Magdalena - Sinu — aquarium fish of this freshwater ecoregion

West of the Colombian Andes, the Magdalena and its great tributary the Cauca run nearly 1,311 mi north from the highlands to the Caribbean, draining the inter-Andean trough that splits the Cordillera Central from the Cordillera Oriental. The river system was sealed off from the rest of South America when the northern Andes rose over the last ten million years, and the fishes trapped behind that wall evolved into something found nowhere else — a trans-Andean fauna heavy with endemic genera and migratory giants. At its heart sits the Mojana, a vast floodplain of shifting channels and ciénagas where the Cauca, San Jorge and Magdalena braid together. That same fauna now carries the weight of a collapsed fishery, mercury from the goldfields, and the Hidroituango dam.

Geography & hydrology

The Magdalena – Sinú ecoregion takes in the entire Río Magdalena system, its largest tributary the Río Cauca, and the separate Río Sinú to the west, all of it draining north into the Caribbean Sea through Colombia and Colombia alone (FEOW Ecoregion 302). The Magdalena rises in the Andean massif of southern Colombia and runs roughly 1,311 mi down the structural valley between the Cordillera Central and the Cordillera Oriental; the Cauca runs a parallel course one ridge to the west, between the Central and Occidental cordilleras, before the two join in the lowlands. Ringing the basin are three Andean ranges and, near the coast, the volcanic Sierra Nevada de Santa Marta, an isolated pyramid that climbs past 5,0 ft straight up from the sea. The drainage itself is young in geological terms — it took its modern form only around 10 million years ago, when the rising Cordillera Oriental cut it off from the Amazon and Orinoco systems to the east (FEOW 302; Maldonado-Ocampo et al., 2005).

The defining feature of the lowlands is the Momposina Depression, a slowly subsiding tectonic basin where the Cauca, San Jorge, Cesar and Magdalena converge. Here the rivers lose their banks and spread into the Mojana, a floodplain of braided channels, swamps, marshes and shallow lakes that Colombians call ciénagas. The depression has been sinking and filling at the same pace for thousands of years — radiocarbon work in the Momposina puts both sedimentation and subsidence at about 0.5 in a year over the last 7,500 years — so the floodplain keeps accreting rather than cutting down, and the result is one of the largest and least-studied wetland systems of any major South American river. Close to 1,900 floodplain lakes exist across the basin, and the lowlands below about 984 ft make up the largest alluvial fans in the northern Andes (Frontiers in Environmental Science, 2022; FEOW 302).

Flow follows two rainy pulses a year rather than the single flood of the Amazon, so the floodplains fill and drain twice annually and the connection between river channel and ciénaga opens and closes on that rhythm. Those pulses — their height, timing and how long the water stays up — are the cue that triggers the spawning migrations of the basin's commercial fishes. North of the Mojana the Magdalena finishes as a deltaic system fronting the Ciénaga Grande de Santa Marta, a brackish lagoon complex pinched between the river mouth and the Sierra Nevada. The Sinú, draining the Abibe highlands to the west, runs its own shorter course to the Gulf of Morrosquillo and shares much of the Magdalena's fauna without being part of the same channel network (FEOW 302; Frontiers, 2022).

Water & habitat

These are Andean-fed rivers that change character completely between the mountains and the coast. In the headwaters the Magdalena and Cauca are high-gradient, rocky and fast, fed in good part by streams that start in high-elevation páramo — the wet tropical alpine grassland above the treeline — and run cold and clear off the cordilleras. There are a few natural mountain lakes, but most of the upland habitat is moving water over rock and cobble, the domain of suckermouth catfishes (Astroblepidae, Loricariidae), pencil catfishes (Trichomycteridae) and small characins built to hold position in current (FEOW 302).

Lower down the main stems turn turbid, shallow and silty as they pick up the erosional load of a steep, deforested, heavily farmed catchment. By the time the Magdalena reaches the Momposina it is carrying an enormous sediment burden — the basin drains hilly contributing zones covering roughly 168,0 mi², about 62% of the total area, and dumps that material into the subsiding lowland trough. The floodplain lakes themselves are warm, shallow and productive; they generate something like half of the basin's fishery output, acting as nurseries and dry-season refuges where 'black' (resident) fishes ripen and spawn while the migratory 'white' fishes move through (Frontiers, 2022).

Water chemistry across the system is broadly soft to moderately hard and near-neutral to slightly alkaline, warm year-round in the lowlands. Cichlids of the basin are kept in conditions that mirror their home water — roughly pH 7 to 8, soft to medium hardness, temperatures around 75–82 °F — and FishBase records the redhump eartheater across pH and hardness ranges consistent with the Magdalena's slightly hard, neutral-to-alkaline lowland water (FishBase; FEOW 302). What controls life in the ciénagas is less the chemistry than the pulse: when the rivers rise, water and sediment flood the lakes and set off a burst of productivity; when they fall, the lakes shrink, warm and can crowd, and the fish that depend on the flood for spawning depend just as much on the flood arriving on time. Decades of dikes, drainage for cattle and agriculture, and channelization have severed many of those river–lake connections — more than 30% of the floodplain has been altered — and an estimated 88% of the wetland area in the Cauca floodplain was lost between 1950 and 1980 alone (Frontiers, 2022).

