Non-lethal effect of the invasive predator Bythotrephes longimanus on Daphnia mendotae

1. We evaluated the antipredator behaviour of Daphnia mendotae to the invasive invertebrate predator, Bythotrephes longimanus, and the consequent effect of the predator on prey growth rate (referred to as a non-lethal effect of the predator). 2. In a laboratory experiment, Daphnia in the absence of Bythotrephes kairomones remained in the top, warmer regions of experimental columns, whereas in the presence of Bythotrephes kairomones, Daphnia migrated vertically, occupying a middle region by night and a low, cold region during the day. Over a 4-day experiment, the vertical migration induced by Bythotrephes caused a 36% reduction in the somatic growth rate of Daphnia, a level that is sufficient to have an effect on prey population growth rate. 3. A second laboratory experiment indicated that concentrations of Bythotrephes kairomones in water taken directly from the field (Lake Michigan) were high enough to induce behavioural shifts that led to these large reductions in somatic growth rate. 4. Our results identify a means by which Bythotrephes has substantial effects on native prey populations other than through direct consumption.