The Sierra Madre del Sur freshwater ecoregion, FEOW ecoregion 170, is a tropical and subtropical coastal-river ecoregion on the Pacific slope of southern Mexico. It includes short, steep Pacific-draining basins associated with the Sierra Madre del Sur, especially in Guerrero and Oaxaca, with adjacent influence from neighboring western Mexican and Central American Pacific-slope systems. The ecoregion lies between the Balsas region to the northwest and the Chiapas-Fonseca region to the southeast, making it an important transition zone for freshwater fauna in southern Mexico.

The landscape is rugged and strongly dissected. Rivers descend from mountains, foothills, dry forests, canyons, and narrow coastal plains before reaching the Pacific Ocean. Important systems include the Río Verde, Río Ometepec, Río Papagayo, Río Atoyac, and other smaller Pacific coastal drainages, depending on the boundary used. Many basins are short and isolated, with big seasonal differences between wet-season floods and dry-season contraction.

The ecoregion’s habitats include rocky headwater streams, mountain creeks, canyon rivers, riffles, runs, pools, springs, seasonal lowland channels, reservoirs, coastal lagoons, estuaries, mangroves, and brackish lower reaches. Upper streams may be clear, shaded, rocky, and fast-flowing, while lower reaches are warmer, slower, more turbid, and more influenced by flood pulses, dry-season low flow, coastal wetlands, and estuarine conditions. Because the mountains lie close to the coast, aquatic habitats can change rapidly over short distances.

The fish fauna is characteristic of southern Mexican Pacific-slope drainages. It includes poeciliids, atherinopsids, catfishes, gobies, sleepers, mullets, eels, snooks, estuarine-associated fishes, and cichlids. Compared with large tropical Atlantic-slope basins such as the Grijalva-Usumacinta or Papaloapan, the fauna is generally less species-rich, but it is biogeographically important because it combines Mexican Pacific-slope elements with northern Middle American lineages.

Poeciliids are important in lowland channels, pools, springs, vegetated margins, coastal lagoons, and disturbed or seasonally variable habitats. Species of Poecilia, Poeciliopsis, Gambusia, Heterandria, and related genera may occur in different parts of the broader region, depending on drainage and locality. These livebearers are often among the most conspicuous small fishes in warm, shallow, or variable waters and form an important part of the native freshwater assemblage.

Peripheral and estuarine-associated fishes are also important. Gobies, sleepers, mullets, eels, snooks, and other coastal fishes may use lower reaches of rivers, lagoons, mangrove margins, and brackish waters. In short Pacific-slope basins, these species can represent a substantial part of the fish fauna because the distance between headwaters and estuaries is often short. For FFM, these lower-river and coastal-transition fishes should be treated as integral components of the ecoregion rather than marginal additions.

Atherinopsids and catfishes add further diversity. Silversides may occur in slower river sections, lagoons, reservoirs, or coastal plain habitats, while catfishes occupy streams, pools, rivers, and lowland channels. The exact composition varies by drainage, elevation, and habitat. Locality data are especially important because neighboring Pacific-slope basins may harbor different assemblages despite their geographic proximity.

Cichlids are native and important in the Sierra Madre del Sur ecoregion, although they do not form a lake-type species flock. The four native species to emphasize are Amphilophus istlanus, Amphilophus trimaculatus, Astatheros macracanthus, and Vieja guttulata. Together, they represent a southern Mexican Pacific-slope river, canyon-stream, lowland-channel, lagoon, and estuarine-margin cichlid assemblage. This makes the ecoregion more cichlid-rich than the more northern Pacific Mexican regions, but less diverse than the major Atlantic-slope and Central American cichlid centers.

Amphilophus trimaculatus, the three-spot cichlid, is one of the emblematic Pacific-slope Middle American cichlids. It occurs in Pacific-slope drainages from Mexico southeastward into northern Central America and is associated with lowland rivers, lagoons, marshes, and brackish-influenced habitats. Older literature may list it as Cichlasoma trimaculatum, Heros trimaculatus, or other historical combinations.

Amphilophus istlanus links the Sierra Madre del Sur fauna with the Balsas and adjacent southern Mexican Pacific-slope systems. It is associated with rivers, streams, pools, rocky margins, sandy areas, and structurally complex habitats. In this ecoregion, it should be treated as part of the native Pacific-slope cichlid fauna, but records should be checked carefully by drainage because the species is especially well known from the Balsas region.

Astatheros macracanthus, the blackthroat cichlid or mojarra de Guamuchal, is another important native Pacific-slope cichlid. It is associated with southern Mexican and northern Central American Pacific-slope waters and helps connect the Sierra Madre del Sur fauna with the Chiapas-Fonseca and northern Central American Pacific-slope assemblages. Its presence reflects the increasing importance of Middle American cichlid lineages toward the southeast.

Amphilophus istlanus is associated with river valleys and different substrate types, including mud, sand, and rocks, and should be distinguished from Atlantic-slope Vieja species or older broad Cichlasoma records. Together with Amphilophus trimaculatus and Astatheros macracanthus, it indicates that the Sierra Madre del Sur belongs to the broader Middle American Pacific-slope cichlid zone.

Taxonomic caution is essential. Older Mexican and Central American cichlid literature often used broad Cichlasoma names or other historical combinations for species now placed in Amphilophus, Astatheros, Vieja, and related genera. Records from Guerrero, Oaxaca, the Río Verde, Río Ometepec, Río Papagayo, Río Atoyac, and adjacent Pacific drainages should be checked carefully by locality and current taxonomy. Species from the Balsas, Chiapas - Fonseca, or more northern Pacific systems should not be transferred automatically into the Sierra Madre del Sur without drainage evidence.

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 lowland waters, but they are not part of the native Sierra Madre del Sur fish fauna. Other introduced fishes may arrive through aquaculture, stocking, aquarium releases, or accidental movement between basins.

Conservation priorities include protecting mountain streams, canyon rivers, perennial pools, springs, riparian vegetation, coastal lagoons, estuaries, mangroves, and water quality. Major threats include deforestation, agriculture, cattle ranching, road construction, mining, sedimentation, water extraction, dams and small reservoirs, pollution, sewage, invasive fishes, riparian degradation, coastal lagoon alteration, and climate-driven changes in rainfall and dry-season flow. Because many basins are short and steep, disturbance in upland catchments can quickly affect lower river, lagoon, and estuarine habitats.

For FFM, Sierra Madre del Sur should be treated as a southern Mexican Pacific-slope ecoregion with a mixed fish fauna of livebearers, silversides, catfishes, peripheral coastal fishes, and native cichlids. Its cichlid fauna is represented by Amphilophus istlanus, Amphilophus trimaculatus, and Astatheros macracanthus, but the ecoregion’s full importance also includes short-river ecology, seasonal flow, estuarine connections, and the transition between western Mexican and Central American Pacific-slope freshwater faunas.

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.