Upper Lualaba

The Upper Lualaba is where the Congo River begins to behave like the Congo: a young, vigorous river that has just descended off the Katanga highlands and spread itself across a vast graben valley, the Kamalondo (Upemba) Depression, before knitting back into a single channel and heading north toward the rapids that will rename it. Some 8,0 mi² of shallow lakes and papyrus swamp fill the floor of that depression, flooding out to nearly 12,0 mi² when the rains come. It is one of Africa's great floodplain-and-lake mosaics, mostly contained within Upemba National Park, and despite a century of collecting it remains one of the least-studied corners of the Congo system — with new fishes still turning up on almost every expedition.

Geography & hydrology

The Lualaba is the Congo's main headstream — the river that carries the name 'Congo' only after it clears the cataracts far downstream. The Upper Lualaba ecoregion (FEOW Ecoregion 545) is the stretch where that headstream, having dropped off the copper-bearing Katanga plateaus of the southern Democratic Republic of Congo, fans out across the Kamalondo Depression. Cartographers also call it the Upemba Depression, after its largest lake. The ecoregion is the depression itself plus the wetlands, swamps and lakes strung along the Lualaba below it, together with the adjacent Lululwe, Kilubi and Lovoi rivers draining in from the west (FEOW Ecoregion 545).

The depression is a tectonic feature, a graben pulled open by crustal extension probably during the Miocene-Pliocene, walled by the Hakansson Mountains to the west (to about 1,656 ft) and the Manika Plateau and Mulumbe Mountains to the east (to 1,2917 ft). Its floor runs roughly 249 mi long and 62 mi wide. At the southwestern end the land sits near 1,0 ft and falls steeply to about 1969 ft, where it flattens and the river slows into a chain of lakes — Kabwe, Kabale, Mulenda, Upemba, Lukanga, Kisale, Kalombwe, Zimbambo and Kabamba, from south to north. Geologists read the depression as the bed of a vanished ancient lake, infilled over time by sediment carried in by the Lualaba, Luvua, Lufira and Luvoi (FEOW Ecoregion 545; Bailey 1986; Hughes & Hughes 1992).

The lakes are uniformly shallow, all of them between 1 and 23 ft deep, and several are simply wide spots in the river: Kabwe and Kisale are expansions of the main channel, with the Lualaba flowing straight through them. Lake Upemba, the largest, spreads over 311–497 mi² along a 70-km basin yet bottoms out at only about 10 ft. Below Upemba the Lualaba takes on its biggest tributary, the Lufira, a right-bank river fed in part by calcareous subterranean streams and warm saline springs, which gives the Lufira a different water signature from the main stem. Past the confluence the Lualaba narrows to roughly 328 ft wide and threads an 80-km swamp belt holding a further cluster of small lakes. The two rivers flood on the rains: the Lualaba runs highest from February to April and lowest from August to October, with an annual swing near 9 ft in the south, while Lake Upemba peaks a little later, March to June (FEOW Ecoregion 545; Bailey 1986). The whole ecoregion lies almost entirely within the 17,730-km² Upemba National Park, which the Lualaba/Kamalondo and the Lufira split into a northern and a southern sector (Katongo et al., Diversity 2023).

Water & habitat

Functionally this is a floodplain-and-lake system, the antithesis of a clearwater rapids river. The character of the place is set by shallow, sun-warmed, wind-stirred water sitting over a swamp floor. Lake Upemba is the type case: nowhere more than waist-to-chest deep, churned daily by wind so that even the bottom water becomes oxygen-supersaturated by day, heavily eutrophic, and the scene of intense algal production. Its northern half is roofed almost completely by stands of cattail (Typha domingensis) (FEOW Ecoregion 545; Beadle 1981; Hughes & Hughes 1992).

What holds the system together is the seasonal pulse. By some estimates the depression carries about 8,0 mi² of lakes and wetland in the dry months, with another 497 mi² of wetland along the Lululwe, Kilubi and Lovoi to the west; at flood the wetlands swell to roughly 11,522 mi², the figure long used as the depression's maximum flooded area. During the dry season the Lualaba runs through a navigable, usually dredged channel, but as the floods rise the surrounding country drowns and floating papyrus mats break loose and clog the channel (FEOW Ecoregion 545; Welcomme 1979; Hughes & Hughes 1992). Tall emergent vegetation lines river and lake alike — papyrus (Cyperus papyrus) and cattail above all — and the open water carries Nile lettuce (Pistia stratiotes), water chestnut (Trapa natans), the blue and white water-lilies Nymphaea caerulea and N. lotus, and Nymphoides indica. Where sandy ground rises above the swamp, patches of woodland take hold, dominated by ambatch (Aeschynomene elaphroxylon) and Hibiscus diversifolius, themselves flooded under 15.5–23.5 in of water for part of the year (FEOW Ecoregion 545; Marlier 1973).

The ecoregion is not uniform, and that variety is what makes it interesting for fishes. The lowland lakes and swamps offer warm, plant-choked, eutrophic shallows; the inflowing rivers off the plateaus bring cooler, faster, rockier water. Falls and rapids — the Kyubo Falls on the Lufira, the cataracts that isolate river sections above the Nzilo Dam on the upper Lualaba — chop the drainage into compartments. The Lufira in particular runs over rapids that hold a specialised current-loving fauna quite unlike the swamp fishes a few dozen kilometres downstream, and these barriers, by keeping upstream populations apart, are a large part of why the ecoregion's endemism is concentrated in its rivers rather than its lakes (FEOW Ecoregion 545; Katongo et al., Diversity 2023).

