The San Juan River freshwater ecoregion, FEOW ecoregion 138, is a northeastern Mexican freshwater ecoregion centered on the Río San Juan drainage. This is the Mexican Río San Juan system, mainly associated with Nuevo León and adjacent northeastern Mexico, and should not be confused with the San Juan River of Nicaragua and Costa Rica. The Río San Juan is an important tributary system in the lower Río Bravo/Rio Grande region, draining parts of the eastern slopes and foothills of the Sierra Madre Oriental before flowing through the lower plains toward the Río Bravo.

The ecoregion’s habitats include springs, mountain creeks, plains rivers, and a few lagoons. Some semi-isolated springs, such as Apodaca, Mezquital, Infiernillo, Huasteca, and others, contain more strongly Neotropical elements, while most headwaters are more Nearctic in character. Several springs have dried out, showing how vulnerable the ecoregion’s aquatic habitats are to groundwater extraction, urban growth, drought, and flow alteration. The aquatic landscape includes rocky mountain streams, spring runs, foothill creeks, pools, lowland channels, reservoirs, irrigation canals, drainage ditches, and modified river sections.

The fish fauna is a mixture of northeastern Mexican, Río Bravo, Sierra Madre, spring-associated, lowland, and plains-river elements. Important groups include cyprinids, catostomids, ictalurids, centrarchids, cyprinodontids, poeciliids, atherinopsids, darters, and cichlids. The fauna changes strongly with elevation, flow, water permanence, and habitat type. Mountain creeks and headwaters may support more Nearctic stream fishes, while springs, lagoons, lowland channels, and warmer reaches contain more Neotropical or Río Bravo-associated elements.

Cyprinids are among the important non-cichlid fishes of the ecoregion. Species of Notropis, Cyprinella, Dionda, Campostoma, and related genera occur in the broader San Juan and Río Bravo context, depending on locality and drainage. These fishes are associated with flowing streams, spring runs, pools, riffles, and river channels. Some species are more widespread in northeastern Mexico, while others may be localized or shared with neighboring Río Salado and lower Río Bravo systems.

Poeciliids and cyprinodontids are also important in springs, vegetated margins, pools, lowland waters, and slow or seasonally variable channels. Species of Gambusia, Poecilia, Xiphophorus, Cyprinodon, and related genera may occur in the broader regional context, but locality and drainage should be checked carefully. In this ecoregion, springs and permanent shallow waters can be especially important refuges for small fishes, particularly where surrounding streams are intermittent or heavily modified.

Darters, catfishes, suckers, sunfishes, and other Nearctic groups contribute to the headwater and stream component of the fauna. Darters and small benthic fishes are associated with flowing-water habitats, while catfishes, suckers, and sunfishes may occur in rivers, reservoirs, pools, and larger channels. The presence of both headwater stream fishes and warm lowland species reflects the transitional character of the San Juan River ecoregion between Sierra Madre streams and the lower Río Bravo plains.

Cichlids are native to the San Juan River ecoregion, but the native cichlid fauna is limited. The principal native species is Herichthys cyanoguttatus, the Río Grande cichlid or Texas cichlid. This species is native to northeastern Mexico and southern Texas in the Río Bravo / Rio Grande region and is the characteristic native cichlid of this ecoregion. It should be distinguished from Herichthys minckleyi, which is endemic to Cuatro Ciénegas, and from Pánuco-associated Herichthys species such as Herichthys labridens and Herichthys bartoni.

Introduced fishes should be treated separately from the native fauna. Tilapiines such as Oreochromis niloticus, Oreochromis aureus, Oreochromis mossambicus, and related aquaculture forms may occur in reservoirs, ponds, canals, irrigation systems, or modified waters, but they are not part of the native San Juan River fish fauna. Other introduced fishes may include stocked sport fishes, carps, aquarium releases, or aquaculture escapees. These species can affect native fishes through predation, competition, disease, habitat alteration, and changes in the food web.

Conservation priorities include protecting springs, groundwater systems, perennial stream reaches, mountain creeks, lagoons, riparian vegetation, and water quality. Major threats include groundwater extraction, spring drying, river regulation, reservoirs, irrigation withdrawals, salinization, urban expansion, pollution, sewage, agricultural runoff, livestock pressure, channel modification, invasive fishes, and climate-driven drought. For FFM, the San Juan River should be treated as a northeastern Mexican Río Bravo-associated ecoregion, important for spring fishes, cyprinids, livebearers, darters, catfishes, and the native Río Grande cichlid Herichthys cyanoguttatus.

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.