The transitional ecotones between lakes and rivers have received less attention than those of terrestrial ecosystems and can meet particular characteristics of great importance for freshwater biodiversity conservation. In order to evaluate this, we studied the spatio-temporal dynamics of the abundances and body sizes of the populations of 13 species of Chilean native fishes, in the ecotonal area Rinihue Lake - San Pedro river, located in the Valdivia river basin (Chile). Thirty five sites were sampled monthly, located in three hydromorphological zones of the study area (number of sites by area: M1 = 9, M2 = 17 M3 = 9). The results of this study show the presence of a dynamic ecotone both spatially and temporally, supporting the hypothesis that these transitional zones have special characteristics that must be taken into account for conservation of biodiversity.