Eburneo

Eburneo is the freshwater ecoregion that drains the heart of Côte d'Ivoire — the Comoé, Bandama and Sassandra rivers and the long string of brackish lagoons behind the surf at Abidjan, Tadio and Aby. The rivers fall off a northern plateau, run south through Sudanian savanna that thickens into Guinean rain forest, and spill into a 300-km belt of coastal lagoons covering roughly 1,124 mi². Its fish fauna is abundant but not especially local: when Hugueny and Lévêque compared West African basins, they found the Ivorian coastal fishes lined up with the Nilo-Sudanian fauna of the interior rather than the Upper Guinean province next door. The cichlids here are the survivors of an environment now squeezed between two dams on the Bandama and the wastewater of a city of five million.

Geography & hydrology

The ecoregion is built around three rivers that run more or less straight south to the Gulf of Guinea. From east to west they are the Comoé, the Bandama and the Sassandra, and they sit almost entirely inside Côte d'Ivoire, with their headwaters reaching north into Burkina Faso and Guinea and the Comoé brushing the Ghanaian frontier. The rivers begin on a plateau in the north of the ecoregion and descend toward the coast on a gradual incline, without the violent cataract zones that define shield rivers elsewhere in the tropics. Floodplains are limited along most of their length; the exceptions are the upper Bandama and upper Comoé, where seasonally inundated areas spread out widely during the rains (FEOW Ecoregion 514; Thieme & Abell, WWF).

The Bandama is the longest river contained wholly within the country, running about 1,31 mi from the savanna of the north to the sea. It drains a basin on the order of 97,0 mi² and is fed by two main branches, the Bandama Blanc (White Bandama) and the Marahoué (Bandama Rouge), which join in the center of the country. The Comoé, to the east, is the longest river crossing Côte d'Ivoire and gives its name to Comoé National Park, a Sudano-Guinean savanna reserve that the river threads for hundreds of kilometers. The Sassandra forms the western edge of the ecoregion; the FEOW boundary follows its lower and middle reaches, with the wetter forest catchments farther west belonging to the Upper Guinea ecoregion.

What ties the system together at the bottom is the lagoon belt. A line of shallow, brackish lagoons runs roughly 186 mi along the coast and covers around 1,124 mi² — the Ébrié, Tadio and Aby complexes the largest among them — separated from the Atlantic by sand barriers and connected to it through narrow passes and the artificial Vridi Canal at Abidjan (Roth & Waitkuwait 1986; FEOW Ecoregion 514). The rivers do not so much build deltas as discharge into these lagoons and estuaries, where their fresh water mixes with seawater pushed in on the tide. The whole landscape grades with the rainfall it sits under: Sudanian savanna woodland in the north, a forest-savanna transition mosaic across the middle, and Guinean rain forest in the south, with granite inselbergs breaking the surface in the southwest and southeast (Sayer et al. 1992). Much of that forest has since been cleared for cocoa, oil palm and other crops, which matters downstream for what the rivers carry.

Water & habitat

Two climates organize the year. The northern, Sudanian part of the ecoregion runs on a single long rainy season and a hard dry season, so the Comoé and the upper Bandama swing between bank-full flood and a chain of isolated dry-season pools; in the southern forest belt the regime softens into roughly four seasons — two wet, two dry — and flow is steadier (Iltis & Lévêque 1982; coastal-river survey, Saudi Journal of Biological Sciences). The rivers themselves are warm, generally above 77 °F, and run over sand, gravel and the occasional rocky shelf rather than the deep clear-rock channels of Amazonian shield rivers. They carry a real sediment load from the savanna and from cleared farmland, so they are turbid rather than transparent, and their chemistry tracks the geology and land use of each sub-catchment.

The more telling habitat is where river meets sea. The lagoons and lower estuaries are a moving brackish-water mosaic: salinity climbs near the ocean passes and during the dry season, then falls sharply when the rivers flood and push fresh water seaward. A survey of the Bandama estuary recorded a fairly well-oxygenated but stressed system — water temperatures above 77 °F, an acidic pH, high conductivity, and salinity above 10 parts per thousand — a chemistry that sorts the fishes by their salt tolerance as much as anything else (Bandama estuary water-quality survey, Journal of Ecology and the Natural Environment, 2022). The Ébrié, the largest lagoon in West Africa, ranges from nearly fresh at its inner ends to almost marine near the Vridi pass, and that gradient is what lets a euryhaline cichlid like the blackchin tilapia thrive across the whole of it.

In the smaller coastal rivers behind the lagoons, the fauna divides cleanly into a freshwater contingent and a marine-and-brackish one that moves up from the sea. A detailed inventory of the Nero River, a small coastal stream of about 612 mi² and 50 mi, turned up 46 species in 33 genera and 24 families; roughly thirty were freshwater fishes and fifteen were marine or brackish-water species — mullets, mojarras, snappers and the like — that penetrate the lower river. Dead wood, leaf litter and submerged roots, along with conductivity, dissolved oxygen, nitrite and channel width, were the variables that best explained which fish lived where (Konan et al. 2014, Nero River ichthyofauna). It is a fauna shaped less by current and rock than by the salt wedge, the seasonal flood, and the cover the banks provide.

