Brucet, Sandra. 2016. Effects of fish predation on density and size spectra of prey fish communities in lakes [in special issue: Size-based approaches in aquatic ecology and fisheries science: a symposium in honour of Rob Peters]. Canadian Journal of Fisheries and Aquatic Sciences, 73 (4). 506-518. 10.1139/cjfas-2015-0034 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Planktivorous and benthivorous fish have been documented to influence the density and size 32 structure of their prey communities in lakes. We hypothesized that piscivorous fish modify their prey 33 fish communities in the same way and sought to find evidence for such predation effects from a 34 comparison across 356 lakes located in nine European ecoregions. We categorized individual fish as 35 being either piscivore, non-piscivore or prey of piscivores, depending on species and individual size. 36We calculated piscivore, non-piscivore and piscivore prey densities, respectively, and fit linear 37 abundance size spectra (SS) on lake-specific piscivore, non-piscivore and piscivore prey size 38 distributions. Multiple linear regressions were calculated to quantify the effect of piscivore density 39 and SS slopes on non-piscivore and piscivore prey densities and SS slopes, by accounting for 40 potentially confounding factors arising from lake morphometry, productivity and local air 41 temperature. Piscivore density correlated positively with piscivore prey density, but was 42 uncorrelated to density of non-piscivores. Across a subset of 76 lakes for which SS slopes of 43 piscivores were statistically significant, SS slopes of piscivores were uncorrelated with SS slopes of 44 either non-piscivores or piscivore prey. However, densities of piscivores, non-piscivores or piscivore 45 prey were a significant negative predictor of SS slopes of the respective groups. Our analyses suggest 46 that direct predation effects by piscivorous fish on density and size structure of prey fish 47 communities are weak in European lakes, likely caused by low predator-prey size ratios and the 48 resulting size refuges for prey fish. In contrast, competition may substantially contribute to between-49 lake variability in fish density and size. 50 51