Automatgenererad bild.

Foto: Matilda Miljand

Published

18 June 2015

Biological method effective in reducing algal blooms

Eutrophic lakes can be restored by reductions in their roach and bream stocks. This is stated by Mistra EviEM in a systematic review of ‘reduction fishing’. The results confirm that biological methods — ‘biomanipulation’ — can be key environmental management tools.

Oxygen depletion, turbidity and algal blooms: these problems of eutrophication beset many lakes, mainly in the south of Sweden. ‘Reduction fishing’ has been used in many places as a simple, low-cost way of restoring a eutrophic lake by biological means. The method prevails above all in Denmark and the Netherlands, but it has also been used in Finland and North America, for example. In Sweden, reduction fishing has been used on a relatively limited scale since the1980s.

‘Although reduction fishing has shown good results in many places, it has long been questioned whether the method really is effective,’ says Professor Per Larsson of Linnaeus University, who headed the review carried out by the Mistra Council for Evidence-Based Environmental Management (EviEM).

Reduction fishing means using various types of fishing equipment to remove large numbers of cyprinids (fish in the carp family), such as roach and bream. With smaller bream and roach populations, the whole lake ecosystem can be modified: zooplankton numbers rise and, since they eat phytoplankton, the latter decline in number in the lake and, by the same token, the risk of algal blooms decreases. Moreover, it results in clearer water and reduced oxygen depletion.

Two-year review

This is the theory. And Mistra EviEM has thus now obtained solid scientific evidence that reduction fishing is indeed a useful way of improving water quality in eutrophic lakes. A group of experts have devoted two years to compiling all the academic literature in this area. In an initial inventory, some 14,000 relevant studies were found. The final report is based on studies of a total of 123 lakes in Europe and North American where reduction fishing has been applied.

‘The key conclusion is that the method works as it was believed to do. This is not true of all lakes, but only of a few. Water transparency increases and the quantity of plant plankton in the lake decreases. These are the two parameters we have studied,’ says Per Larsson.

The effects may last for several years. But if the basic problem is not remedied, i.e. the continuous influx of phosphorus from, for example, farmland and sewerage is reduced, the intervention will have to be repeated regularly.

Making biological methods possible

Today, reduction fishing is used in about 20 lakes, according to Larsson.

‘It’s not a final solution to the problem. It may serve as a supplementary method, but there’s no point believing it should be spread on a larger scale.’

Traditionally, stakeholders in environmental conservation have had great faith in engineering treatments of lakes, such as dredging and liming, while a biomanipulative method like reduction fishing has encountered more scepticism. Larsson sees great value in EviEM’s review in terms of establishing the principle. Public agencies and others will be made aware of biomanipulation as a practical tool in environmental management.

‘We’ve shown that it’s feasible to use biological methods to achieve various aims in the management of lakes. Reduction fishing contributes, for example, both to improved scope for bathing and more varied fishing.’

Text: Henrik Lundström, Vetenskapsjournalisterna

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