Historical Changes in Large River Fish Assemblages of the Americas

Abstract

<em>Abstract.</em>—The Rio das Velhas is a tributary of the Rio São Francisco, one of Brazil’s largest rivers. It is the Rio São Francisco’s second most important tributary in water volume (mean annual discharge of 631 m<sup>3</sup>/s), with a drainage area of 27,867 km<sup>2</sup>, length of 761 km, and mean width of 38 m. Like many other rivers around the world, it became heavily polluted in the 1900s. The Rio das Velhas is the most polluted river of Minas Gerais state because the basin contains approximately 4.5 million people. Unlike other Brazilian rivers, its fish fauna was studied from 1850 to 1856. Fifty-five fish species were recorded; 20 of them were first described at that time, when there were previously no more than 40 known species in the entire São Francisco basin. Recent fish collections, approximately 150 years later, indicate 107 fish species, but some may be locally extinct. There are good prospects of rehabilitating this fauna because of the connectivity of the Rio das Velhas with the São Francisco main stem, its well-preserved tributaries, and increased investments in sewage treatment.