Reversed Diel Horizontal Migration of Fish: Turbidity Versus Plant Structural Complexity as Refuge

ABSTRACT

Diel horizontal migration (DHM) by small perch (Perca fluviatilis) between the littoral vegetation zone and open water was studied in the eutrophic Kirkkojärvi basin (southern Finland). Small perch showed reversed DHM by occupying the open water area during daylight and moving into the littoral zone at dusk. They avoided the vegetation zone during daylight because water clarity among the plants was much higher than in the open water, which was attributed to the reductive effect of macrophytes on sediment resuspension. The migration of perch into the littoral zone at dusk was explained by the higher availability of zooplankton among the plants. The results indicated that the importance of structural complexity of vegetation as a refuge decreases with increasing turbidity of the water. The effects of macrophytes on water turbidity may create circumstances where predation threat for small planktivorous fish during daylight is highest among the vegetation, and such circumstances may thus induce reversed DHM.