AGAINST THE FLOW - CHEMICAL-DETECTION OF DOWNSTREAM PREDATORS IN RUNNING WATERS

In running waters, chemical cues have generally been assumed to always come from upstream locations. Here, we present field and laboratory e vidence that Gammarus pulex can use chemical cues from downstream pred ators to adaptively adjust drifting behaviour. In the field, significa ntly fewer Gammarus migrated into stream enclosures where brown trout (Salmo trutta) were present than into control enclosures. In a subsequ ent laboratory experiment, Gammarus actively avoided live trout and tr out chemicals placed downstream in an artificial stream, whereas no ef fects were found in response to control or visual cues. We suggest tha t the mechanism explaining the ability of Gammarus to detect downstrea m predators is use of backflows, which locally transport fish chemical s against the main flow Such backflows are both created by the Gammaru s itself and by surrounding substrate heterogeneity. These results pro foundly affect the way in which we view the chemical environment of ru nning waters and have important implications for empirical and theoret ical work evaluating predator effects in running waters, as they demon strate that prey immigration rates can depend on downstream predator d ensities.