Change in fish fauna as indication of aquatic ecosystem condition in Rio Grande de Morelia-Lago de Cuitzeo Basin, Mexico

The Rio Grande de Morelia-Lago de Cuitzeo basin in west central Mexico has experienced major increases in water pollution from a rapidly growing human population. We examined changes in the long-term distribution of fishes in relation to water quality and quantity in order to assess the condition an d health of aquatic ecosystems in the basin. Sampling between 1985 and 1993 revealed that five (26%) of the 19 native fish species known from the basi n had been extirpated. Two of these were endemics, Chirostoma charari and C . compressum, and they are presumed extinct. Twelve (63%) of the remaining species had declines in distribution. Sixteen (80%) of the 20 localities sa mpled had lost species. The greatest declines occurred in Lago de Cuitzeo p roper and in the lower portion of the Rio Grande de Morelia watershed. Spec ies losses from the lake were attributable to drying and hypreeutrophicatio n of the lake because of substantial reductions in the amount and quality o f tributary inputs, whereas losses from the Rio Grande de Morelia watershed were the result of pollution from agricultural, municipal, and industrial sources, especially in the region around the city of Morelia. Three localit ies in the upper por lion of the Rio Grande de Morelia watershed-Cointzio r eservoir, La Mintzita spring, and insurgente Morelos stream-contained most of the remaining fish species diversity in the basin and deserve additional protection. Fish faunal changes indicated major declines in the health of aquatic ecosystems in the Morelia-Cuitzeo basin.