Large female fish important for fish population recovery

Our most-fished predatory fishes – perch, pike and zander – produce better offspring when the female fish increase in size. Intensive fishing changes the size structure of fish populations and the function of the entire aquatic ecosystem. However, it is possible to change the characteristics of fish fry, as the regulation of fishing impacts the structure of fish populations.

These were the findings from Project KESKALA: Studies on Ecologically Sustainable Fishing, carried out by the University of Helsinki and the Natural Resources Institute Finland between 2005 and 2017.

In perch, intense fishing decreased the quality of the roe eggs, as there were fewer large spawning female fish, and the females reached sexual maturity at an earlier age, when smaller. In addition, the roe eggs were smaller in relation to the size of the females than they were before fishing commenced.

–  It is also alarming that fishing can cause genetic changes in fish populations if only one type of fish is able to reproduce, explains Professor Hannu Lehtonen from the University of Helsinki’s Department of Environmental Sciences.

In pike, fishing increased the amount of food available to the remaining pike, and larger female pike produced heavier roe eggs. However, even among pike, the number of large spawning females fell as fishing continued.

–  Intensive fishing regulated with minimum size limits can remove all large female fish in four years, states Lehtonen.

The density, biomass and size structure of the pike population were better protected through slot limits, in which only individuals between 40cm and 65cm in length could be fished.

One of the conclusions from the study is that the structure of fish populations can be controlled by regulating fishing.

–  Regulating through minimum size limits alone may not be the most sustainable solution. The characteristics of the fish population in question must also be considered, emphasises Lehtonen.

For example, for rapidly growing populations of zander, a minimum size limit of 50cm is appropriate. However, when fishing is particularly intense, even slot limits cannot solve the common problem in which fish that spawn when small produce the most fry, leading to a smaller average size at which fish reach sexual maturity and ultimately lower population productivity.

Clear water is important

In the project, it was found that if the water got darker, the competitive relationships between the species altered. Darker water makes it difficult for perch to feed on fish and hinders predation. Consequently, the fish are smaller than average. In clear water, the perch are more active and more efficient at eating animal plankton than the common roach. However, when visibility in the water becomes lower, the perch lose to the roach in the competition for food.

Climate change is making lakes warmer and darker. The exposure of female fish to poorer living conditions and other environmental disorders may have a compound effect on the fish populations, as larger females produce the most young which are most likely to survive. Female fish are also more sensitive than males to environmental pressures, such as darker water, higher temperatures and fishing.

As fishing targets large perch, larger organisms which live on the lake bottom become significantly more numerous. Meanwhile, if smaller perch are culled, the bottom community remains relatively stable. Changes to the animal plankton community are more dependent on the behaviour of perch and the internal dynamics of the animal plankton community than the number of the perch.

–  Fishing may result in very unexpected changes in the food chain, states Lehtonen.


Project SUSFISH – The principles of sustainable fishing of percids and pike

Results from the KESKALA project have been published in the special issue of the Boreal Environment Research series,


more information:

University of Helsinki (

DSc (Agriculture and Forestry) Mikko Olin, tel. 050 4486452

Professor Hannu Lehtonen, tel. 050 4150625

DPhil Satu Estlander, tel. 044 5020608

Natural Resources Institute Finland (

Researcher Jukka Ruuhijärvi, tel. 0400 219613