Studies of the ichtyofauna prior to the formation of man-made lakes, generally have towo main purposes: a) to get basic data which allow a monitoring of events after dw clousure, and b) to make predictions about the probable effects of impoundment on fish. As the ecological stress of impoundment will effect all the elements of the ecosystem in a complex and interactive manner, we considered, in this paper the fish community as a whole. The research was conducted in the lower Tocantins river, a tributary of the Amazon estuary. The stretch of the river under direct influence of the dam was divided in three sections with different physical characteristics. A section of rapids, at the site of the future lake, is surrounded by tow stretches of calmer waters, a small floodplain up river with numerous lakes, and a stretch of rougher water downstream in the upper part of Atlantic tidal influence. In each of these sections, fish communities were sampled by a gill net battery, Duice a year, during 3 consecutive years. A good fit of these samplings to the community structure models show that, in each of the localities, aunique and equilibrated community exists. The ecommunities are characterized by i) a great number of species probably as a result of a great long term stability of the environment, ii) a high equitability which can be related to the richness of the water in nutrients, and iii) a very high fish density. A spacial heterogencity of the communities is demonstrated and attributed to the limited geographical distribution of some esturine species and the specifie adaptations to particular biotopes.
This study represents the first stage of an attempt to analyse the effects of a dam in tropical region on the whole fish community, and particularly, to establish the sequences of events and the duration of the equilibration phase. In this stage, the results suggest on and long term reduction of fish species.