Antipredator behaviour of 0þ year Perca fluviatilis: effect of vegetation density and turbidity M. S NICKARS *†, A. S ANDSTRO ¨ M *‡ AND J. M ATTILA * *Huso ¨ Biological Station and the Department of Biology, A ˚ bo Akademi University, Akademigatan 1, FI-20500, Turku, Finland and ‡The Institute for Coastal Research, National Board of Fisheries, Sta ˚ngholmsva ¨gen 2, SE-17983, Drottningholm, Sweden (Received 6 November 2003, Accepted 7 September 2004) In this study, the combined influence of vegetation density and water turbidity on habitat utilization of a prey fish, 0þ year perch Perca fluviatilis, under predation risk (pike, Esox lucius) was investigated. The vegetated habitat was overall preferred over the open habitat in the presence of a predator. The level of turbidity, and to a lesser extent vegetation density, however, influenced the response of 0þ year perch. The use of the vegetated habitat was lower in very turbid than in clear and turbid conditions, suggesting reduced antipredator behaviour in very turbid water. The effect of vegetation density on antipredator behaviour was only present in clear water, where the use of a structural refuge decreased with increasing vegetation density. No such effect was observed in turbid and very turbid water. The results showed that the structuring role of vegetation or habitat complexity may diminish with increased turbidity. The observed masking effect of turbidity suggests that predator-prey interactions in vegetated habitats are more complex than what has generally been thought. # 2004 The Fisheries Society of the British Isles Key words: antipredator behaviour; habitat complexity; percid; predator-prey interaction; turbidity. INTRODUCTION Fishes are normally most susceptible to predators during early life-stages and give precedence to avoidance of predation when selecting habitat (Fraser & Cerri, 1982; Werner et al., 1983). Since mortality of prey is often negatively correlated with size (Werner et al., 1977), they also need to optimize foraging and growth, and as a consequence endure higher predation risk in order to utilize the most profitable habitats. Prey animals face a complex trade-off between mortality risk and availability of resources in different types of habitats (Werner et al., 1983; Christensen, 1997; Turner, 1997). When exposed to predators, fishes often seek shelter in structurally complex habitats, normally consisting of submerged vegetation, and tend to avoid open habitats (Werner et al., 1983; Gotceitas & Golgan, 1989). Such active decision- making, i.e. antipredator behaviour, plays a crucial role in the interaction †Author to whom correspondence should be addressed. Tel.: þ35822153426; fax: þ35822153428; email: masnicka@abo.fi JournalofFishBiology (2004) 65, 1604–1613 doi:10.1111/j.1095-8649.2004.00570.x,availableonlineathttp://www.blackwell-synergy.com 1604 # 2004TheFisheriesSocietyoftheBritishIsles