Bight Drainages

The Bight Drainages are the short coastal rivers and lagoons that empty into the Gulf of Guinea — the old Bight of Benin — across southern Togo, southern Benin and the southwestern corner of Nigeria. The Mono, the Ouémé and the Couffo run only a few hundred kilometres off low plateaus before fraying into swampy deltas and a near-continuous chain of brackish lagoons: Lake Togo, Lake Ahémé, the Porto-Novo lagoon and, largest of all, Lake Nokoué at Cotonou. This is a savanna coast, not a rainforest one — the rivers cut through the Dahomey Gap, a wedge of dry country that splits the West African forest belt in two — and the fish here are mostly hardy, wide-ranging tilapias and their relatives rather than narrow endemics.

Geography & hydrology

FEOW places these rivers in Ecoregion 517, the Bight Drainages, a 'tropical and subtropical coastal river' unit that takes in the catchments draining into the Gulf of Guinea between the Volta to the west and the Niger delta to the east. It sits squarely inside the Dahomey Gap — the band of savanna, running roughly from the Cavally in Côte d'Ivoire to the Cross River, that interrupts the Guinean forest along this stretch of coast. Geographically the ecoregion covers the southern halves of Togo and Benin, the southwest of Nigeria, and a sliver of southeastern Ghana (FEOW Ecoregion 517).

The rivers are short and modest by African standards. The Mono drains the southern and central parts of the Chaîne du Togo and runs about 249 mi to the sea, forming much of the Togo–Benin border before reaching its delta. The Ouémé is the longest river wholly within Benin at around 317 mi, draining most of the country's low southern plateau (656–984 ft above sea level); its main tributaries include the Okpara and the Zou, and permanent swamps sit in the Okpara headwaters. The Couffo is smaller still, feeding Lake Ahémé. East of Benin, the Ogun and Oshun rivers drain the southwest corner of Nigeria into the Lagos and Lekki lagoon system. None of these is a large-discharge river; what defines the ecoregion is less any single channel than the way they all behave near the coast (FEOW Ecoregion 517; Hughes & Hughes 1992).

Where the rivers reach the flat coastal plain they slow, spread and connect. Each ends in a swampy delta, and the deltas merge into a chain of lakes and lagoons strung behind the beach: the Lagos (311–373 mi²) and Lekki (153 mi²) lagoons in Nigeria, Lakes Nokoué (93 mi²), Ahémé (53 mi²) and the Porto-Novo lagoon (19 mi²) in Benin, and Lakes Togo (29 mi²) and Vogan (5 mi²) in Togo. Channels link many of them, but those channels are seasonal — some dry out completely in the dry months — so the whole coastal system tightens and loosens through the year. Lake Nokoué, the largest standing water in Benin, is the clearest example: it is fed by the Ouémé from the east and the Sô from the north, but its flood does not arrive with the rains. The lake fills about a month after the main June rains, swelling from August to November as the two rivers deliver their runoff (FEOW Ecoregion 517; Houngnigbo et al. 2022, Ecologies).

Water & habitat

The signature of this coast is shifting salinity. The lagoons are tidally or seasonally connected to the Atlantic, and the balance between river runoff and seawater swings hard across the year. Through the wet season the freshwater pulse from the Ouémé, the Sô, the Mono and the Couffo pushes the lagoons fresh — Nokoué can desalinate almost completely and take months to recover — while in the dry season, with the rivers low, seawater intrudes through the inlets and salinity climbs. The result is that a single lagoon can read as a freshwater lake for part of the year and a brackish estuary for the rest, and the fish living in it have to tolerate both (FEOW Ecoregion 517; Houngnigbo et al. 2022).

Lake Nokoué shows how extreme that can get. The lake averages only about 5 ft deep and connects to the sea through the dredged Cotonou channel cut in the 1880s, which lets Atlantic water reach it on the tide. In a year of monitoring around 2020–2021, researchers logged the saline intrusion of the dry season as strong enough to encourage shipworms (Teredo) that rot the wooden brush parks the fishers build, while the August–November flood from the Ouémé and Sô drives the water fresh again. The lake is shallow, warm and turbid, with clay-and-sand bottoms, and it silts steadily as river sediment and decaying vegetation accumulate. Eutrophication is heavy: water hyacinth can blanket up to 40% of the surface at flood, and dense mats of it choke oxygen out of the water beneath (Houngnigbo et al. 2022).

Upriver the habitats are more conventionally freshwater but still strongly seasonal. The Ouémé and Mono are floodplain rivers — the flood spreads onto inundated forest and grassland along the Ouémé and through narrow valley strips in the more mountainous parts of Togo, and fish move out onto the floodplain to feed and spawn as the water rises, then retreat to channels and pools as it falls. Streams like the Okpara, a clear-water Ouémé tributary, carry a dry-season fish community concentrated into shrinking pools, which is part of why a top predator such as Hemichromis can come to dominate there. Mangrove (Rhizophora and Avicennia) fringes the lagoons on the sandy coastal soils, grading into reed-swamp of Typha and Cyperus and grassy floodplain behind. It is a patchwork of fresh, brackish and swamp water rather than one uniform river system (FEOW Ecoregion 517; Hughes & Hughes 1992; Montchowui et al. 2019).

