Feeding behaviour of two functional groups of 0+ perch Perca fluviatilis (epilimnetic, staying all 24 hours in epilimnion; hypolimnetic, daily migrating between hypolimnion and epilimnion) were investigated in the deep canyon-shaped Slapy Reservoir (Czech Republic) during two 24-h periods in late May and mid June 2002. Densities of most favoured cladocerans and copepods were generally higher in epilimnetic than in hypolimnetic zones. The two 0+ perch groups fed predominantly on cyclopoid copepods during the daytime in May. In June, epilimnetic perch fed on cladocerans (Daphnia sp., Diaphanosoma brachyurum), whereas hypolimnetic perch preferred calanoid copepod Eudiaptomus gracilis. Throughout darkness, when nearly all perch occupied upper strata, their gut contents were clearly dominated by cladocerans Daphnia sp. and Diaphanosoma brachyurum in May and June, respectively. Digestive tract fullness (DTF) of hypolimnetic perch was 2.0–2.8-times lower than the DTF of epilimnetic perch, and a higher share of perch with empty digestive tracts was found in the hypolimnion. Maximum DTF occurred in the epilimnion during the day and/or dusk, whereas at night and dawn progressive evacuation of guts was recorded and migrants returned with low DTF back to the hypolimnion. Low zooplankton abundance, unfavourable light and temperature conditions in the hypolimnetic zone are suboptimal both for prey searching and for overall metabolic processes.
Many species of chironomids undergo their immature development in habitats that rapidly change in suitability, such as rain pools, phytotelmata, freshly filled ponds or soil layers that seasonally dry out. Strategies for the utilization of these habitats can be divided into two groups: i) physiological or behavioral adaptations of larvae, which enable them to survive unsuitable conditions (in situ resistance) or ii) repeated recolonization of temporarily suitable habitats. In situ resistance, includes desiccation or frost resistance, often in combination with cocoon building and migration of larvae into the sediment. Generally, the species that use the recolonization strategy tend to be better at migration and have a higher fertility and shorter development. Recolonization may include only temporary habitats or temporary habitats and some more stable habitats as well.