Co-existence of a pair of pelagic planktivorous coregonid fishes Ingeborg Palm Helland, 1 * Chris Harrod, 2,3 Jörg Freyhof 1 and Thomas Mehner 1 1 Department of Biology and Ecology of Fishes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany, 2 Department of Evolutionary Genetics, Max Planck Institute for Limnology, Plön, Germany and 3 School of Biological Sciences, Queen’s University Belfast, Belfast, UK ABSTRACT Hypothesis: Ecological specialization facilitates co-existence of Coregonus spp. in Lake Stechlin. A difference in trophic ecology is the dominant means by which the species are ecologically segregated. Background: Sympatric fish species pairs in post-glacial lakes often feed on different resources, segregating available trophic resources. Organisms: Sympatric European winter-spawning Coregonus albula and the local endemic dwarf-sized spring-spawning Coregonus fontanae. Time and place: March–December 2005, Lake Stechlin, North Germany. Methods: By combining stomach content analyses and stable isotope analyses we compared consumption patterns of the two species at different depths over a 10-month period. Results: Stable isotope analyses and stomach content analyses both showed little trophic difference between the two species, but a significant effect of capture depth and body size on individual diet. Conclusions: The sympatric species pair in Lake Stechlin does not follow the expected pattern of niche segregation. Trophic divergence is not the dominant grounds for co-existence. Keywords: δ 13 C, δ 15 N, niche overlap, reproductive isolation, resource competition, stable isotope analysis, stomach content analysis, sympatric species. INTRODUCTION Resource competition is a fundamental concept in ecology and classical competition theory predicts that sympatric species cannot have completely overlapping niches. The principle of competitive exclusion states that one competing species will always have an advantage that will lead either to extinction of the poorer competitor or to an evolutionary shift towards a *Address all correspondence to I.P. Helland, Department of Biology and Ecology of Fishes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, PO Box 850 119, D-12561 Berlin, Germany. e-mail: helland@igb-berlin.de Consult the copyright statement on the inside front cover for non-commercial copying policies. Evolutionary Ecology Research, 2008, 10: 373–390 © 2008 Ingeborg Palm Helland