Chiapas–Fonseca

Chiapas-Fonseca is the Pacific face of southern Mesoamerica: a ribbon of short, steep rivers that drop off the Sierra Madre and the Central American volcanic chain straight into the sea, from the Golfo de Tehuantepec in Oaxaca down through Chiapas, Guatemala and El Salvador to the Choluteca basin at the Gulf of Fonseca in Honduras. None of these rivers is long, but they fall hard — from peaks near 4,0 ft to mangrove coast in well under 124 mi — and they end in a string of brackish lagoons and crater lakes rather than a single great delta. The cichlids here are a Pacific-slope cast led by the blackthroat (Astatheros macracanthus), an endemic the ecoregion is partly defined by, set against tilapia, dams and the runoff of sugar and coffee country.

Geography & hydrology

This is a coastal ecoregion in the strict sense — Freshwater Ecoregions of the World numbers it 201 and calls its major habitat type 'tropical and subtropical coastal rivers.' It lies entirely southwest of the Continental Divide, draining the Pacific slope from the Golfo de Tehuantepec in Oaxaca, Mexico, across Chiapas and the Guatemalan and Salvadoran lowlands, to the Choluteca basin on the Gulf of Fonseca in southern Honduras. The land it drains is young and steep: a narrow alluvial plain at the coast, a piedmont, and behind them a mountainous volcanic belt where elevations run from sea level to nearly 4,0 ft. Off the Salvadoran coast the Cocos plate dives beneath the Caribbean plate along the Middle America Trench, and the same subduction that builds the volcanoes makes this one of the more earthquake-prone coastlines in the Americas (FEOW Ecoregion 201; Dewey et al., 2004; White et al., 2004).

Because the divide sits close to the sea, almost every river here is short and fast. Mexico's side is a fringe of minor coastal basins, the largest being the Tehuantepec, which empties into the linked coastal lagoons of Mar Muerto, Superior and Inferior. South of there the Suchiate marks the Mexico-Guatemala line, and the Paz divides Guatemala from El Salvador. The one genuinely large river is the Lempa: it rises in Guatemala, clips southwestern Honduras, then crosses the central plateau of El Salvador before cutting through the coastal volcanic range to reach the Pacific — about 261 mi, with a watershed that covers roughly half of El Salvador. At the southern end the Goascorán, Nacaome and Choluteca rivers of Honduras drain into the Gulf of Fonseca (FEOW Ecoregion 201).

The other half of the hydrology sits in volcanic craters. The interior highlands of Guatemala and El Salvador hold a set of deep lakes with no large river feeding them — Atitlán and Chicabal in Guatemala, and Güija, Coatepeque and Ilopango in El Salvador — plus shallow Laguna de Olomega and the Laguna de Metapán wetlands. These crater and tectonic lakes, isolated and often chemically distinct from the rivers around them, give the ecoregion a second kind of freshwater habitat layered over the coastal drainages and the brackish lagoons of the shore (FEOW Ecoregion 201).

Water & habitat

Habitat in Chiapas-Fonseca runs the full gradient from cold mountain headwater to hypersaline coastal flat in the space of a single river. FEOW describes the mix as mountain streams, canyon creeks, coastal lagoons and crater lakes, with mangroves and tidal flats dominating the estuaries and salt marshes forming inland where sediments turn hypersaline. The shore is lined with red mangrove — Rhizophora mangle, R. harrisonii and R. racemosa — so the lower reaches of these rivers are tidal, brackish and turbid before they meet the surf (FEOW Ecoregion 201; Chapman, 1992; Yáñez-Arancibia & Lara-Domínguez, 1999).

The upper and middle reaches are warm, hard-water rivers. Pacific-slope Chiapas drains limestone and volcanic terrain, and the cichlids that live in it sit comfortably in neutral-to-alkaline water: aquarium and field figures for the dominant species cluster around pH 7.0-8.0, moderate to high hardness, and temperatures in the mid-20s Celsius, with the blackthroat noted as preferring hard, alkaline water above about 77 °F. The water carries dissolved carbonate off the catchment rock rather than the soft, acidic chemistry of a shield river, which is part of why this fauna tolerates — and in the lagoons, requires — a slide toward brackish conditions downstream (Pacific-slope Chiapas surveys; STRI Shorefishes of the Tropical Eastern Pacific).

The coastal end is its own world. Astatheros macracanthus is a demersal, soft-bottom fish recorded over mud, sand, gravel and into estuary and mangrove, across a depth range of roughly 0–16 ft and into brackish water — a cichlid as much at home in the lower river and lagoon as in fresh current. The big lagoon complexes around the Golfo de Tehuantepec, and the tidal stretches of the Lempa and the Fonseca rivers, are productive nursery habitat where freshwater cichlids overlap with euryhaline gobies, mojarras and snook. Set against the rivers are the crater lakes, which behave nothing like the streams: deep, often stratified, and in cases such as Güija and the Salvadoran maars holding their own restricted fish populations. The result is a patchwork — fast hard-water canyon, brackish mangrove lagoon, isolated volcanic lake — within one short coastal ecoregion (STRI Shorefishes; FEOW Ecoregion 201).

