Taxonomy & naming
George Albert Boulenger described this fish in 1912 as Tilapia macrochir, from material collected in the upper Zambezi River, in a short paper in the Annals and Magazine of Natural History introducing three new African cichlids. Trewavas later selected a lectotype, BMNH 1908.11.6.35, held at the Natural History Museum in London, and in her 1983 revision of the tilapiine genera moved the species into Oreochromis — the lineage of maternal mouthbrooders separated from the substrate-spawning Tilapia and the paternal/biparental Sarotherodon. It sits in the cichlid subfamily Pseudocrenilabrinae, tribe Oreochromini.
The species name is a tidy piece of description: from the Greek makros (large) and cheir (hand), the latter standing for the pectoral fin — a nod to the long fins that give the fish its English name. Several historical names fall into its synonymy, including Castelnau's Chromys chapmanii and Coptodon sparmanni (1861) from Lake Ngami, and Tilapia intermedia and Tilapia sheshekensis of Gilchrist & Thompson (1917). Common names track its broad use: 'longfin tilapia' and 'greenhead tilapia' in English, 'green-headed bream' or 'Kafue bream' in the Zambian fishery, and 'Igihwati' in Kinyarwanda.
One genuine taxonomic snag is worth flagging, because it bears directly on Lake Kivu. Oreochromis macrochir is easily confused with Oreochromis mweruensis Trewavas, 1983, the Mweru tilapia of the Luapula–Mweru–Congo system. The two were long treated as strains or subspecies of one species and hybridize freely in captivity; Schwanck (1994) argued they are distinct, separable partly by nest shape (a volcano-shaped mound in macrochir versus a star-shaped one in mweruensis). FishBase still notes that the boundary, and the identity of many stocked populations, is 'not clear.'
Morphology
This is a deep-bodied, laterally compressed tilapia built on the standard Oreochromis plan, reaching a maximum of about 17 in (17 in) total length, though fish in the 8–11 in (8–10.5 in) range are far more typical in surveys and catches. The head profile is steep and rounded. Fin counts run to roughly 15–17 dorsal spines with 11–14 soft rays, three anal spines with 9–12 soft rays, and 29–32 vertebrae.
Color is the muted olive-to-silver of a floodplain grazer rather than the electric blues of a rift-lake rock dweller, often with a greenish cast over the head — the source of 'greenhead.' Trewavas's diagnosis emphasizes dark brown to black flecks scattered over the temporal region, gill cover and below the eye, mostly tied to the openings of the lateral-line system, and the absence of the conspicuous mid-lateral blotches some congeners show. Breeding males flush darker and develop the long, trailing fins that give the species its name, along with a notably long, tassel-like genital papilla used in spawning. The single most reliable in-the-field separator from its near-twin Oreochromis mweruensis is behavioral, not anatomical — the shape of the nest the male builds.
Habitat
The natural range is firmly south-central African: the upper Zambezi, Kafue and Okavango systems, the upper Kasai, and the Lake Bangweulu–Chambeshi region, with the Cunene to the west. The IUCN's native-range list runs across Angola, Botswana, Namibia, Zambia, Zimbabwe, Eswatini and South Africa. This is a fish of quiet water: river margins, backwaters, floodplain lagoons, swamps and man-made impoundments, usually shallow — recorded depths of about 16–46 ft (16–46 ft) — and often associated with aquatic vegetation. It tolerates a wide temperature band (roughly 64–95°F / 64–95 °F) but has very low salinity tolerance, which keeps it strictly freshwater.
Lake Kivu is not part of that natural range. The longfin tilapia is an introduced species there, and the introduction is well documented in the De Vos, Snoeks and Thys van den Audenaerde checklist of Rwandan fishes: a Luapula–Mweru stock was brought into Rwanda from 1949, and a Kafue stock after 1952, as part of pond-culture and stocking efforts. From those plantings it established in Lake Kivu and the upper and middle Akagera. So on this site it belongs in the 'introduced to Kivu' column — useful to understand the lake, but not a member of its native fauna. Lake Kivu's own endemic cichlids are a small flock of roughly fifteen haplochromines (Snoeks and colleagues), confined to the thin oxygenated surface water that is the only habitable layer of this unusual lake.
Feeding
Oreochromis macrochir is a low-trophic-level grazer, sitting at about trophic level 2.1 — close to a strict herbivore-detritivore. Adults feed mainly on detritus and the periphyton and phytoplankton that come with it: diatoms, other algae and blue-green algae (cyanobacteria), scraped and filtered from soft sediments and vegetation. Juveniles will take small invertebrates and zooplankton, but they lose that appetite as they grow into the adult diet. Food-habits work on the upper Zambezi tilapiines (Winemiller & Kelso-Winemiller) found heavy dietary overlap among the floodplain tilapias, which is part of why introducing a competitor matters so much for this species.
Ecologically it is a productive, fast-turnover fish — high resilience, a generation time of only two to three years, and a population-doubling time under fifteen months — the profile that makes tilapias such reliable aquaculture and forage species and such effective colonizers when moved into new waters. In its native floodplains it converts detritus and algae into a substantial standing crop of fish that supports both subsistence fisheries and larger predators.
