The Santiago River freshwater ecoregion, FEOW ecoregion 164, is a tropical and subtropical coastal-river ecoregion on the Pacific slope of western Mexico. It is largely defined by the watersheds of the Río San Pedro and Río Santiago, including tributaries such as the Río Mezquital, Río Grande de Santiago, Río Bolaños, and Río Juchipila. The Río Santiago drains Lake Chapala, although Lake Chapala itself belongs to the neighboring Lerma - Chapala ecoregion. The Santiago River ecoregion includes parts of Durango, Nayarit, Jalisco, Zacatecas, Aguascalientes, and Guanajuato, as well as the Islas Marías.

The ecoregion is shaped by the western slope of the Sierra Madre Occidental, deep canyon systems, volcanic landscapes, and the transition from the Mexican Plateau to the Pacific coastal lowlands. Elevations range from sea level to more than 3,000 m. Its habitats include large rivers with sand and rocks, canyon rivers, upland tributaries, seasonal streams, springs, reservoirs, irrigation-influenced channels, coastal wetlands, estuaries, and brackish lower reaches. The Río Santiago is one of the major outlets of the Mexican Plateau to the Pacific, but its hydrology has been significantly altered by dams, reservoirs, water extraction, agriculture, urban growth, and industrial pollution.

The fish fauna is less cichlid-rich than that of southeastern Mexico, but it is important to western Mexican freshwater fish diversity and endemism. FEOW lists several strict endemics, including Algansea avia, Algansea monticola, and Ictalurus ochoterenai. Near-endemic or regionally important fishes include additional cyprinids, catfishes, silversides, goodeids, poeciliids, gobies, sleepers, mullets, and estuarine-associated species. The fauna reflects both the history of the Mexican Plateau and Pacific-slope drainage connections.

Cyprinids are among the important native fishes of the ecoregion. Species of Algansea, Yuriria, Notropis, and related genera occur in different parts of the broader Santiago and adjacent Lerma-Chapala-Santiago region, depending on drainage and locality. Algansea avia and Algansea monticola are especially important because they are strict endemics of the ecoregion. These fishes are associated with river channels, tributaries, upland streams, and canyon habitats, and they are vulnerable to flow alteration, pollution, introduced fishes, and habitat fragmentation.

Catfishes also contribute to the fauna's distinctiveness. Ictalurus ochoterenai, the Chapala catfish, is listed by FEOW as a strict endemic of the Santiago ecoregion. Other ictalurids may occur in the broader Lerma-Chapala-Santiago context, but drainage and locality should be carefully checked because Lake Chapala, the Río Lerma, the Río Santiago, and adjacent Pacific drainages may represent distinct faunal contexts. The catfish fauna is especially relevant in larger river channels, reservoirs, and deeper pools.

Goodeids and other livebearers are important in the broader western Mexican context. Species of Goodea, Skiffia, Yuriria, Xenotoca, and related genera may occur in tributaries, springs, reservoirs, vegetated margins, and slower waters, depending on locality and drainage assignment. Some records may reflect connections to the Lerma-Chapala region or neighboring western Mexican ecoregions, so they should be treated with caution. Western Mexican livebearers are often highly sensitive to habitat degradation, the introduction of fishes, and the loss of aquatic vegetation.

Atherinopsid silversides, poeciliids, gobies, sleepers, mullets, and other peripheral or estuarine-associated fishes add to the ecological diversity of the ecoregion. Silversides may occur in lakes, reservoirs, lowland waters, and slower river habitats. Gobies, sleepers, mullets, and other coastal fishes are especially relevant in the lower reaches and estuarine transition zones. The ecoregion, therefore, includes both inland upland-river species and coastal-margin species, with big differences among canyon rivers, reservoirs, tributaries, and lower Pacific-slope habitats.

Atherinopsids are a major component of the Santiago River ecoregion and include several endemic or regionally restricted silversides. Species of Chirostoma and related atherinopsid genera occur in lakes, reservoirs, slow river reaches, and broader lowland or plateau-associated waters within the Lerma-Chapala-Santiago regional complex. Several forms are endemic or near-endemic to this western Mexican drainage context, reflecting the long isolation and fragmentation of highland and Pacific-slope aquatic habitats. These silversides are ecologically important and, in some cases, culturally or economically significant as “charales” or “pescados blancos.” Their taxonomy and drainage limits should be handled carefully, because Lake Chapala, the Río Lerma, the Río Santiago, and neighboring basins may contain closely related but distinct species or local forms.

Cichlids are native to the Santiago River ecoregion, but the native cichlid fauna is limited. The only native cichlid to emphasize is Mayaheros beani, the green guapote or Sinaloa cichlid. This species represents the northern Pacific Mexican cichlid element and occurs on the Pacific slope of Mexico. In the Santiago River ecoregion, it should be treated as the characteristic native cichlid, especially in lower river-valley sections, warm channels, pools, reservoirs, and lowland habitats. Older literature may list it as Cichlasoma beani.

Introduced cichlids and other non-native fishes should be treated separately from the native fauna. Tilapiines such as Oreochromis niloticus, Oreochromis aureus, and related aquaculture forms may occur in reservoirs, ponds, canals, irrigation systems, or modified lowland waters, but they are not part of the native Santiago River fish fauna. Other introduced fishes, including carps, stocked sport fishes, aquarium releases, or aquaculture escapees, may also affect native fishes through predation, competition, disease, habitat alteration, or changes in the food web.

Conservation priorities include protecting canyon rivers, tributaries, springs, native fish refuges, riparian vegetation, estuarine connections, and water quality. Major threats include dams, reservoirs, water extraction, irrigation, urban and industrial pollution, agricultural runoff, sedimentation, invasive fishes, riparian degradation, and altered seasonal flow. The Río Santiago has some of the most heavily impacted waters in western Mexico, but its tributaries and canyon reaches may still preserve important native fish habitats.

For FFM, the Santiago River should be treated as a major western Mexican Pacific-slope ecoregion with a distinctive but unevenly known fish fauna. Its importance lies in endemic cyprinids such as Algansea avia and Algansea monticola, endemic or regionally important catfishes such as Ictalurus ochoterenai, western Mexican livebearers, silversides, coastal-associated fishes, and the native cichlid Mayaheros beani. It is not primarily a cichlid ecoregion, but it is an important transitional system linking the Mexican Plateau, deep western canyons, and the Pacific coastal lowlands.

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.