The cichlid fauna

The fish fauna of the Bight Drainages is Nilo-Sudanian in character and short on endemics. FEOW's own reviewers note the low endemism here and explain it historically: the Dahomey Gap is dry country, and during the arid phases of the Ice Ages — most recently around 18,000 years ago, with the present gap reforming about 4,000 years ago — many of these small coastal rivers are thought to have dried out, then to have been recolonised by fish spilling in from the Niger system. What survives is a fauna of tough, wide-ranging species rather than locally evolved specialists, and the cichlids are the clearest example (FEOW Ecoregion 517; Lévêque 1997).

The atlas roster for the ecoregion is a compact set of these generalists. Coptodon guineensis (long known as Tilapia guineensis) is the classic euryhaline tilapia of the West African coast, at home in fresh lakes and brackish lagoons alike and a mainstay of the lagoon catch. Pelmatolapia mariae (the spotted tilapia, formerly Tilapia mariae) is a substrate-spawning, more vegetation-associated tilapia of the rivers and coastal lakes. Chromidotilapia guntheri is a small, biparental mouthbrooder of streams and river margins — it turns up in inventories of the Ouémé system, including clear tributaries like the Okpara — and Thysochromis ansorgii is the lagoon-and-coastal dwarf cichlid characteristic of exactly this Bight-of-Benin belt of brackish water. Hemichromis guttatus, one of the red 'jewel' cichlids, rounds out the roster as a small, pugnacious predator of vegetated margins (Adite et al. on the Mono lakes; Montchowui et al. 2019; FEOW Ecoregion 517).

What these fish lack in rarity they make up for in sheer weight. Cichlids dominate the fisheries of southern Benin, making up close to half — around 50% — of total inland fish production, and a single brackish-tolerant tilapia, Sarotherodon melanotheron, can account for two-thirds or more of the catch in places like Lake Toho on the Mono. That dominance is tightly bound to the region's most distinctive fishery, the acadja. An acadja is a brush park: fishers drive bundles of branches into the soft lagoon bottom, the submerged wood grows a film of algae and invertebrates, fish gather to feed and shelter, and after some months the whole plot is encircled with netting and harvested. On Lake Nokoué these brush parks cover a large share of the lake — on the order of 15% or more of its surface — and they have turned much of the lagoon into a patchwork of semi-cultivated fish habitat. Studies of the population dynamics of Sarotherodon melanotheron and the predatory Hemichromis fasciatus under acadja pressure have become a small literature of their own, because so much of the lagoon's cichlid biology now plays out inside these wooden enclosures (Adite et al.; KMAE 2010; Houngnigbo et al. 2022).

Conservation

The pressures on these waters are concentrated where the people are: the coastal lagoons. Lake Nokoué sits inside Benin's densest urban cluster — Cotonou, Porto-Novo and Abomey-Calavi, with average densities around 4,000 people per km² along its shore, and on the order of a million and a half people depending on the lagoon for fishing and transport. The famous stilt village of Ganvié, often called the 'Venice of Africa,' is built out over the water. Untreated urban and household waste from Cotonou drains straight into the lake, and a hydrological quirk makes it worse: modelling shows pollutants build up through the dry season and then flush out rapidly to sea on the rains, concentrating the load in time (Houngnigbo et al. 2022; IRD, Nokoué, a lagoon under pressure).

The lagoons are filling in. Nokoué silts at roughly 0.5–1 in a year in places, dropping to an average of about 4 ft deep in the dry season, as river sediment, decaying water hyacinth and the rotting branches of abandoned acadjas pile up. The acadja fishery is itself part of the problem and the point: brush parks are highly productive, but the volume of cut wood they require drives mangrove and woodland clearance around the lagoons, and the parks accelerate sedimentation and crowd the open water. Overfishing layers on top — the gear used in and around the acadjas, including fine-mesh nets, takes fish small, and catch-per-effort on the cichlid stocks has been under chronic pressure. Mangrove loss removes the nursery cover that euryhaline tilapias like Coptodon guineensis and Sarotherodon melanotheron use, and water-hyacinth blooms periodically strip oxygen from large areas (IRD, Nokoué, a lagoon under pressure; Houngnigbo et al. 2022).

Upstream, the rivers carry their own legacy. The Mono is dammed at Nangbéto (commissioned 1987, built without an adequate resettlement plan) and faces the prospect of the larger Adjarala project (96 MW) downstream, which would displace several thousand more people across Togo and Benin. Dams on a short coastal river of this kind flatten the seasonal flood that the floodplain fishery depends on, cut the sediment and freshwater reaching the delta and Lake Ahémé, and can sharpen seawater intrusion at the coast. The Mono catchment is also flagged for worsening flood hazard under climate and land-use change, a reminder that the same flood pulse that sustains these fisheries is becoming harder to predict. For a fauna that is already poor in endemics and leans on a handful of robust, salt-tolerant cichlids, the real risk here is not extinction of unique species but the steady degradation of the lagoons and floodplains that carry one of West Africa's most productive inland fisheries (International Rivers, Africa projects; MDPI Sustainability 2023, Mono flood hazards; FEOW Ecoregion 517).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Chromidotilapia · 1

Coptodon · 1

Pelmatolapia · 1

Rubricatochromis · 1

Thysochromis · 1