Abstract Nile tilapia, Oreochromis niloticus (L.), has been introduced in most tropical and subtropical regions of the world to improve inland fisheries and aquaculture. This species is generally viewed as beneficial where introduced, but the environmental impacts of its widespread introductions are uncertain and not well understood. In this study, a 30-year time series of fisheries records from a tropical reservoir was divided into five equal periods of 6 years and compared. The introduction of Nile tilapia did not increase the total catch-per-unit-of-effort (CPUE), the number of fishers actively fishing or their per capita income. Conversely, a significant reduction in the CPUE of other commercially important species was observed after the introduction of Nile tilapia in the reservoir. Although other factors cannot be rule out as possible explanations of the observed changes in the reservoir fisheries, the results are consistent with the hypothesis that these changes may have been caused, at least partially, by the introduction of the Nile tilapia.