This work tests the null hypothesis that the coefficients of the total landings, landed values, mean catches and price per kg of migratory and resident species are constant over time following the installation of two large run- of-the-river hydroelectric dams in a large tropical river. To identify shifts in catches and economic returns due to river impoundment, we inspected daily landing data (25-year time series) and wholesale prices (19-year time series) for the Madeira River, the largest tributary of the Amazon River. Our results show that the period of decreasing catches and increasing prices observed for fisheries in the Madeira River matched the timings of the construction of the two dams. According to the results, both dams quickly changed catches and fish supply to market, which were immediately echoed in the price per kg of exploited fish species. Following the dam con- struction, prices rose for both fish that became scarce and fish that became abundant. Though catches declined 58% in 25 years, the price increased 49% over the same period, representing a high economic cost for the local population. Further, there was a clear decline in the catches of some species (e.g., the dourada and the curimatã), but increased catches of others (e.g., the sardine and the tucunaré). Moreover, some fluctuation patterns across years showed natural oscillations, or changes, in local habitats and even fishing efforts.