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Homepage Ruedi Müller

Academic & research interests

Ecology and management of fish in lakes, particularly coregonids

Fish in eutrophic lakes:
I address the general question how lake eutrophication affects fish, in particular which fish species are affected in which way. Thereby, the whitefishes (coregonids) are of primary interest because their natural reproduction is impeded or even completely interrupted in eutrophic lakes. In addition, growth of most fish species increases with increasing trophic state which may require an adjustment in fishing practice. Proper understanding of such processes is a prerequisite to adequately manage coregonids and other fish species in eutrophic lakes. It allows, e.g., to optimize the success of stocking. Investigations on the functioning of natural reproduction of coregonids will also help to judge the success of internal lake restoration measures, such as hypolimnetic oxygenation.

Fish in re-oligotrophicating lakes:
Trophic state in terms of phosphorus has significantly decreased over the past 20 years in almost all Swiss lakes. Declining trophic state leads to slower growth of many fish species, and to changes in the structure of species assemblages. Most affected are plankton-eating species, such as most coregonids. The fish become more slender and stay smaller when a lake shifts from mesotrophic to oligotrophic. As a consequence, females produce fewer eggs and larval fish suffer higher mortality due to the scarcity of suitable food. This affects year class strength and ultimately fishing yield. Proper monitoring programs, combined with the analysis of catch statistics, allow to follow such developments. Laboratory experiments may help to explain and quantify certain phenomena that have been observed. The results will be used to adjust management of the populations, if needed. It is hoped to transfer the results to other lakes in a similar situation. The general aim is to maintain, in spite of decreasing productivity, a certain fishing yield without over-exploiting the stocks.

Population genetics and evolutionary aspects
Coregonids are very fertile and easy to reproduce. This makes them excellent objects for studying certain questions, such as the potential modification of genetic integrity by artificial reproduction in hatcheries, or the significance of mate choice for offspring viability. This part of the work is carried out in close cooperation with Claus Wedekind from the University of Bern.

Curriculum vitae

2000-06, Chairperson of the European Inland Fisheries Advisory Commission (EIFAC) of FAO
1986-99, Head of the Department of Fisheries Sciences at EAWAG
1983, Visiting Scientist, Institute of Freshwater Research, Drottningholm, Sweden (7 months)
1975-2007, Senior researcher at EAWAG, Department of Fisheries Sciences
1974-75, Research Associate, Department of Zoology, University of Toronto, Toronto, Canada (11 months)
1974, Ph.D. Swiss Federal Institute of Technology (ETH) Zürich

1973, Visiting Scientist, Freshwater Fisheries Research Laboratory,
Tokyo, Japan (7 months)
1968, M.Sc. Swiss Federal Institute of Technology (ETH) Zürich

Consulting

Fisheries biological investigations on lakes Hallwil, Sempach, Lucerne and Brienz, under contracts from Cantons Aargau, Lucerne and Bern. Monitoring of whitefish populations, assessment of the fisheries in relation to lake trophic state, elaboration of recommendations regarding fisheries management. Assessment of the effects of lake internal restoration measures (hypolimnetic oxygenation, forced mixing with compressed air) on the fish populations.