Effects of brown and turbid water on piscivore–prey fish interactions along a visibility gradient LYNN RANA ˚ KER, MIKAEL JO ¨ NSSON, P. ANDERS NILSSON AND CHRISTER BRO ¨ NMARK Department of Biology, Aquatic Ecology, Ecology Building, Lund University, Lund, Sweden SUMMARY 1. Environmental changes such as eutrophication and increasing inputs of humic matter (brownification) may have strong effects on predator–prey interactions in lakes through a reduction in the visual conditions affecting foraging behaviour of visually oriented predators. 2. In this experiment, we studied the effects of visual range (25–200 cm) in combination with optically deteriorating treatments (algae, clay or brown humic water) on predator–prey interactions between pike (Esox lucius) and roach (Rutilus rutilus). We measured effects on reaction distance and strike distance for pike and escape distance for roach, when pike individuals were exposed to free-swimming roach as well as to roach held in a glass cylinder. 3. We found that reaction distance decreased with decreasing visual range caused by increasing levels of algae, clay or humic matter. The effect of reaction distance was stronger in turbid water (clay, algae) than in the brown water treatment. 4. Strike distance was neither affected by visual range nor by optical treatment, but we found shorter strike distances when pike attacked roach using visual cues only (roach held in a cylinder) compared to when pike could use multiple senses (free-swimming roach). Escape distance for roach was longer in turbid than in brown water treatments. 5. Changes in environmental drivers, such as eutrophication and brownification, affecting the optical climate should thus have consequences for the strength of predator–prey interactions through changes in piscivore foraging efficiency and prey escape behaviour. This in turn may affect lake ecosystems through higher-order interactions. Keywords: behaviour, escape distance, foraging, reaction distance, visual range Introduction Predation by piscivorous fish is one of the most important structuring forces in lake ecosystems. However, individ- ual properties of predators and prey as well as environ- mental conditions may affect the relative strengths of predator–prey interactions (Miner & Stein, 1996; Fryxell & Lundberg, 1997; Persson et al., 1997). Most fish have a well-developed visual system and use it as their primary source of information (Guthrie & Muntz, 1993), including detection of prey. Their foraging may be impaired by environmental changes affecting the efficiency of the visual system, such as reductions in water transparency (Beauchamp et al., 1999; Engstro ¨m-O ¨ st et al., 2005; Shoup & Wahl, 2009) and ambient light intensity (Reid, Fox & Whillans, 1999; Mazur & Beauchamp, 2003). Several studies have shown that degraded visibility has a negative effect on search efficiency in fish because of a reduction in reaction distance (Gerritsen & Strickler, 1977; Reid et al., 1999; Utne-Palm, 2002; De Robertis et al., 2003). The effects of degraded visibility may, however, differ between piscivores and their prey, introducing a visual refuge for prey fish in turbid waters (Jacobsen, Berg & Jepsen, 2004; Lehtiniemi, Engstro ¨m-O ¨ st & Viitasalo, 2005). Moreover, when prey fish are attacked, their escape success can be heavily reduced in turbid waters owing to poor timing of the escape response (Meager et al., 2006). Depending on differences in response to degraded visibility conditions for piscivores and prey, the relative advantages ⁄ disad- vantages may change and affect structural patterns in the Correspondence: Lynn Rana ˚ker, Department of Biology, Aquatic Ecology, Ecology Building, Lund University, SE-223 62, Lund, Sweden. E-mail: lynn.ranaker@biol.lu.se Freshwater Biology (2012) 57, 1761–1768 doi:10.1111/j.1365-2427.2012.02836.x Ó 2012 Blackwell Publishing Ltd 1761