The cichlid fauna

Cichlids are one of the dominant families in these rivers. In the Nero inventory the Cichlidae were the single most diverse family, with nine species, and across the wider region they make up a consistent share of the catch — alongside the tetra-like Alestidae, the schilbeid catfishes and the cyprinids. None of the Eburneo cichlids are showpiece endemics, which fits the ecoregion's profile: the coastal rivers are rich in fishes but carry low endemism, and their cichlid genera are the same broadly West African ones that range across the Nilo-Sudanian zone (FEOW Ecoregion 514; Hugueny & Lévêque 1994).

Günther's mouthbrooder, Chromidotilapia guntheri, is the small, even-tempered workhorse of these waters — a biparental mouthbrooder of vegetated banks, root tangles and slow margins that turns up as a cosmopolitan, high-tolerance species throughout coastal rivers like the Nero. Hemichromis guttatus, one of the red jewel cichlids, fills the role of the pugnacious bank-side predator; its close relative the banded jewelfish (H. fasciatus) is a near-constant in the same surveys, and the two are among the cichlids most tolerant of disturbed, modified water. These are the fishes a hobbyist would recognize: hardy, adaptable, at home where the banks are messy and the chemistry shifts week to week.

The tilapiine cichlids carry the fishery. Coptodon dageti — long known as a redbelly tilapia of the Tilapia zillii group — is a substrate-spawning, largely herbivorous fish of the rivers and their margins, the kind of tilapia that fed reservoir fisheries after the dams went in. The blackchin tilapia, Sarotherodon melanotheron, is the lagoon specialist: a paternal mouthbrooder that tolerates the full salinity range of the Ébrié and Aby, from fresh river water to near-seawater, and is among the most heavily fished and most studied fishes in the system — the subject of repeated work on its diet, parasites and contaminant load in the polluted inner lagoon.

The largest and most distinctly lagoon-bound cichlid on the roster is Tylochromis jentinki, a heavy-bodied, sand-and-mud-sifting species of the Ébrié lagoon and the lower estuaries. It feeds on benthic invertebrates and detritus across the brackish bottom, spawns mainly through the late-year months in the lagoon, and has been studied in detail there for its feeding ecology and reproduction. Together these five — a riverine mouthbrooder, two jewel cichlids, a tilapia of the streams and reservoirs, a euryhaline lagoon tilapia, and a big benthic sifter — map the whole gradient of the ecoregion, from forest creek to salt-laced lagoon, without a single narrow endemic among them.

Conservation

The clearest mark humans have left on Eburneo is the staircase of dams on the Bandama. The Kossou dam closed across the White Bandama in February 1971, drowning the center of the country under Lake Kossou — planned for 1,460 mi² but never reaching even half that, partly for want of rain. A second dam downstream created the Taabo reservoir in 1978, and the Sassandra was likewise impounded at Buyo in 1980, flooding some 559 mi². These projects rebuilt the rivers as fisheries: Lake Kossou's fleet grew from 41 canoes to more than 1,300 in four years, peak catches hit around 9,300 tonnes in 1977, and Nile tilapia (Oreochromis niloticus) and Heterotis niloticus were stocked in 1971–72 to bolster them (FAO, Côte d'Ivoire reservoir fisheries; Lake Kossou). For the cichlids the effect was a wholesale change of habitat — flowing river traded for warm, stratified standing water, where catfishes and tilapias that tolerate low oxygen do well and the riverine specialists do not.

Downstream, the problem is the lagoon. The Ébrié receives the solid, liquid and industrial waste of Abidjan, a metropolis of roughly five million people, and the result is heavy-metal and organic loading severe enough to have depleted the fishery; in parts of the once-productive lagoon, fishing has reportedly been abandoned altogether (UNEP/GRID Interactive Country Fiches, Côte d'Ivoire; UNEP Post-Conflict Environmental Assessment 2015). The fishes that remain carry the evidence. Blackchin tilapia and Tylochromis jentinki from the Ébrié have been used as monitors of contamination — studies repeatedly report elevated trace-metal and arsenic burdens in their tissues and heavy parasite loads in the most polluted sectors — because they are abundant, sedentary and eaten locally. Plastic is now a second front: Côte d'Ivoire ranks among Africa's worst for plastic leakage into the ocean, and lagoon and beach waters between Abidjan and Grand-Bassam are thick with it.

The rest of the pressure comes from the land. Côte d'Ivoire cleared much of its forest for cocoa and other export crops through the late twentieth century, and the sediment, fertilizer and pesticide that wash off cropland raise turbidity and nutrient loads in rivers that were already sediment-rich. Overfishing presses on the same stocks the dams and pollution have squeezed, and enforcement of the country's environment code is uneven. The cichlids of Eburneo are not, on the whole, threatened species — they are tough, widespread fishes that have absorbed a lot — but the system they live in has been re-plumbed by dams at the top, fouled by a major city at the bottom, and stripped of forest in between. The blackchin tilapia going hungry in a polluted Ébrié is a fair emblem of the whole ecoregion: still full of fish, but a long way from intact (UNEP/GRID; FAO; Bandama estuary survey 2022).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Chromidotilapia · 1

Coptodon · 1

Rubricatochromis · 1

Sarotherodon · 1

Tylochromis · 1