice

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