The Biology and Culture of Tilapias

and Lake Turkana into Israel and the Jordan Valley. T. rendalli, one of the
forms formerly included in 'T. melanopleura', has a wide distribution in the
Upper Zai're and Kasai, Lakes Tanganyika and Malawi, the Zambezi basin
and the Limpopo. At Yangambi, near Kisangani, where the River Zaire turns
westwards, the natural distributions of the two species overlap. In west
Africa T. rendalli is replaced by T. dageti, which appears to be less abundant
than T. zillii.
At Yangambi T. zillii is in nature less abundant than T. rendalli. Gosse
(1963), who administered the fishponds at Yangambi, did not at first realize
that his T. rendalli ponds included an admixture of T. zillii, but after a while
the more robust T. zillii dominated the pond populations. The same thing
evidently happened in the ponds at Yaounde, Cameroon, to which Gosse had
inadvertently sent a mixture of the two species.
In Lake Victoria T. zillii was introduced from Lake Albert and T. rendalli
either from Shaba or Zambia. At first they were believed to have interbred,
but later, when introductions were made into many farm dams in Uganda
and Kenya, the Tilapia were named as T. zillii.
Gosse was emphatic that T. rendalli was by far superior as a food fish. It is
therefore important for pisciculturists to know one from the other. The
differences are:
Color. T. zillii usually has two horizontal dark stripes, one mid-lateral,
the other nearer the dorsal outline. These are crossed by vertical bars and
the strongest marks are blotches at the intersections. The caudal fin is
usually covered by a grey network with pale interstices. The eggs are
green.
T. rendalli usually shows dark vertical bars only and the scales have
each a dark basal crescent. The caudal fin has a spotted upper half and a
red or yellow lower half. This red or yellow pigment may also color the
anal fin. The eggs are yellow.
Both species frequently have a carmine flush on the lower flanks,
behind the pectoral fin, and this is not confined to one sex or to mature
fishes.
General shape. T. rendalli is typically more deep-bodied than T. zillii
but this is an average difference and not suitable for individual identifica-
tion.
Meristic characters (counts). The number of gill-rakers is the same in
both species: 8-10 below the joint of the first arch. This distinguishes
them at once from Sarotherodon. The mean numbers of scales, vertebrae
and dorsal fin-rays provide good distinctions and the modal numbers of
dorsal spines are sufficiently well-marked to allow a quick field test on a
number of specimens. T. zillii usually has 28 vertebrae, T. rendalli 29.
In T. zillii the commonest number of dorsal spines is XV, in T. rendalli
XVI (Table 1). For the whole dorsal fin, among 130 T. rendalli I found
26% with XVI 12, 66.9% with XVI 13; other combinations were found in
only one or two fishes. Among 40 T. zillii 18 (45%) had XV 12 and 10
(25%) XV 13.
There are about 29 species of Tilapia arid most of this speciation has taken
place in the western rivers of Africa. The euryhaline T. guineensis is found in
lagoons and estuaries along the coast from Angola to Senegal. Other species