Mitigating eutrophication effects by use of bio-manipulation

There are studies that suggest that diminishing stocks of predatory fish could exacerbate symptoms of eutrophication in sea areas. This project has studied if it is possible to increase the amounts of predatory fish by restocking pikepearch and thereby decrease the symptoms of eutrophication such as plankton blooms and filamentous algae. The project restocked 870 000 juvenile pikepearch during a period of 4 years but in spite of this, no substantial increases of pikepearch has been achieved in the area. It is therefore concluded that restocking of pikepearch is not a suitable method. The final report “Mitigating eutrophication by means of biomanipulation” can be downloaded under project material below (in Swedish).

Background
Experiments in lakes suggest that removal of top-predatory fish results in trophic cascades with increased abundance of small fish feeding on zooplankton, and an increased production of phytoplankton, with turbid water and algal blooms as some of the consequences. Taking measures to strengthen the populations of large predatory fish may thus, besides the obvious effects on fisheries, potentially have positive effects on water quality and ecosystem status.

Objectives
The project is the first of its kind in a sea area and the purpose is to research if an increase of predator fish will change the relation between the organisms in the food web so that the effects of eutrophication are reduced. The total amount of released pikeperch will be around 900 000 in three years time. The result could be an essential tool to reduce eutrophication that might supplement other actions and water treatment plants. This could also show the importance of a well managed fish stock.