Deforestation, overfishing, pollution, and construction of dams have severely impacted migratory fishes of western Venezuela. The coporo, Prochilodus mariae (Prochilodontidae), has supported the largest commercial fishery among the species that have seasonal long-distance migrations between rivers of the Andean piedmont and the llanos floodplains. During the period July 1988 to November 1990, coporo ecology was studied in the Río Boconó and the Boconó-Tucupido reservoir in estado Portuguesa. Coporo females mature at age 2 at 23 cm SL. Relatively few coporos were captured from the reservoir and the R. Boconó upstream. No coporos were encountered in the river > 3 km upstream from the reservoir nor in hypoxic regions of the reservoir > 4 m deep. In the river segment immediately downstream (0–4 km) from the Boconó Dam, the stock was comprised of mostly juveniles (78% < 17 cm SL), and this size/age structure was relatively stable over time. In recent years, overfishing downstream from the dam has reduced the densities and sizes of coporos in the R. Boconó. The river segment 4–16 km below the dam was comprised of 78% adults. Few coporos > 3 yr and > 30 cm SL were found in the rivers, whereas none were < 4 yr and < 30 cm in the reservoir. Detritus was consumed more than algae by coporos in the reservoir and by adults relative to juveniles among river-dwelling fish. In both reservoir and river fish, gonadal development was initiated during late November and peaked during May-June. We found no evidence of successful reproduction in the reservoir, and mature adults in the lower river segment disappeared during June, presumably having migrated to the floodplains of the low llanos for spawning. From mid-November 1989 until March 1990, coporo ‘ribazóns’ (schools of ascending migrants) of diminishing densities were surveyed from the R. Apure upstream to the R. Boconó. ‘ribazóns’ have been eliminated or greatly diminished in nearly 80% of the principal rivers of the Andean piedmont in western Venezuela. Management options to assist this economically and ecologically important species are discussed.