Journal of Fish Biology (2010) 76, 787–799 doi:10.1111/j.1095-8649.2009.02475.x, available online at www.interscience.wiley.com Morphological variation of perch Perca fluviatilis in humic lakes: the effect of predator density, competition and prey abundance J. Kek¨ al¨ ainen*†‡, J. K¨ ahk ¨ onen*, V. Kiviniemi§ and H. Huuskonen* *Ecological Research Institute, Faculty of Biosciences, University of Joensuu, P.O. Box 111, FI-80101 Joensuu, Finland, †Department of Biological and Environmental Science, University of Jyv¨ askyl¨ a, P.O. Box 35, FI-40014 Jyv¨ askyl¨ a, Finland and §Information Technology Centre, University of Kuopio, P.O. Box 1627, FI-70211 Kuopio, Finland (Received 2 March 2009, Accepted 2 October 2009) Between and within-lake variations in morphology of perch Perca fluviatilis were studied in four humic lakes in eastern Finland. P erca fluviatilis were more streamlined and smaller headed in a lake with the highest abundance of cyprinids, but lowest abundance of predators (Lake Tuopanj¨ arvi), indicating adaptation to planktivorous feeding and low predator density. Highest bodied fish were found from a lake with the lowest cyprinid but highest predator abundance (Lake Koppeloj¨ arvi), which conversely indicates adaptation to more effective predator avoidance. Furthermore, the length of the paired fins was longest in Lake Kinnasj¨ arvi and Lake Tuopanj¨ arvi, where the abundance of benthic macroinvertebrates was lowest, suggesting selection for more effective benthivory. Clear morphological differences of P. fluviatilis between habitats were found only in Lake Kinnasj¨ arvi, whereas in Lake Koppeloj¨ arvi and Lake Tuopanj¨ arvi only the length of the paired fins differed and in Lake Harkkoj¨ arvi no differences were found. Taken together, these results suggest that inter and intrapopulation morphological differences are probably highly dependent on different biotic factors (i.e. predation risk, resource availability and competition). Spatial and temporal variations in these factors may have a great effect on body morphology of P. fluviatilis. © 2010 The Authors Journal compilation © 2010 The Fisheries Society of the British Isles Key words: morphology; predation; population; resource availability. INTRODUCTION Morphological differences between species and populations often reflect adaptations to spatial differences in ecological conditions (Svanb¨ ack & Ekl¨ ov, 2002). This intra or interspecific polymorphism is often a consequence of divergent selection regimes between alternative environments. For example, fish species or subpopulations living in open habitats or structurally more complex environments often have a different body morphology, because the different selection regimes between habitats favour different phenotypes (McPhail, 1984; Webb, 1984; Webb & Weihs, 1986; Hjelm et al., 2000). Usually fish species that are specialized for pelagic conditions and ‡Author to whom correspondence should be addressed. Tel.: +358 13 251 4577; fax: +358 13 251 3449; email: jukka.kekalainen@joensuu.fi 787 © 2010 The Authors Journal compilation © 2010 The Fisheries Society of the British Isles