Oreochromis esculentus (Graham 1928)

Diagnosis: A large bodied tilapia with a small rounded head, short snout, forward-facing mouth, deep body, relatively small eye for the body size, and generally lacking any dark stripes or blotches (Ref. 118638). Females and non-territorial males typically pale grey, white ventrally; males are typically darker, especially in the head/dorsal region; breeding males have a reddish-pink head and flank, dark fins with red/pink dorsal margins, and red/pink caudal region (Ref. 118638). In contrast to Oreochromis rukwaensis, mature males of O. esculentus do not show enlarged jaws (Ref. 27292). Oreochromis esculentus differs from O. niloticus as follows: no vertical stripes on the caudal fin; lower numbers of soft dorsal rays and therefore of total dorsal rays; lower modal number of lower gill-rakers; finer teeth in jaws and pharynx; a longer lower pharyngeal bone, 33.0-35.5% length of head; caudal peduncle as long as or, usually, longer than deep; O. niloticus, in common with many other species, is often preserved with three or four midlateral spots and another on the top of the caudal peduncle, a pattern not found in O. esculentus; and male genital papilla opening between a pair of often spongy lobes, vs. papilla conical or with a slight distal notch in O. niloticus (Ref. 2).Description: A somewhat pointed head and snout with a relatively steep forehead and low positioned eyes; the preorbital bone is rather shallow, its depth not exceeding 21% length of head (Ref. 2, 27292). Length/depth ratio of the caudal peduncle 0.9-1.1 (Ref. 34290, 55020). Teeth in 2-3 series in juveniles, 3 at 100-150 mm standard length and 4-5 above 240 mm standard length; outer bicuspid, with an occasional tricuspid in some adults; inner tricuspid; 60-92 in outer series of upper jaw (Ref. 2). Gill-rakers 15-21 on lower part of anterior arch; total number on the arch 18-27; microbranchiospines on outer sides of 2nd, 3rd and 4th arches (Ref. 2, 34290, 54840, 55055). Lower pharyngeal bone in adult longer than wide; length 33.0-35.5% and width 29.0-32.5% length of head; teeth fine and crowded; blade 1.3 to twice median length of dentigerous area (Ref. 2). Scales on cheek usually in 3 rows, occasionally 2; lateral line series with 29-35 scales, upper lateral line with 20-23 scales, lower lateral line with 12-17 scales; between origin of dorsal and lateral line 4-5.5, usually 4.5 or 5; over top of caudal peduncle above lateral line 8 or 9; scales around caudal peduncle 16-18 (Ref. 2, 27292, 34290, 55055). Dorsal fin with XVI-XVIII spines and 10-11 soft rays; anal fin with III spines and 10-12 soft rays; dorsal and anal fins posteriorly pointed, but not filamentous; pectoral fin extending beyond origin of anal fin (Ref. 2, 54979). Caudal truncate, often rounded by wear in adult, not densely scaled, but in large specimens small scales may extend on the rays nearly to the margin and are rather dense on the upper and lower rays (Ref. 2). Vertebrae 30-31 (Ref. 2). Genital papilla of male not prolonged, but may be bluntly bifid and have a number of short lobes of a spongy texture (Ref. 2). There is no genital tassel in this species (Ref. 363, 27292). Otoliths fairly smooth (Ref. 56279).Colouration: General colour of female and non-breeding male olive-brown to watery green, grading to cream or silvery white below; soft vertical fins usually with round white spots; young with a clear-ringed, conspicuous tilapia-mark, which may still be detected up to 170 mm standard length, although then not ringed (Ref. 2, 56136). The tilapia-mark is a well-defined black spot outlined in pale yellow (Ref. 4903, 34290, 55020). Bars are rarely present except in individuals from 1.3 to 2.0 cm length, where 4-5 broad diffuse bars may be present, occasionally these take the form of horizontal blotches on the flanks (Ref. 55020). Breeding male colours predominantly black and red, the black involving the whole ventral surface from mouth to anus, the pelvics and the anterior part of the anal fin, as well as the dorsal fin; in contrast to Oreochromis variabilis the dorsal fin typically has no bright red or orange border, although in a few specimens the posterior lappets and the upper edge of the soft dorsal are white as preserved, sometimes dark crimson lappets (Ref. 2, 363, 4903). The white or greenish white spots on dorsal and caudal may be intensified in the male; the sides of head and body above the black area are crimson and elsewhere the scales are grey with pinkish-fawn edge; the pectorals are clear and slightly pink (Ref. 2, 363). The tail is of a plain colour: no vertical stripes on the caudal fin (Ref. 2, 36900). On death the colours fade rapidly but the general black of the ventral surface, pelvics and anal, and the red sides to the head and body and red lappets of the dorsal fin remain; the sides are often dark red with greenish-white spots (Ref. 363). Preserved specimens: olive grey, darker dorsally, whitish ventrally; head dark; opercular spot clearly visible; dorsal, anal and caudal fin greyish (Ref. 54979).

Dactylosoma Infection 1. Parasitic infestations (protozoa, worms, etc.)

Acanthogyrus Infestation. Parasitic infestations (protozoa, worms, etc.)

Alitropus Infestation. Parasitic infestations (protozoa, worms, etc.)

