IES research development of natural fish resources in Latvian and Estonian lakes

During the autumn and winter seasons of 2020 Institute for Environmental Solutions (IES) post-doctorate Katrit Karus continued comprehensive research on food webs in Latvian and Estonian lakes by estimating the development of natural fish resources. Research is carried out in 8 lakes, 4 in Estonia – Akste, Nohipalo Valgjärv, Kaiavere, Prossa and 4 in Latvia – Riebinu, Auciema, Laukezers, Varzgune.


In the summer IES post-doctorate Katrit Karus together with researchers from Estonian University of Life Sciences carried out lake fieldwork campaigns in Estonia and Latvia. After completion of the fieldwork campaigns, researchers started laboratory analysis of gathered samples. Results of sample analysis and collected data will help to understand how food webs function in different types of lakes and how it impacts fish stock. Thereby, helping to develop research data-based plans for lake management that includes both economic and ecological solutions.

The researchers carried out lake sampling campaigns during spring, summer and autumn in 2019, as well as repeated campaigns during summer in 2020. The main tasks were to estimate the development of natural fish resources, to collect data about the general characterization of the studied lakes, as well as to gather physico-chemical data and to collect fish community samples.


In 2020, researchers repeated fish community studies to measure the development of the young fish. The focus was on 0+ fish (hatched in 2019) year class strength – fish survival and stock development after 1 year. During sampling campaigns (both in 2019 and 2020) researchers used Nordic gillnets for 12 hours in each of the studied lakes. All the gathered samples were analysed by measuring the total length and weight of all fish specimens caught with Nordic gillnets. Results were calculated and expressed in grams – weight per unit effort and number per unit effort. Thus, allowing to easily compere 0+ fish stock (hatched in 2019) development after one year. The summary of fish stock development data is highlighted in Image 1.


The mortality of newly hatched fish larvae is extremely high, eliminating up to 99% within a few days after the fish first feeding. Besides predation, fish larvae may die for many reasons.  During the early stages of life, starvation could be one of the main causes of high mortality. It happens because of the shortage of suitable food objects (size, morphology).

To evaluate importance of larval and juvenile fish prey the researcher K.Karus calculated food object consistency and feeding strategies at the population level. Fish feeding data gathered in 2019 showed several dietary importance indicators:

  • Relative importance of food type percent by number (%N)
  • Percent by weight (%W)
  • Percent frequency of occurrence (%O)
  • Index of food items relative importance (IRI)
  • Percent index of food items relative importance (%IRI)
  • Food consumption index (Ti%)
  • Ivlev`s selectivity index (E)
  • Hurlbert's standardized niche breadth index (Ba)


Sample analysis and data collection is still in progress. The researchers will continue to implement them until the spring of 2021. The next step of the research project will be dissemination of results. The researchers plan to develop an article about the comparison of larval fish feeding in littoral and open-water sites. It will be presented in the 10th International Shallow Lakes Conference, which will take place in a virtual format from 1st to 5th March 2021.

The researchers also will present the results in the 14th International Congress on the Biology of Fish. The event will take place from 6th to 9th July 2021 in Montpellier, France.


The research is a part of the project „Fish feeding conditions in lakes with different planktonic food web structure and microvegetation “(MICROFISH), No. Agreement with State Education Development Agency of the Republic of Latvia No. Programme number The project is financed by the European Regional Development Fund, the State budget of the Republic of Latvia and the foundation „Institute for Environmental Solutions”.

Find out more about the project here.