Mating
The longfin tilapia's courtship is one of the more architecturally interesting in the genus. In the breeding season — September to March in the southern part of the range — males move into shallow water, generally less than about 5 ft (5 ft) deep, and excavate a nest that is not a simple pit but a central volcano-shaped mound with a flat or slightly dished top serving as the spawning platform, ringed by a ditch and a raised boundary wall. These nests are typically clustered into communal arenas — a classic lek — with the cone of each rising above its surrounding wall. This volcano form is the trait used to tell the species apart from Oreochromis mweruensis, whose nest is star-shaped.
Spawning is brief and promiscuous, with no pair bond: partners associate only for the act itself. A male courts females in succession at the center of his mound; a female may lay only a few to a few dozen eggs at a time before taking them into her mouth, and may move between several males in a season, ultimately brooding a mixed clutch. The male's long, trailing fins and dark breeding dress, together with the conspicuous tassel-like genital papilla, are the signals he advertises from his bower.
The tassel itself is more than an ornament. In several maternal-mouthbrooding Oreochromis, oral rather than substrate fertilization is inferred from the way a female snaps at the male's genital region while she is laying and collecting eggs, and researchers have suggested the fringed tassel works on the same principle as the anal-fin egg-spots of haplochromine males — a lure the female mouths as though it were an egg of her own, which puts her in position to take up sperm. Observations of Oreochromis macrochir do not fully agree on how this plays out: on the Lufira River, males and females were seen gliding together over the nest in the more familiar pattern of laying and fertilizing directly on the mound, while separate captive observations recorded a female mouthing the male's tassel itself. One proposal, from Wolfgang Wickler, is that the tassel holds a film of seminal fluid at its surface — effectively a crude, external analogue of a spermatophore — long enough for the female to pick up a working dose when she takes it into her mouth. It is a plausible mechanism rather than a settled one, and the field and captive accounts for this species have not yet been reconciled.
Breeding
Like all Oreochromis, Oreochromis macrochir is a maternal mouthbrooder: once eggs are fertilized over the male's mound, the female alone takes on incubation and there is no further male involvement. Depending on her size a female can carry up to about 1,300 greenish-brown eggs roughly 0.5 in across, holding them in the buccal cavity through development.
The brooding female retreats into thick vegetation to incubate. Fry begin leaving the mouth before the yolk sac is fully absorbed but are re-taken into it at night or whenever threatened, and become independent at around 1 in and three weeks of age. A single female may raise several broods over a season at roughly five-week intervals — exactly the high reproductive turnover that lets stocked populations build so quickly once established in new water.
In the aquarium
Be honest about what this is: a food-and-pond fish, not an ornamental cichlid, and almost no one keeps it as a display animal. It reaches well over a foot long, grazes constantly, digs, and breeds prolifically, so the practical requirements are those of any large tilapia — a big tank or, more realistically, a pond, with strong filtration to cope with a heavy, plant-and-detritus diet. Across the broader tilapia-keeping community the consistent reports are the same regardless of species: rapid growth, relentless reproduction (broods every few weeks once mature), and territorial squabbling that intensifies around spawning, even if these fish are not the relentless killers some rift-lake mbuna can be.
Where it does show up in the aquarium and pond trade is functional. In Zambian aquaculture the greenhead bream is grown in polyculture alongside the three-spotted bream Oreochromis andersonii and the redbreast Coptodon (Tilapia) rendalli, and it has a long history as a stocked sport and food fish. Water-wise it is undemanding — warm, fresh, near-neutral to alkaline (about pH 6.5–8.5), with essentially no salt tolerance — but the real cautions are ecological rather than husbandry: because it hybridizes readily with relatives (Oreochromis mweruensis, and the invasive Oreochromis niloticus) and breeds explosively, it should never be released or allowed to escape into wild waters. Most of the harm this species has done to native fishes came from exactly that kind of well-intentioned stocking.
Conservation
On the IUCN Red List, Oreochromis macrochir is assessed as Vulnerable under criterion A3e (Marshall & Tweddle, assessment published 2007, flagged as needing updating), with the population trend listed as unknown and the species itself described as 'common and widespread.' The Vulnerable status rests on a single projected threat: competition with, and hybridization by, the introduced Nile tilapia (Oreochromis niloticus), which already displaces macrochir where the two meet on the Zimbabwean plateau and is spreading through the Zambezi, Kafue and Limpopo systems. The assessors anticipated a decline of at least 30% over a decade as Oreochromis niloticus advances, and suggested the Cunene may end up the last safe refuge. There is no notable aquarium-collection pressure; the risk is entirely about a more competitive farmed relative loose in the wild.
Lake Kivu adds a different and sharper kind of context — though it cuts the other way, since here macrochir is itself the introduced fish. Kivu is a meromictic Rift Valley lake: its deep water never mixes with the surface, and seasonal overturn reaches only about 200 ft (197–213 ft) down. Below that lies one of the planet's most unusual hazards — a vast dissolved-gas reservoir holding on the order of 60 km3 of methane and 300 km3 of carbon dioxide (Eawag/Schmid and colleagues), enough to support both a recognized limnic-eruption hazard and a growing commercial methane-extraction industry on a densely populated shoreline. Every fish in Kivu, native or stocked, is confined to the thin oxygenated surface layer above that gas, and the pelagic zone has already been reworked once by people — by the 1959 introduction of the Tanganyika sardine Limnothrissa miodon. So the framing for this species is twofold and should not be overstated: as a fish, macrochir is globally Least-Concern-adjacent — 'Vulnerable' only because of an invasive cousin, not because it is rare — while the lake it was planted into is genuinely strained by shoreline population pressure, industrial gas activity, and the physical narrowness of its habitable surface water.