Taxonomy & naming
Oreochromis squamipinnis was described by the British ichthyologist Albert Günther in 1864 as Chromis squamipinnis, in his report on the fishes Dr. John Kirk collected during Livingstone's Zambezi expedition to the Nyassa (Malawi) region; the type locality is Lake Malawi. The species has since passed through the long taxonomic reshuffling of the African tilapias. Ethelwynn Trewavas, in her landmark 1983 monograph Tilapiine Fishes of the genera Sarotherodon, Oreochromis and Danakilia, placed it in the genus Oreochromis — the maternal mouthbrooders — and within the subgenus Nyasalapia she had earlier erected for the Lake Malawi mouthbrooding tilapias whose males develop elaborate branched genital 'tassels'. Four Nyasalapia species are endemic to the Malawi system and are caught together as chambo: Oreochromis squamipinnis, Oreochromis karongae (the Karonga tilapia), Oreochromis lidole, and Oreochromis saka. Trewavas treated Oreochromis saka as distinct, but later morphometric work (Turner & Robinson 1990; Turner, unpublished) folded the southern 'saka' populations into a variable Oreochromis karongae, so the chambo are now usually counted as three full species plus the questionable saka. The chambo are believed to be more closely related to one another than to any other tilapia and to have radiated within the lake (Sodsuk et al. 1995). Females and non-territorial males of squamipinnis are effectively inseparable in the field from Oreochromis karongae; only the breeding male's coloration and certain dental and pharyngeal characters reliably diagnose it, and even then only above about 8 in. Vernacular Malawian names recorded for the species across sexes and life stages include Kasawala (used on FishBase), Lisanga, Chinganga, Nchisichesi, Ngwalu, and Zeya.
Morphology
This is a large, heavily built tilapia with a wide, rounded head and a deep, laterally compressed body — generally the deepest-bodied of the chambo. FishBase records a maximum standard length of about 14 in; Trewavas (1983) gives roughly 15.5 in total length and a weight to about 1.1 kg. The fin counts run to 16–17 dorsal spines and 10–11 dorsal soft rays, with 3 anal spines and 8–10 anal soft rays. Females and juveniles are a plain silvery-grey crossed by six or more faint vertical bars and are very hard to separate from young Oreochromis karongae. The breeding male is unmistakable: he develops a brilliant pale 'mask' over the head — usually blue, occasionally green or white — while the underside of most of the body turns black and the upper surface a conspicuous white to pale blue. The other diagnostic flourish is the genital papilla, which in ripe Nyasalapia males extends into a long, branched, pinkish-to-bright-yellow 'tassel' that functions as an egg dummy during spawning; in squamipinnis these tassels can be exceptionally long, a single 6.5 in male having borne paired tassels up to 3.5 in in length (Trewavas 1983). Dwarf 'hunger-form' populations with bony, stunted body shapes occur in some small crater lakes of the catchment. Compared with its siblings, squamipinnis has small-to-medium jaws and a medium-sized pharyngeal toothed area, intermediate between slim, big-jawed Oreochromis lidole and the variable Oreochromis karongae.
Habitat
Oreochromis squamipinnis is endemic to the Lake Malawi system, occurring around the lake's shores and through its catchment, the connected Lake Malombe and the Upper Shire River, and several small satellite crater lakes (Kingiri, Ilamba and Massoko). Lake Malawi spans Malawi, Mozambique and Tanzania, and the species is native to all three. It is a semi-pelagic, benthopelagic fish of warm, alkaline Rift-lake water — Lake Malawi runs roughly pH 7.5–8.8 and about 73–82 °F in the surface layers the species frequents. Although it ranges across many habitats, it is most abundant in shallow, vegetated bays and is especially concentrated in the productive south-eastern arm of the lake and in shallow, turbid Lake Malombe. Depth use shifts through life: fry shoal in water less than a metre deep, juveniles move progressively deeper as they grow (8–8.5 in fish dominate trawl catches at 131–164 ft), and maturing adults of about 9.5 in return to shallower water. Breeding males build their nests over sand and mud at depths reported from about 7 ft up to 66 ft or more, with ripe fish historically taken as deep as 72 ft. There is also a seasonal pattern: in the cold season (May–August) the fish largely vacate the shallows, and few chambo over 4 in are seen above 66 ft.
Feeding
Chambo are microphagous grazers near the base of the food web — FishBase assigns squamipinnis a trophic level of about 2.1. It feeds chiefly on phytoplankton, with diatoms forming the major part of the diet, supplemented by detritus scraped from sand and rock and by zooplankton. The mix shifts with size and place. The smallest juveniles (2–3 in) scrape algae from rock surfaces; at 3–4.5 in they switch to grazing submerged plants; and larger fish feed increasingly on sediment and suspended plankton (Turner et al. 1991a). In the open south-eastern arm of Lake Malawi the diet is dominated by the filamentous diatom Aulacoseira (formerly Melosira), whereas in shallower, more eutrophic Lake Malombe the large solitary diatom Surirella and cladoceran zooplankton (especially Bosmina) bulk much larger — zooplankton can make up around 38% of stomach contents there versus about 8% in the lake proper. Chambo positively select large diatoms and cladocerans over small algae and fast copepods. As an abundant planktivore the species channels primary production into the lake's most valuable fishery, and it has long shared these inshore grazing grounds with the other chambo, which avoid competition more by habitat than by diet.
