The Llanos El Salado freshwater ecoregion, FEOW ecoregion 166, is a xeric, freshwater, endorheic (closed-basin) ecoregion of central Mexico. It lies primarily in San Luis Potosí, with portions in Zacatecas, Nuevo León, Tamaulipas, and Guanajuato. The ecoregion is part of the Mexican Plateau and consists mainly of dry, internally draining basins where streams have no outlet to the sea. Water is lost mostly through evaporation, and many areas have little or no permanent surface drainage.

The ecoregion is mostly flat to hilly, with mountains mainly along its boundaries. Its habitats include isolated springs, spring pools, short spring runs, temporary waters, shallow marshy areas, and small closed basins. Permanent aquatic habitats are scarce and highly fragmented. Many basins are arid or nearly so, with no clear surface drainage except after rains. Because aquatic habitats are small and isolated, individual springs or groups of springs can contain very localized fish populations.

The fish fauna is low in species richness but exceptionally important for endemism. FEOW notes that about 75% of the fishes found in the ecoregion are endemic. The endemic fauna is centered on cyprinodontid pupfishes, including Cyprinodon alvarezi, Cyprinodon ceciliae, Cyprinodon inmemoriam, Cyprinodon longidorsalis, Cyprinodon veronicae, and the Catarina pupfish, now generally treated as Megupsilon aporus. These species are associated with isolated desert springs, spring pools, short runs, and small wetland habitats.

Pupfishes are the defining fishes of Llanos El Salado. Their distributions are often extremely restricted, and some species are known from a single spring, spring complex, or small valley system. This makes them highly vulnerable to groundwater extraction, spring drying, pollution, introduced fishes, and habitat modification. The extinction of Megupsilon aporus illustrates the severity of the conservation crisis affecting Mexican desert spring fishes.

Introduced fishes are a serious threat. Tilapiines such as Oreochromis niloticus, Oreochromis aureus, or related aquaculture species would be non-native if present. Other introduced fishes, including mosquitofish, carp-like fishes, or aquarium releases, may also affect native pupfishes through predation, competition, disease, or habitat disturbance. In isolated spring systems, even a small introduction can have major consequences because native fishes often have nowhere else to retreat.

Conservation priorities include protecting groundwater, spring discharge, spring pools, short spring runs, marshy areas, and water quality. Major threats include groundwater extraction, spring drying, agricultural water use, livestock impacts, pollution, habitat modification, invasive fishes, and climate-driven drought. For FFM, Llanos El Salado should be treated as one of Mexico’s most fragile desert-fish ecoregions: a low-diversity but highly endemic closed-basin system whose conservation importance rests mainly on its localized pupfishes and isolated spring habitats.

This account is based in part on information from Freshwater Ecoregions of the World (FEOW), © WWF/TNC, used as a reference for educational and scientific purposes. See: http://www.feow.org.