abstract: The regularly inundated region of the middle Amazon river together with its marginal lakes is characterized by special hydrographic, hydrochemical and biological conditions [1-4]. It is called "Várzea". Regular investigations of such Várzea lakes revealed several correlations between changes in water level, thermic and chemical layering of the water, and fish behaviour. 1. Starting in August, the lakes begin to dry out. Consequently large amounts of plant material from floating plant mats undergo decomposition. 2. The rising of the water level in the Solimões (R. Amazon) which begins in December, results in filling of the lagoons with "Weißwasser". This leads to a rich primary production in the uppermost water layer whereas the layers at the bottom become heavily reduced in O2. 3. Special weather conditions (persistent winds from the south) may cause a complete mixing of the epi-and hypolimnion, accompanied by an abnormal drop in air and water temperature ("friagem"). Thus water poor in O2and rich in H2S is brought to the surface causing typical signs of asphyxia in all fish and even mass mortality in some species. 4. Fish living in the Várzea lakes show a conspicious migration pattern related to the change of wet and dry periods. 4.1. Spawning migrations from the lakes into the Solimões in the starting phase of the inundation period (February-April). Cause: the better oxygenated spawning grounds in the river and the utilization of the rich food sources in the floating plant masses. 4.2. Back migration of adult fish (a few days after spawning) and of the growing fry (during the subsequent period of high water). Cause: utilization of the rich terrestrial food sources in the Igapós (=inundated forest). 4.3. Mass migration from the Igapós and lakes in the dry season (September). Cause: the decrease in space and food supply. 4.4. Gradual back migration from the river at the beginning of the wet season (December-January). Cause: the rich primary production in the newly filled lakes.
authors.affiliation: Instituto Nacional de Pesquisas da Amazônia, Manaus, Brasilien.
authors.forename: Umberto
authors.initials: U
authors.lastname: de Menezes Santos