Diurnal distribution and behavioral responses of fishes to extreme hypoxia in an Amazon floodplain lake

Because of the need for surface access for aquatic surface respiration (ASR), fish density increases were demonstrated for the open water of a floodplain lake during severe hypoxia. This indicates an O2-induced diurnal pattern of horizontal migrations between the zone of macrophyte cover and open water. Supplemental experimental investigations seem to suggest that species such as characoids, Colossoma macropomum and Schizodon fasciatum, deviate from this pattern. During long periods of oxygen depletion, they return to the region of macrophyte growth and survive there without displaying the usual kind of ASR. Mortality studies in net cages exposed in natural water bodies confirmed that only these two species are able to survive severe hypoxia beneath macrophyte cover. The possibility of an O2-input through the root system of plants is discussed. The O2-concentration has a significant influence on the locomotory behavior and the frequency of opercular movement in characoids. There is significantly less locomotory activity beneath the macrophytes during periods of oxygen depletion among those species not forced to migrate than among those in the open water regions, where normal ASR behavior is possible. © 1987 Dr W. Junk Publishers