Against the flow: chemical detection of downstream predators in running waters

In English In running waters, chemical cues have generally been assumed to always come from upstream locations.
Here, we present ¢eld and laboratory evidence that Gammarus pulex can use chemical cues from down-
stream predators to adaptively adjust drifting behaviour. In the ¢eld, signi¢cantly fewer Gammarus
migrated into stream enclosures where brown trout (Salmo trutta) were present than into control enclo-
sures. In a subsequent laboratory experiment, Gammarus actively avoided live trout and trout chemicals
placed downstream in an arti¢cial stream, whereas no e¡ects were found in response to control or visual
cues.We suggest that the mechanism explaining the ability of Gammarus to detect downstream predators is
use of back£ows, which locally transport ¢sh chemicals against the main £ow. Such back£ows are both
created by the Gammarus itself and by surrounding substrate heterogeneity. These results profoundly a¡ect
the way in which we view the chemical environment of running waters and have important implications
for empirical and theoretical work evaluating predator e¡ects in running waters, as they demonstrate that
prey immigration rates can depend on downstream predator densities.