Reproduction probably triggered by the rains (Ref. 2771), with the time of maximum spawning activity coinciding with the wettest months of the year (Ref. 363). Males form a crater-like spawning nest without a distinct wall (Ref. 27292). The pit is about 30cm in diameter and 10cm deep, and is probably made in the early morning (Ref. 27292). Ovaries show that a female may have a succession of three or more broods in a spawning period; brooding females often shelter in weed beds and swampy places (Ref. 2, 363). Males defend their breeding territory (Ref. 2) for weeks or on and off for several months, while females only make short visits to the spawning grounds and leave the territory immediately after spawning (Ref. 363). Males eat little while actively guarding the nest (Ref. 363). Papyrus swamp channels (Ref. 363, 34921, 55020) and beaches with weed grown swamps (Ref. 34921) function as nursery areas. Young become independent at a length of about 1.5cm (Ref. 2, 363)by which size the yolk sac is occluded and they have strated to feed (Ref. 363), and at about 12cm TL they move from the nursery areas to the open water (Ref. 2).

Dorsal spines (total): 16 - 18; Dorsal soft rays (total): 10 - 11; Analspines: 3; Analsoft rays: 10 - 12; Vertebrae: 30 - 31

Most abundant at less than 13 m in Lake Victoria (Ref. 3), in sheltered gulfs and bays (Ref. 363, 2771, 4903) where the lake bottom is composed of soft (algaceous) mud (Ref. 363, 2771, 4903, 52061, 55020). Occasionally forms schools (Ref. 2, 363, 36900), mainly diurnal (Ref. 2). Tolerant of low oxygen concentrations, less than 1 ppm, and high levels of carbon dioxide (Ref. 3, 52016). A plankton-feeder using the mucus-trap mechanism combined with the combing action of the pharyngeal teeth (Ref. 2); the species is non-selective, in the sense that all organisms of a size capable of being retained by these means are passed into the stomach, but it is selective in that the schools follow concentrations of diatoms in the lake (Ref. 2). Food consist almost entirely of phytoplankton (Ref. 3, 6316), mainly diatoms (Ref. 4903, 30832, 34291), but also higher plants, insects and their larvae, crustaceans and worms like Tubifex (Ref. 27292); insect larvae and planktonic crustaceans occur less frequently although they may contribute to the diet of young fishes (Ref. 4903, 34291).

Occasionally forms schools (Ref. 2, 363, 36900), mainly diurnal (Ref. 2). Tolerant of low oxygen concentrations, less than 1 ppm, and high levels of carbon dioxide (Ref. 3, 52016). A plankton-feeder using the mucus-trap mechanism combined with the combing action of the pharyngeal teeth (Ref. 2); the species is non-selective, in the sense that all organisms of a size capable of being retained by these means are passed into the stomach, but it is selective in that the schools follow concentrations of diatoms in the lake (Ref. 2). Food consist almost entirely of phytoplankton (Ref. 3, 6316), mainly diatoms (Ref. 4903, 30832, 34291), but also higher plants, insects and their larvae, crustaceans and worms like Tubifex (Ref. 27292); insect larvae and planktonic crustaceans occur less frequently although they may contribute to the diet of young fishes (Ref. 4903, 34291). A maternal mouthbrooder (Ref. 363, 4903, 12468, 27292, 34291, 54840, 118638); males make simple nests in shallow water (Ref. 118638). Ecologically, an offshore-living plankton feeder, which complemented the more inshore-living Oreochromis variabilis in the original ecosystem of Lake Victoria; these niches are now filled by the introduced Oreochromis niloticus and O. leucostictus respectively (Ref. 118638). Was a major component of the fisheries catch in Lake Victoria; supports artisanal fisheries in satellite lakes of Lake Victoria and forms a large part of the catch in Lake Rukwa, Nyumba ya Mungu and the central Tanzania lakes (Ref. 118638). Excellent taste, firm meat and suitable for sundrying (Ref. 54860). IUCN conservation status is critically endangered (Ref. 118638).

fisheries: commercial; aquaculture: experimental

Oreochromis esculentus, the Singida tilapia or Graham's tilapia, is a species of cichlid endemic to the Lake Victoria basin, including some of its satellite lakes such as Kyoga, in Tanzania, Uganda, and Kenya. Its common name refers to Lake Singida, but this population is the result of an introduction that happened in the 1950s. This fish is highly valued by local fishermen, who know it as ngege.

In 1927-1928 Michael Graham conducted the first ever systematic Fisheries Survey of Lake Victoria. In his official report of the expedition, Graham wrote that "The ngege or satu Tilapia esculenta, is the most important food fish of the lake, whether for native or non-native consumption. No other fish equals it in the quality of the flesh. It is convenient size for trade, travels well and is found in much greater numbers than other important fish, such as semutundu (Luganda), Bagrus sp.''. Furthermore, Graham noted that "The introduction of the European flax gill-net of 5 inch mesh has undoubtedly caused a diminution in the number of ngege in those parts of the Kavirondo Gulf, the northern shore of the lake, the Sesse Islands and Smith's Sound which are conveniently situated to markets".

Oreochromis esculentus has been introduced into the Pangani River basin, where it presents a threat to the native relative O. jipe.