Mating
Breeding males are lek spawners: they gather and each defends a territory centred on a self-built nest, displaying their pale heads and dark bodies to schools of females passing overhead. The male digs a simple circular pit — a sand or mud 'bower' — and makes brief forays toward females shoaling at the surface, but a receptive female generally descends to the nest on her own initiative. Courtship at the pit includes mutual sand-digging and the characteristic tilapiine 'T-stand', in which one fish places its snout against the other's genital region, the partners alternately forming the horizontal bar of the T (Trewavas 1983, after Berns et al.). Because males play no part in raising young and spawn polygamously, a male returns to courting almost immediately after one female departs — one reason that heavy fishing of nesting males, which concentrate conspicuously on the breeding grounds, has comparatively little direct effect on total population fecundity. Breeding is seasonal: in Lake Malawi it runs roughly October–April, peaking December–February, and begins somewhat later in squamipinnis than in the other chambo. Males do not nest on the steeply shelving rocky coasts of the Nankumba Peninsula, favouring gentler sand and mud bottoms instead.
Breeding
Like all Oreochromis, Oreochromis squamipinnis is a maternal mouthbrooder — only the female incubates and tends the brood. Eggs are laid in batches on the raised central platform of the male's pit and are picked up by the female almost immediately. She then mouths the male's long branched genital tassel, the 'egg dummy', as he glides over and releases milt, so that fertilisation occurs both on the substrate and inside her mouth, where the eggs are already gathered. The female leaves the nesting arena soon after spawning and broods the developing eggs and larvae for about two weeks. As the fry absorb their yolk she moves into shallow water and releases them, but continues to guard the school and lets the young dart back into her mouth when threatened; fry are taken back until they reach about 0.5 in in squamipinnis (compared with roughly 1 in in Oreochromis karongae and 2.5 in in Oreochromis lidole), so the fry-guarding phase lasts several weeks. Fecundity is modest, as in all mouthbrooders, and scales with female size; squamipinnis from shallow, productive Lake Malombe produce significantly more eggs at a given length than those from the south-eastern arm of Lake Malawi. The reliance on shallow water during the fry-guarding stage makes brooding females and their young acutely vulnerable to beach seining — a key link between the breeding biology and the fishery's collapse.
In the aquarium
Oreochromis squamipinnis is essentially a fisheries and food-fish species rather than an ornamental one; it is bred and grown in Malawian aquaculture and conservation programmes but is rarely if ever kept by hobbyists outside its native range, and it is not a community-aquarium fish. Anyone who did house it would have to treat it as a large, robust, warm-water mouthbrooding tilapia. Adults approach 14–15.5 in and over a kilogram, so a genuinely large tank or pond — on the order of several hundred litres per adult, with a long footprint for the lek-style spawning behaviour — would be the minimum, paired with powerful filtration and the heavy water changes that high-biomass tilapia demand. Water should mirror the lake: warm (about 75–82 °F), hard and distinctly alkaline (pH roughly 7.5–8.5), over an open sand or fine-substrate bottom into which males can dig a spawning pit. The diet is straightforward for a herbivore-leaning planktivore: a spirulina- and vegetable-based staple with algae and the occasional small live or frozen invertebrate keeps condition and colour. Behaviourally these are seasonally territorial, polygamous fish, and breeding males become pugnacious around their bowers, so crowding small numbers in a confined space invites trouble. The honest summary is that this is a species for aquaculture facilities, research stations and restocking hatcheries rather than the display aquarium — its importance is on the dinner plate and in the lake, not the living room.
Conservation
The IUCN Red List assesses Oreochromis squamipinnis as Critically Endangered under criterion A2d (assessed 22 May 2018, published 2018, by B. Phiri of the Monkey Bay Fisheries Research Centre and G.Z. Kanyerere of the Malawi Department of Fisheries), with a decreasing population trend. The driver is decades of overfishing of a once-vast resource. Chambo were the most valuable food fish in Malawi: over 3,000 tonnes a year were taken from the south-eastern arm alone in the 1950s, and the chambo catch peaked at an estimated 21,000 tonnes in 1983, concentrated in the south-east arm and Lake Malombe. Through the late 1980s and 1990s tilapiine landings collapsed — falling from around 40% of the total Malawi catch in the late 1970s to under 20% by 1990 — as effort intensified, mesh sizes shrank, and beach seines repeatedly stripped the shallow nursery grounds where brooding females shelter their fry. Habitat degradation, sedimentation from catchment deforestation, and the disruption of the Upper Shire–Lake Malombe migration corridor compounded the decline. Three of the four chambo species are now assessed as Critically Endangered. The species carries no CITES or CMS listing. Recovery hinges on enforced closed seasons, gear and mesh-size regulation on the breeding and nursery grounds, and aquaculture and restocking efforts — set against the reality that more than a third of Malawians depend on Lake Malawi for protein and livelihoods.