Ecology, fisheries and management of wild brown trout populations in boreal inland waters

Abstract

Eutrophication of aquatic ecosystems is an increasing problem, especially threatening small and shallow freshwater lakes. While the extreme inputs of nutrients (phosphorus and nitrogen) can nowadays be reduced and controlled rather efficiently, additional measures to reverse eutrophication may be needed. These often involve biomanipulation, typically mass removal of cyprinid fish. However, the success of biomanipulations has many times been limited or short lived. The reasons for this are not well known, perhaps because the more subtle impacts of biomanipulation on the ecosystem-wide processes of lakes have not been thoroughly studied. Natural abundance stable isotopes may provide a cost-effective tool to study such ecosystem level impacts of food web perturbations. In this thesis, I used stable isotope analysis (SIA) to study the impacts of biomanipulation on lake ecosystem properties in Jyväsjärvi, a lake recovering from severe eutrophication. Variability in stable isotope signatures within the studied lake was examined as a prerequisite for reliable SIA. Temporarily lower δ15N values of fish in one area of Jyväsjärvi resulted from extensive fish migration into Jyväsjärvi from the connected lake Päijänne. Stable isotope data indicated that the migration was considerably reduced following biomanipulation and the immigrant fish from Päijänne did not quickly recolonise Jyväsjärvi. Isotopes also indicated significant changes in feeding niche widths of perch and roach after fish removals. From a wider perspective, contribution from pelagic energy sources to higher trophic levels in the Jyväsjärvi ecosystem increased after biomanipulation, in contrast to expectation. To enhance the potential of SIA in ecological studies, I also tested the possibility of using archived sample materials in retrospective SIA, which could allow for longer time perspectives in such ecosystem studies. I found that typical archived freshwater samples do provide valid materials for SIA.