The Ameca - Manantlán freshwater ecoregion, FEOW ecoregion 168, is a tropical and subtropical coastal-river ecoregion on the Pacific slope of west-central Mexico. It includes several short, independent Pacific-draining basins from the Río Ameca in the north to the Río Ahuijallo in the south. The ecoregion covers a small part of southern Nayarit, southern Jalisco, northwestern Michoacán, and all of Colima. Important river systems include the Río Ameca, Río Armería-Ayuquila, Río Coahuayana-Tuxpan, Río Marabasco-Cihuatlán, Río Purificación, and smaller coastal drainages.

The ecoregion is mountainous, deeply dissected by canyons, and ranges in elevation from sea level to nearly 4,000 m. Its freshwater habitats include cool upland streams, springs, canyon rivers, rocky runs, riffles, pools, short coastal rivers, reservoirs, irrigation-influenced channels, coastal wetlands, estuaries, and brackish lower reaches. FEOW describes the region’s rivers as coastal or short-course systems with narrow or almost absent floodplains, especially in the northern part of the ecoregion. The surrounding landscapes include dry forests, pine-oak forests, and coastal mangroves.

The fish fauna is highly distinctive and exceptionally important for the conservation of Mexican freshwater fish. FEOW notes that more than half of the fishes of the Ameca - Manantlán ecoregion are endemic, and many endemics are restricted to a single small basin. The ecoregion is especially important for goodeid livebearers. Endemic or highly characteristic goodeids include Allodontichthys hubbsi, Allodontichthys polylepis, Allodontichthys tamazulae, Allodontichthys zonistius, Ameca splendens, Ilyodon furcidens, Ilyodon xantusi, Skiffia francesae, Xenotaenia resolanae, Xenotoca melanosoma, and Zoogoneticus tequilae. Important endemic poeciliids include Poecilia chica, Poeciliopsis baenschi, and Poeciliopsis turneri. The endemic cyprinid Algansea aphanea occurs in the Coahuayana and Armería systems.

The Río Ameca and its spring-fed tributaries are especially famous for Ameca splendens, the butterfly splitfin, and Zoogoneticus tequila, the tequila splitfin. These species illustrate the conservation value of small spring systems such as those around Teuchitlán. The Coahuayana-Tuxpan and Armería-Ayuquila systems are also important for localized goodeids such as Allodontichthys hubbsi and related species. In this ecoregion, small streams, springs, and short tributaries often contain a large part of a species’ known distribution, so locality data are especially important.

Cichlids are native to Ameca-Manantlán, but they are not the dominant fish group. The two native cichlid species to mention are Mayaheros beani and Amphilophus istlanus. Mayaheros beani, the green guapote or Sinaloa cichlid, represents the northern Pacific Mexican cichlid element. Amphilophus istlanus, historically treated as Cichlasoma istlanum, links the ecoregion with the Balsas and adjacent Pacific-slope cichlid fauna and is associated with flowing rivers and stony habitats. These native cichlids should be clearly separated from introduced tilapia and other non-native cichlids.

Other native fishes include cyprinids, poeciliids, goodeids, ictalurids, atherinopsids, gobies, sleepers, mullets, and estuarine-associated species in the lower reaches. Fish assemblages vary strongly among basins and habitats. Mountain streams, canyon rivers, spring-fed habitats, lowland rivers, reservoirs, and estuaries each support different combinations of species. Some fish tolerate saline or brackish conditions in lower coastal reaches, while many endemic goodeids and poeciliids are associated with clearer, permanent, spring-fed, or upland waters.

Introduced fishes are a major threat. Tilapiines such as Oreochromis aureus and Oreochromis niloticus may occur in reservoirs, ponds, irrigation systems, or disturbed lowland waters, but they are not part of the native fauna. Other non-native fishes may arrive through aquaculture, stocking, aquarium releases, or accidental introductions. These species can affect native fishes through predation, competition, disease, habitat disturbance, or other ecological effects.

Conservation priorities include protecting springs, headwaters, canyon rivers, perennial stream reaches, riparian vegetation, estuaries, and water quality. Major threats include water extraction, spring modification, dams and reservoirs, pollution, eutrophication, agricultural runoff, sewage, and drying or degradation of small streams. Because many native fishes are highly localized, the loss of a single spring, tributary, or short river reach can mean the loss of an entire local population or even a major part of a species’ range.

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.