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The Río Lerma basin, one of the most important hydrographic regions of Mexico, has a distinctive fish fauna. In recent years, the basin has experienced major increases in human populations and industrial and agricultural development. To assess the health of aquatic ecosystems in the Lerma basin, we analyzed the current status and long-term trends in fish species occurrence at 116 widely distributed sites in relation to water quality and land use information. Our results reveal a staggering and unprecedented level of environmental degradation. Over 50% of our sites are no longer capable of supporting fish life. Many sites have completely disappeared because of groundwater extraction, water diversions, or urbanization. The Alto Lerma subprovince has experienced the greatest negative impacts, but no region of the basin has escaped significant damage. Only 15% of our sites currently support species that we classify as sensitive to environmental degradation, and most of these sites are on small headwater streams or isolated highland lakes that are protected as parks or municipal water supplies. Only one lowland river, the Río Turbio, retains relatively good environmental quality. Forty percent of the 42 species native to the basin have experienced major declines in frequency of occurrence, and three endemic species, Algansea barbata, Chirostoma charari, and Chirostoma compressum may be extinct. Conservation efforts should focus on protecting all sites that still hold native fishes, with particular emphasis on the Río Turbio.

A checklist based on previously published records and original data is presented for the helminth parasites reported in 35 fish species from nine families from the Rio Papaloapan basin, east Mexico. The checklist contains 85 taxa from 39 helminth families. Trematodes and nematodes were the most abundant taxonomic groups. The helminth fauna in the fish of the Papaloapan River basin predominantly consists of Neotropical species that are largely autogenic. The introduced species Centrocestus formosanus was the most widely distributed helminth, infecting 16 host species. Ten of the recorded helminth species have only been found in fish from the Papaloapan. This inventory contributes 157 new host records, and reports the presence of 30 helminth species in the Papaloapan for the first time . This inventory shows the richness of helminth parasite species in the fish of the Papaloapan River basin in comparison with the other hydrological basins in Mexico. It also demonstrates that this fauna is typically Neotropical and quite similar to that from the neighboring basins of the Grijalva-Usumacinta system and the Yucatan Peninsula. The data also suggest highly effective transmission between environments within the same basin and that the regional parasite fauna is strongly influenced by fish community composition.

An inventory based on previously published records and original data is presented for the helminth parasites reported in 54 fish species from 17 families from the state of Chiapas, southeast Mexico. This survey reports the presence of 43 helminth species in Chiapas for the first time. The actualized inventory of helminth parasites of freshwater fish from Chiapas contains 88 species from 67 genera and 40 helminth families. Trematodes and nematodes were the most abundant taxonomic groups. The helminth fauna in freshwater fishes from Chiapas consists of Central American species. All the helminth species recorded in Chiapas have also been collected from bodies of freshwater between the Isthmus of Tehuantepec and the Isthmus of Panama. This inventory shows that this fauna is quite similar to that from the neighboring basins of the lower Grijalva-Usumacinta system and the Yucatan Peninsula. The taxonomic composition and distribution data reported here for the helminth fauna of Chiapas' freshwater fish, contribute to a better understanding of this faunal component in Central America.

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