The cichlid fauna

Colombia's trans-Andean basins hold a fauna shaped by isolation. When the Cordillera Oriental finished rising and closed the Magdalena off from the Amazon and Orinoco, the cichlids on the western side were stranded; what they have radiated into is a smaller, more distinctive set of lineages than the cichlid riot of the cis-Andean lowlands. The Magdalena is famous among ichthyologists for its endemic non-cichlid genera — the catfish Eremophilus and Centrochir, the characins Genycharax and Carlastyanax — but its cichlids are the part most aquarists meet, and the atlas roster captures the core of that west-of-Andes assemblage (FEOW 302; Musilová et al., 2015).

The two acaras anchor it. Andinoacara latifrons, the platinum or smooth-headed blue acara, ranges through the Magdalena, Cauca, Sinú and San Jorge — the platinum sheen and red-edged fins make it one of the most recognizable lowland cichlids of northern Colombia. Its close relative Andinoacara pulcher, the blue acara of the aquarium trade, occupies much the same waters and the basins immediately around them. The genus Andinoacara is itself a trans-Andean signature, its distribution mapping onto exactly the country the Andes walled off, from the Magdalena across to Lake Maracaibo (FishBase; trans-Andean cichlid biogeography, Vertebrate Zoology). Sifting the bottom alongside them is Geophagus steindachneri, the redhump eartheater — a sand-winnowing mouthbrooder from the Magdalena, Cauca and Sinú in which courting males swell a fatty red bulge on the forehead. It is one of only three species in the steindachneri group, all tied to this northwestern corner of South America, and it has been a hobbyist favorite for decades.

The basin's heroine cichlids run from the merely large to the largest in the New World. Caquetaia kraussii, the yellow-and-black 'mojarra amarilla,' is a deep-bodied, big-mouthed predator endemic to Colombia and Venezuela that thrives across the ciénagas and slow channels and shows up in artisanal catches. Towering over it is Kronoheros umbrifer (long known as Caquetaia umbrifera, and listed in some references as umbriferus) — the turquoise cichlid, whose genus Říčan and colleagues named in 2016 for Kronos, leader of the Titans, because it grows into the largest-bodied cichlid in the Neotropics, past 17.5 in. It hunts open water for Astyanax and other fishes, shrimp and crabs, ranging from the Magdalena and Atrato across to the Tuíra and Chucunaque drainages of Panamá, a distribution that traces the old land connections of the trans-Andean region (FishBase; Říčan et al., 2016). Cichlids are a modest slice of a basin fauna dominated by characins and catfishes — the headline commercial fishes, the bocachico and the bagre rayado striped catfish, are not cichlids at all. The basin's cichlids are the resident generalists of the ciénaga edge, the eartheaters and acaras and predators that hold their ground while the migratory giants come and go (Frontiers, 2022; FEOW 302).

Conservation

The Magdalena's fishery has already collapsed once. Landings ran near 80,000 tons a year in the 1970s; by the 2000s they had fallen to roughly a tenth of that, and catches across the basin are estimated to have dropped about 90% between 1970 and 2000. The bocachico Prochilodus magdalenae — a migratory detritivore that has supplied 80–95% of the catch by weight for decades — has been shrinking in size and age at capture even as fishing pressure climbs, and the bagre rayado or Magdalena striped catfish Pseudoplatystoma magdaleniatum has seen its landings fall by roughly 90% and is now red-listed as critically endangered in Colombia. The drivers are layered: overfishing by a growing, often landless riverine population; deforestation and the resulting sediment load; the diking and draining of the ciénagas that cut migratory fish off from their nurseries; and dams that block the river's longitudinal connections. The cichlids of the basin are caught up in the same fishery — Andinoacara latifrons and the predatory Caquetaia kraussii both turn up in artisanal landings — and they depend on the same flood pulse and ciénaga refuges now being severed (The Magdalena River fresh water fishes and fisheries, 2007; Frontiers, 2022).

The most acute recent blow has been Hidroituango, the 2,400-MW dam on the lower Cauca and the largest hydroelectric project in Colombia. During construction a diversion tunnel collapsed amid heavy rain in April–May 2018, triggering what authorities called the worst socio-environmental disaster in the country's history and forcing mass evacuations downstream. Then in early February 2019, EPM closed the floodgates to fill the reservoir and abruptly cut the Cauca's flow — reducing it from a normal dry-season 1476–1640 ft³/s to as little as 115 ft³/s, around a tenth of normal — and stranded fish along the dewatered channel. Estimates of the kill from those days run from several thousand to tens of thousands of fish, and the reservoir now traps the Cauca's sediment and reworks the lower basin's flood pulse (Mongabay, 2019; The Bogotá Post, 2019; AIDA).

Pollution compounds the loss. The Mojana floodplain sits downstream of decades of artisanal and small-scale gold mining, and the mercury used to amalgamate that gold has worked into the food web: studies of the most-consumed fish in the region find the ichthyofauna contaminated with methylmercury at levels treated as a public-health problem, the heaviest burden falling on the piscivores — the feeding niche of Caquetaia kraussii and Kronoheros umbrifer. Invasive fishes pile on top: more than 40 alien species have been logged in the basin, and Nile tilapia (Oreochromis niloticus) has climbed to among the most-landed fishes in the system, competing with native cichlids in the warm, shallow ciénaga habitat they favor. The river's fishes are squeezed between a broken flood pulse, a contaminated floodplain, introduced competitors and a fishery that already overshot its limits (SciELO/Rev. U.D.C.A, 2022; Frontiers, 2022).

Magdalena - Sinu

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Andinoacara · 2

Caquetaia · 1

Geophagus · 1

Kronoheros · 2