The cichlid fauna

Cichlids are common and conspicuous here without being the most diverse family — the recent Upemba checklist counted sixteen cichlid species against fifty-one cyprinids — but they include the fishes that fill the lakes' nets and the rapids' crevices alike. The roster for this water area gathers nine of them, and they sort cleanly into the ecoregion's two worlds: the open, eutrophic floodplain lakes, and the broken, current-swept river reaches (FEOW Ecoregion 545; Katongo et al., Diversity 2023).

The tilapias are the lake-and-swamp generalists. Oreochromis upembae, the Upemba tilapia, is the signature fish of the depression — abundant in the lakes and so closely tied to this country that it is recorded from only three ecoregions in all (FEOW Ecoregion 545; Banister 1986). Beside it swim the longfin tilapia Oreochromis macrochir (assessed Vulnerable by IUCN), the redbreast tilapia Coptodon rendalli, a robust herbivore of vegetated shallows, and the small swamp-loving Tilapia ruweti, the Okavango tilapia, whose presence here in the upper Congo is a fingerprint of the old wetland connections between this basin and the upper Zambezi. That link is not merely historical: redbreast tilapia and other species can still cross between the Kamafwafwa (upper Lualaba) and the Kanyita (upper Zambezi) at flood, through the swamp that joins the two headwaters (Katongo et al., Diversity 2023). Rounding out the broad-niche cichlids are two earth-eating Tylochromis — T. variabilis and the heavier-jawed T. mylodon, both products of Stiassny's 1989 revision of the genus — sand-sifters of the lower, slower reaches.

The predators and the rheophiles tell the other half of the story. Serranochromis thumbergi, the brownspot largemouth, is a large ambush piscivore of the floodplains, one of the Zambezi-affinity serranochromines that reach their northern limit in this corner of the Congo. Pseudocrenilabrus nicholsi is a small, intensely coloured mouthbrooder of the depression's quieter waters. Lamprologus mocquardi represents the riverine lamprologines that, in the Congo proper, fill the rock-dwelling roles their relatives made famous in Lake Tanganyika. The Lufira rapids carry the ecoregion's true cichlid endemics — fishes like Lamprologus symoensi, a small lamprologine described from and confined to these fast Lufira reaches, and the torrent-adapted Orthochromis species that wedge into the current above and below the Kyubo Falls (FEOW Ecoregion 545; Katongo et al., Diversity 2023). The Lualaba/Upemba and the Lufira are treated as two distinct fish communities, each with at least fourteen endemic taxa, and the cichlids straddle both — supplying the eutrophic lakes their tilapias and the rocky cataracts their specialists. New forms still emerge from museum drawers and recent collections: undescribed Oreochromis and Sargochromis from the Lufira await formal names, so the cichlid list here is provisional rather than closed (Katongo et al., Diversity 2023).

Conservation

Most of the ecoregion sits inside Upemba National Park, gazetted in 1939 and, at 17,454 mi², the DRC's second-largest protected area after Salonga. On paper that is real protection, and the park's core does shelter its fishes by limiting access. In practice the protection is uneven: the annex and buffer zones, where most people live and fish, are under heavy pressure, and the park has spent decades short of funding and security in a region scarred by armed conflict (Katongo et al., Diversity 2023).

The gravest threat reaches the depression from upstream, off the Katangese copper belt. Industrial copper and cobalt mining above the park discharges untreated effluent straight into the rivers that feed it, and the hydroelectric reservoirs on those same rivers — Nzilo and Nseke on the upper Lualaba, Tshangalele and Koni on the upper Lufira — double as settling basins for that contaminated load. The danger is not abstract: in 2005 the opening of the Nseke Dam floodgates sent muddy, metal-laden water down into the Kamalondo Depression and killed fish across the annex zone of the park. Because more active mining means faster siltation of those reservoirs, and flushing the gates is the cheap way to clear the sediment, the researchers who compiled the modern checklist expect such releases — and such fish kills — to recur for decades as Katanga's mineral economy keeps expanding (Katongo et al., Diversity 2023).

Local fishing presses on the fauna from within. Surveys through the 2010s documented seine nets dragged through the lakes, mosquito-net seines fine enough to strip out juveniles, brush and fishing weirs strung across rivers, and the widespread use of plant ichthyotoxins — fruits of Amblygonocarpus andongensis and the leaves of Tephrosia vogelii — to stun fish in the smaller waters. Water is being pulled off the system too: stretches of rivers such as the Luvilombo have been left dry by upstream diversion for irrigating bean fields. Each of these chips at the same floodplain pulse and the same shallow, vegetated habitats that the depression's tilapias and mouthbrooders depend on for spawning and nursery cover (Katongo et al., Diversity 2023).

The deeper problem is how little is firmly known. FEOW rates the ecoregion's level of taxonomic exploration as poor, and the 2023 Upemba checklist — which raised the park's tally to 247 native species, 18% of them endemic and a further 14% still awaiting formal description — only sharpened the point that this is a biodiversity hotspot being altered faster than it is being catalogued. Its authors call for fish-specific conservation guidelines for the park, the kind of management that has never really existed here. For now the endemic cichlids of the Lufira rapids and the tilapia fishery of the Upemba lakes share the same predicament: protected in name, squeezed in fact, and racing a mining frontier that is reshaping the water before science has finished reading it (FEOW Ecoregion 545; Katongo et al., Diversity 2023).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Coptodon · 1

Lamprologus · 1

Oreochromis · 2

Orthochromis · 1

Pseudocrenilabrus · 1

Serranochromis · 2

Tilapia · 1

Tylochromis · 2