The Grijalva - Usumacinta freshwater ecoregion, FEOW ecoregion 173, is a tropical and subtropical coastal-river ecoregion on the Atlantic slope of southern Mexico and northern Guatemala. It includes the lower and middle Río Grijalva and Río Usumacinta systems, their tributaries, floodplains, wetlands, lagoons, swamps, estuaries, and coastal outlets to the southern Gulf of Mexico. It lies between the Coatzacoalcos ecoregion to the west and the Upper Usumacinta and Yucatán ecoregions to the east and southeast.

The ecoregion includes one of the largest river and wetland complexes in Mesoamerica. The Grijalva drains the highlands of Guatemala and Chiapas before flowing through Chiapas and Tabasco, while the Usumacinta receives major tributaries such as the Pasión, Salinas/Chixoy, and Lacantún and then flows northward along parts of the Mexico–Guatemala boundary into the Tabasco lowlands. The lower Grijalva-Usumacinta plain contains extensive floodplains, marshes, swamps, seasonally flooded forests, lagoons, mangroves, estuaries, and brackish lower reaches.

The fish fauna is one of the richest in Mexico and Middle America. The Grijalva River basin alone has been reported to contain 92 fish species across 13 orders, 25 families, and 50 genera, including 83 native, 9 exotic, and 15 endemic species. Cichlidae and Poeciliidae are the most diverse families in the Grijalva basin, with 25 and 23 species, respectively. Secondary freshwater fishes dominate the fauna, while primary freshwater fishes and peripheral or estuarine-associated species are also important.

Poeciliids are especially important in the Grijalva-Usumacinta ecoregion and are one of the main reasons the fish fauna has such a strong Middle American character. They occupy lowland streams, floodplain channels, swamps, lagoons, vegetated margins, spring-fed waters, and disturbed or seasonally variable habitats. Important genera include Belonesox, Gambusia, Heterandria, Poecilia, Poeciliopsis, Priapella, and Xiphophorus, depending on drainage and locality. Several species are widespread and ecologically flexible, while others are more localized or tied to particular tributaries, wetlands, or karstic systems. Their diversity complements that of the cichlids: together, poeciliids and cichlids form the dominant secondary freshwater component of the ecoregion, especially in warm lowland waters and floodplain habitats.

The ecoregion also includes several distinctive endemic or range-restricted non-cichlid fishes that deserve attention. Among the most unusual is Batrachoides goldmani, the Mexican freshwater toadfish, a member of Batrachoididae and one of the few toadfishes strongly associated with freshwater. It is known from the Usumacinta system and has also been recorded in the Grijalva/Mescalapa drainage, illustrating the connection between lowland rivers, deep channels, sandy or muddy bottoms, and the broader Gulf-slope fauna. Other important endemic or localized fishes include Potamarius nelsoni and several poeciliids, catfishes, and cichlids restricted to particular parts of the Grijalva-Usumacinta complex. These taxa show that the ecoregion’s importance is not limited to its conspicuous cichlids; it also includes unusual peripheral-derived freshwater fishes, localized livebearers, specialized catfishes, and riverine species whose distributions depend on the integrity of tributaries, floodplain channels, and lowland river habitats.

The Usumacinta component is also highly important. Recent work on the Río San Pedro and Río La Pasión in the Usumacinta basin reported 70 fish species, including six non-native species. These rivers are part of a globally significant center of freshwater-fish diversity. In that area, Cichlidae and Poeciliidae are again the most species-rich families, showing that the same two families dominate much of the Middle American lowland fish fauna.

Cichlids are among the defining fish groups of the Grijalva-Usumacinta ecoregion. They do not form a lake-type species flock, but they represent one of the richest assemblages of Middle American river, floodplain, lagoon, swamp, karst stream, lowland channel, and estuarine-margin cichlids. Important genera include Chuco, Cribroheros, Kihnichthys, Maskaheros, Mayaheros, Oscura, Parachromis, Paraneetroplus, Petenia, Rocio, Theraps, Thorichthys, Trichromis, Vieja, and Wampaheros, depending on drainage and taxonomy.

Notable native cichlids include Kihnichthys ufermanni, Maskaheros regani, Mayaheros urophthalmus, Oscura heterospila, Parachromis multifasciatus, Parachromis multifasciatus, Paraneetroplus gibbiceps, Petenia splendida, Rocio octofasciata, Thorichthys meeki, Trichromis salvini, Vieja bifasciata, Vieja breidohri, Vieja hartwegi, and Vieja melanurus, although each should be checked carefully by locality and current taxonomy. Predatory species such as Parachromis and Petenia occur in rivers, lagoons, swamps, and structurally complex lowland waters, while Mayaheros urophthalmus is especially associated with lowland, swamp, lagoonal, coastal, and brackish-tolerant habitats.

Taxonomic caution is essential because many older records use broad Cichlasoma names for fishes now placed in several different genera. Records from the Grijalva, Lacantún, Usumacinta, Pasión, San Pedro, Tabasco wetlands, Chiapas karst streams, and Guatemala tributaries should not be treated as interchangeable. Locality data are especially important for cichlids, poeciliids, endemic species, and fishes from karstic or isolated tributary habitats.

Introduced fishes are a significant conservation issue. Exotic species have been recorded in both the Grijalva and Usumacinta portions of the ecoregion. Introduced tilapiines such as Oreochromis niloticus, Oreochromis mossambicus, and related aquaculture forms may occur in reservoirs, ponds, canals, and modified lowland waters. Introduced loricariids or “plecos” have also been recognized in the Grijalva-Usumacinta complex and may affect native habitats and food webs.

Conservation priorities include maintaining natural flood pulses, protecting karstic streams, forest tributaries, floodplain lakes, swamps, marshes, riparian forests, wetlands, lagoons, estuaries, and water quality. Major threats include hydroelectric dams, flow regulation, deforestation, cattle ranching, agriculture, sedimentation, pollution, sewage, oil and industrial impacts, invasive fishes, wetland drainage, riparian degradation, and habitat fragmentation. For FFM, Grijalva-Usumacinta should be considered one of the most important freshwater fish ecoregions in Mexico: a major river-wetland system with exceptional cichlid and poeciliid diversity, many native and endemic fishes, and a strong connection among rivers, wetlands, lagoons, and the Gulf coastal plain.

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.