The Mayrán - Viesca freshwater ecoregion, FEOW ecoregion 163, is a xeric freshwater and closed-basin ecoregion of northern Mexico, centered on the Nazas-Aguanaval interior drainage system. It lies mainly in the Comarca Lagunera region of Durango and Coahuila. The Río Nazas and Río Aguanaval historically drained into the terminal lake and playa systems of Laguna de Mayrán and Laguna de Viesca, which formed the region's endorheic base level. Today, much of this hydrological system has been altered by dams, irrigation, groundwater extraction, and agricultural development.

The ecoregion’s habitats include intermittent rivers, springs, spring-fed reaches, pools, marshes, shallow wetlands, reservoirs, irrigation canals, drainage ditches, agricultural return-flow waters, and former playa-lake environments. Natural flow was historically episodic and strongly seasonal, but modern hydrology is now dominated by water regulation, irrigation withdrawals, groundwater pumping, and habitat fragmentation. Many former river, marsh, and lake habitats have been reduced, dried, or transformed into artificial or highly modified aquatic systems.

The fish fauna is distinctive, reflecting a mixture of Río Bravo-derived elements, isolated, closed-basin evolution, and species divergence driven by fragmentation. FEOW notes that the ecoregion includes cyprinoids, catostomids, a pupfish, and one percid in the upper Río Mezquital context, together with fishes of the Parras basin and related closed-basin systems. This is not a tropical cichlid-rich ecoregion, but a northern Mexican desert-fish region where springs, intermittent streams, and isolated basins are central to fish diversity.

Important fishes recorded from the ecoregion include cyprinids such as Cyprinella alvarezdelvillari, Cyprinella garmani, Dionda melanops, Gila conspersa, Codoma ornata, Campostoma ornatum, and Notropis nazas. These fishes represent the minnow and shiner component of the fauna and may occur in springs, streams, pools, desert rivers, and modified channels, depending on species and locality. Some are associated with restricted basins or particular sections of the Nazas-Aguanaval-Mezquital complex, making locality data especially important.

Pupfishes are among the most important fishes of Mayrán - Viesca. Species such as Cyprinodon meeki and Cyprinodon nazas represent the cyprinodontid component of the fauna, and the broader Mayrán/Viesca region has been associated with localized endemic pupfishes such as Cyprinodon latifasciatus. These fishes are tied to isolated desert waters, springs, shallow habitats, and remnant wetlands, and are highly vulnerable to groundwater depletion, drying, pollution, and introduced species.

Livebearers and related small fishes are also important. Recorded goodeids and poeciliids include Characodon lateralis, Goodea atripinnis, Xenotoca variata, Gambusia senilis, although the status and locality of each record should be checked carefully. In closed-basin systems, some livebearers may be native to particular springs or drainage sections, while others may represent introductions, translocations, or broader ecoregion records.

Other native or regionally recorded fishes include Catostomus nebuliferus, Ictiobus niger, Etheostoma pottsi, Chirostoma mezquital, and Astyanax mexicanus. These species show that the ecoregion includes more than pupfishes and minnows; it also contains suckers, buffalo fishes, darters, silversides, and characids in suitable habitats. However, many of these fishes depend on the persistence of aquatic refuges, which have been reduced or altered by water development.

The native fish fauna of the region is composed mainly of desert cyprinids, pupfishes, livebearers, suckers, darters, silversides, and other fishes adapted to closed-basin environments. Tilapiines such as Oreochromis niloticus, Oreochromis aureus, Oreochromis mossambicus, and related aquaculture forms are not native to the region. The same applies to the widely introduced Xiphophorus hellerii, which should also be treated as non-native in this ecoregional context.

Introduced fishes are a major conservation threat. In closed desert basins, native fishes often persist in small springs, short stream reaches, irrigation-influenced refuges, or isolated wetlands. Non-native fishes may affect these communities through predation, competition, disease, hybridization in some groups, or habitat alteration. Because these systems are isolated, native fishes have very limited opportunities to recolonize habitats once local populations are lost.

Conservation priorities include protecting springs, groundwater, perennial stream refuges, remnant wetlands, marshes, water quality, and natural flow where it persists. Major threats include damming of the Nazas and Aguanaval, irrigation withdrawals, groundwater extraction, drying of terminal lakes and wetlands, spring-flow reduction, pollution, agricultural return flows, salinization, livestock impacts, channel modification, invasive fishes, and climate-driven drought. For FFM, Mayrán-Viesca should be treated as an important northern Mexican closed-basin desert fish ecoregion, notable for localized cyprinids, pupfishes, livebearers, and remnant aquatic refuges rather than for cichlid diversity.

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.