The Coatzacoalcos freshwater ecoregion, FEOW ecoregion 172, is a tropical and subtropical coastal-river ecoregion on the southern Gulf of Mexico slope of Mexico. It is defined mainly by the Río Coatzacoalcos watershed, located at the northern end of the Isthmus of Tehuantepec in northeastern Oaxaca and eastern Veracruz. The basin extends from rugged mountain headwaters in the Sierra Madre de Oaxaca and Chimalapas region to lowland Veracruz floodplains, wetlands, lagoons, estuarine reaches, and the Gulf of Mexico.

The ecoregion’s habitats include shaded mountain streams, rocky tributaries, foothill rivers, lowland channels, floodplain wetlands, swamps, oxbows, lagoons, mangroves, estuarine lower reaches, and brackish coastal margins. Upper tributaries in Oaxaca and the Chimalapas region may be clear, rocky, shaded, and fast-flowing, while lower reaches in Veracruz are warmer, slower, more turbid, and more strongly influenced by floodplain, estuarine, and human-modified environments. The lower basin is also affected by urban, industrial, port, and petrochemical development around Coatzacoalcos and Minatitlán.

The fish fauna is rich and typical of a southern Mexican Gulf-slope basin. It includes poeciliids, characids, catfishes, atherinopsids, gobies, sleepers, mullets, eels, estuarine-associated fishes, and cichlids. The fauna changes strongly along the river continuum. Mountain tributaries, foothill streams, lowland channels, floodplain wetlands, swamps, lagoons, and brackish lower reaches may contain different assemblages. Poeciliids and other livebearing fishes are especially important in lowland streams, pools, wetlands, and disturbed habitats, while gobies, sleepers, mullets, eels, and other peripheral fishes are important in lower river and estuarine habitats.

The Coatzacoalcos ecoregion is especially important for poeciliids, including localized and endemic livebearers. Priapella intermedia is endemic to the Coatzacoalcos basin and represents one of the characteristic poeciliid elements of this southern Mexican Gulf-slope fauna. Xiphophorus clemenciae, the yellow swordtail, is another important endemic species of the region, associated with tributaries of the Coatzacoalcos system. These livebearers illustrate the high conservation value of small streams and tributaries within the basin, where localized species may occupy restricted ranges and be vulnerable to habitat degradation, pollution, flow alteration, and introduced fishes. Other non-cichlid groups, including characids, catfishes, atherinopsids, gobies, sleepers, mullets, and eels, add to the basin’s diversity and reflect the mixture of upland, lowland, floodplain, and estuarine habitats.

Cichlids are native and very important in the Coatzacoalcos ecoregion. They do not form a lake-type species flock, but they represent a rich Middle American Gulf-slope river, stream, lowland-channel, floodplain, swamp, and estuarine-margin assemblage. Important genera include Amatitlania, Cryptoheros, Mayaheros, Parachromis, Paraneetroplus, Rocio, Thorichthys, Trichromis, and Vieja, depending on locality and current taxonomy.

Several cichlids are especially important for this ecoregion. Thorichthys panchovillai was described from tributaries of the Río Coatzacoalcos basin and is one of the most distinctive cichlids associated with the region. Other native or regionally relevant cichlids may include Thorichthys callolepis, Trichromis salvini, Rocio octofasciata, Mayaheros urophthalmus, Paraneetroplus bulleri, Maskaheros regani, Vieja zonata, and related heroine cichlids, but records should be checked carefully by drainage and locality.

Taxonomic caution is essential because many older records use broad Cichlasoma names or species concepts that have since been revised. Records from the Coatzacoalcos main basin, Uxpanapa tributaries, Chimalapas headwaters, lower Veracruz wetlands, or adjacent drainages may not represent the same taxonomic or biogeographic unit. Locality data are especially important for Thorichthys, Vieja, Paraneetroplus, and other heroine cichlids.

Introduced fishes should be treated separately from the native fauna. Tilapiines such as Oreochromis niloticus and Oreochromis mossambicus may occur in reservoirs, ponds, canals, aquaculture systems, or modified lowland waters, but they are not part of the native Coatzacoalcos fish fauna. Other non-native fishes may arrive through stocking, aquaculture, aquarium releases, or accidental movement between basins.

Conservation priorities include protecting mountain tributaries, forested headwaters, lowland wetlands, riparian vegetation, floodplain habitats, lagoons, estuarine reaches, and water quality. Major threats include deforestation, cattle ranching, agriculture, road construction, sedimentation, industrial and petrochemical pollution, urban sewage, sand and gravel extraction, wetland loss, dams and water diversions, invasive fishes, and degradation of riparian vegetation. For FFM, the Coatzacoalcos should be treated as a highly important southern Mexican Gulf-slope ecoregion, with a rich native fish fauna and a particularly significant Middle American cichlid component.

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.