The cichlid fauna

Cichlids are the most diverse fish family on this slope — the Chiapas state checklist alone counts 35 cichlid species among its 311 freshwater fishes, with the Pacific slope holding roughly two-thirds of the state's fauna (Velázquez-Velázquez et al., 2016). The signature fish, and one of the endemics FEOW uses to delimit the ecoregion, is the blackthroat cichlid, Astatheros macracanthus. It runs from southern Mexico to El Salvador on the Pacific coast and nowhere else, a bottom-feeding omnivore to about 10–12.5 in that works soft substrates for insect larvae, snails, crustaceans and small fish, and ranges from fresh river into brackish lagoon and mangrove. As an endemic tied to exactly this stretch of coast, it is the fish that most cleanly defines the region (FEOW Ecoregion 201; STRI Shorefishes; FishBase).

Of the roster's smaller cichlids, the convict, Amatitlania nigrofasciata, is the genuine northern Pacific-slope native — a hardy, stream-and-lake fish of southern Mexico, Guatemala and Honduras that holds rocky, flowing water and breeds in crevices, and the prototype of the whole 'convict' group hobbyists know. The closely related Amatitlania siquia is a more southern, San-Juan-province fish whose core range sits on the Atlantic slope from Honduras to Costa Rica; where it touches this ecoregion it does so at the southern, Fonseca end, and the two convicts were only relatively recently separated as distinct species. Both are small, adaptable substrate-spawners rather than open-water predators (IUCN Red List; cichlid taxonomic literature).

The rest of the roster reads as the Central American predator-and-digger guild reaching the Pacific drainages. Cribroheros longimanus, the red-breast cichlid, is a sand-sifting omnivore that ranges onto the Pacific slope from Guatemala (Río Nahualate) south to Costa Rica, digging and reworking the bottom in neutral-to-alkaline water. The two guapotes are the apex fish: Parachromis managuensis, the jaguar cichlid, and Parachromis dovii, the wolf cichlid — large, toothy piscivores reaching well over 15.5 in (dovii to around 27.5 in) that hunt fish and large invertebrates in lakes, rivers and lagoons. Their natural strongholds are the Atlantic-slope and San Juan lakes of Honduras, Nicaragua and Costa Rica; on this Pacific coast they appear at the Fonseca end and, in Honduras, have been moved by people into Pacific drainages where they were not originally found. That mix — a tightly endemic blackthroat and a few Pacific-slope natives alongside San-Juan-province cichlids spilling or carried over the divide — is exactly what FEOW means when it notes the ecoregion's high endemism set against the northernmost limits of widespread Central American taxa (FEOW Ecoregion 201; FishBase; USGS NAS; Bussing, 1976).

Conservation

The pressures on this coast are agricultural before anything else. The Pacific lowlands of Chiapas, Guatemala and El Salvador are sugar, coffee, cattle and oil-palm country, and the short rivers that cross them carry the runoff straight to the sea — sediment off cleared slopes, nitrogen and phosphorus from fertilizer, and pesticide residues that concentrate in the lagoons where cichlids breed. The pine-oak forests that once held the upper Lempa basin together have been almost entirely cleared on the Salvadoran side, which raises erosion and flashiness in rivers that were already steep and fast (FEOW Ecoregion 201; WWF/Powell et al., 2001).

The Lempa shows what a developed Central American river looks like. It is dammed in series for hydropower — Guajoyo, Cerrón Grande, 5 de Noviembre and 15 de Septiembre — with the Cerrón Grande reservoir the largest artificial water body in El Salvador. Dams in series slice a river into reservoirs, block movement between reaches, drop sediment behind the walls and even out the natural flood pulse, all of which favor open-water and introduced fish over specialists tied to flowing, rocky habitat. The Lempa also takes untreated industrial and domestic wastewater along much of its length, so that water quality, not just flow, is a live threat to its fish (Lempa basin sources; FEOW Ecoregion 201).

Then there is tilapia. African Oreochromis are farmed and stocked across these basins and the crater lakes, and the Chiapas checklist lists multiple established exotic cichlids — O. niloticus, O. aureus, O. mossambicus and Tilapia zillii among them — competing with and preying on native fish in exactly the warm, productive, often degraded waters where tilapia thrive. The crater and tectonic lakes are the sharpest worry: Lake Güija and Laguna de Metapán, a Ramsar-listed wetland, are reported to hold globally threatened native cichlids (the literature flags Cichlasoma guija and 'C.' trimaculatum) while facing invasive fish, pollution, fire and forest loss at once. The blackthroat cichlid is currently assessed Least Concern, but it is a coastal-lagoon and lower-river fish — precisely the habitat most exposed to agrochemical runoff, mangrove conversion and brackish-water aquaculture — so its status is a function of habitat that is itself under steady pressure (Velázquez-Velázquez et al., 2016; FEOW Ecoregion 201; World Bank, 2005; IUCN Red List).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Amatitlania · 3

Amphilophus · 1

Astatheros · 1

Cribroheros · 1

Parachromis · 2