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The reproductive activity of the fish assemblage in a large Neotropical reservoir was evaluated by surveying the occurrence of ichthyoplankton in response to spatial and temporal variation in environmental factors. Fish reproduction was investigated by capturing larvae along representative spatial gradients in the main body (longitudinal) and arms (lateral) of the reservoir. After identification, the captured larvae were classified into different groups according to the functional traits of their parental stock. Sampling occurred monthly at night from October to March over five reproductive periods between 2009 and 2016, using conical‐cylindrical plankton nets with flowmeter. Water samples were taken simultaneously allowing the measurement of environmental factors related to reproductive activity. The relationship between the environmental factors and the functional traits was assessed using RLQ and fourth‐corner analyses. An evaluation of 51,995 larvae revealed there was greater larval abundance in the lacustrine and transition zones of both longitudinal and lateral gradients. Occurrences often correlated with temporal instead of spatial gradients. Higher rainfall correlated with abundances of detritivorous and migratory species. Higher water levels correlated with higher abundances of non‐native species, species without parental care, with external fecundation, and partial spawning. Dry periods, with lower water and higher dissolved oxygen, correlated with higher abundances of insectivores and sedentary species. Although the presence of a dam‐free stretch of river upstream of the reservoir had a positive influence on fish larvae in the fluvial zone, the results did not show differentiation along either gradient. Thus, the composition of the functional groups of fish species that reproduce in the Itaipu Reservoir appears to be predominantly influenced by the temporal variation in environmental factors rather than the formation of spatial gradients. [ABSTRACT FROM AUTHOR]