Journal of Fish Biology (2009) 74, 2115–2131 doi:10.1111/j.1095-8649.2009.02289.x, available online at www.interscience.wiley.com Piscivory and trophic position of Anguilla anguilla in two lakes: importance of macrozoobenthos density H. D ¨ orner*†‡ , C. Skov§, S. Berg§, T. Schulze*¶, D. J. Beare†** and G. Van der Velde††‡‡ *Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Dept Biology and Ecology of Fishes, M¨ uggelseedamm 310, 12587 Berlin, Germany, †European Commission, Joint Research Centre (JRC), Institute for the Protection and Security of the Citizen (IPSC), Maritime Affairs Unit, TP 051, 21027 Ispra (VA), Italy, §DTU AQUA, National Institute of Aquatic Resources, Technical Universityof Denmark, Vejlsøvej 39, DK-8600 Silkeborg, Denmark., ¶Dresden University of Technology, Institute of Hydrobiology, 01062 Dresden, Germany, **Wageningen IMARES, Ijmuiiden, The Netherlands, ††Radboud University Nijmegen, Institute for Wetland and Water Research, Department of Animal Ecology and Ecophysiology, Nijmegen, The Netherlands and ‡‡National Museum of Natural History Naturalis, P.O. Box 9517, 2300 RA Leiden, The Netherlands The feeding habits of the European eel Anguilla anguilla (>300 mm total length, L T ) were compared in two lakes of different environmental state: Lake Großer V¨ atersee (LGV), Germany (clear water, mesotrophic and submerged macrophytes), and Lake Vallum (LV), Denmark (turbid, eutrophic and no submerged macrophytes). The density of macrozoobenthos was higher in LV (3500 individuals m −2 ) than in LGV (1500 individuals m −2 ). The abundance of small prey fishes (40–99 mm L T ) was highest in LV. In LV, A. anguilla fed on macrozoobenthos, in particular, chironomid larvae. In LGV, A. anguilla used fishes as the main food component. Stable isotope analyses confirmed the stomach contents dietary results. The estimated mean ± s.d. trophic positions of A. anguilla in LGV (3·7 ± 0·2) was one level higher than those of fish in LV (2·7 ± 0·2). Based on these results, it is concluded that piscivory among A. anguilla was generally controlled by the density of macrozoobenthos. Stable isotope analysis further indicated that A. anguilla may act as integrators between benthic and pelagic food webs when density of insect larvae is low. © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles Key words: fish; lakes; predation; prey; stable isotopes. INTRODUCTION Throughout its distribution, several populations of the European eel Anguilla anguilla (L.) have been declining for the last 30 years or more (Moriarty & Dekker, 1997; Dekker, 2004). Reasons for the decline are not yet fully understood, but possibilities have been suggested, several of which have an anthropogenic basis (Feunteun, 2002; Dekker, 2004) and recent papers strongly implicate effects of ocean climate on ‡Author to whom correspondence should be addressed. Tel.: +39 0332789343; fax: +39 0332789658; email: hendrik.doerner@jrc.it 2115 © 2009 The Authors Journal compilation © 2009 The Fisheries Society of the British Isles