A species’ reproductive tactics and life history strategy are the result of a complex evolutionary interplay where individuals constantly adjust their energy investment in accordance with the environmental conditions. Fecundity is a key feature of life that reflects the amount of energy that an individual invests in reproduction. Little is known about the response of this life history feature of species that persist in reservoirs, and for this reason, fecundity and some variables related to it were analyzed for some of the most abundant species within the fish assemblage in the area of influence of the cascading reservoirs Porce II and Porce III. Sampling was carried out from September 2011 to December 2016, during contrasting climatic periods (dry and rainy seasons), in 31 sites distributed throughout Porce II and Porce III reservoirs, and also including the river channel and tributary streams of the reservoirs. It was found that Astyanax microlepis had the highest value of total fecundity (20.202 eggs on average), while Poecilia reticulate had the lowest values (15 embryos). A relation was found between the number of eggs and the size of the fish (standard length and body weight). The fecundity of the species analyzed was similar among those phylogenetically related (species of the same family), and in turn, it was determined that this reproductive trait was specific to each species.