blue-green algae in the gut, whereas fish in the inshore regions had ingested
detritus; since this species is more abundant inshore, it is essentially a
detritivore (Moriarty et al. 1973).
Introductions of S. leucostictus showed how rapidly tilapias can be
established in new waters from very few fry. All the S. leucostictus in
Kenya waters appear to have originated from six specimens between 6 and
11 cm TL taken to Kisumu ponds from Uganda. In the Teso dams (Uganda)
where T. zillii and S. leucostictus were stocked simultaneously, T. zillii
predominated in the first few years after stocking, but were gradually
overtaken in numbers by S. leucostictus, although this species lays fewer
eggs at a time (Lowe (McConnell) 195513). Such changes may have been
related to food supply; in one of the dams T. zillii predominated while dense
stands of macrophytes existed, but as it cleared these from the dam, S. leu-
costictus became the predominant species.
S. leucostictus introduced into Lake Naivasha, Kenya, in 1956 hybridized
with another introduced species S. spilurus niger. Hybrids were abundant
in 1961, but had disappeared by 1974 when Siddiqui (1977b) studied the
lake. He found S. leucostictus then to be the dominant species, and to spawn
throughout the year, though Hyder (1970a) had previously maintained they
had a relatively quiescent period here from July to September. Males matured
at 18 cm TL, females at 16 cm. Fecundity ranged from 320 to 1,328 eggs in
16 to 32 cm females. Males predominated in the catches.
The nilotic S. niloticus and T. zillii, together with S. leucostictus, have
been studied in Lake Victoria to see how they have fitted in with, and
taken over from, the indigenous S. esculentus and S. variabilis. Earlier
studies (Lowe (McConnell) 1956a; Fryer 1961a) had indicated that these two
indigenous tilapias were ecologically complementary species with little or
no competition between them; S. esculentus predominated in land-locked
bays where the bottom was of soft flocculent mud from phytoplankton
deposition, while S. variabilis was most abundant off more exposed shores,
in water lily lagoons and in the outflowing River Nile. Both species occupied
different biotopes according to the size and sexual condition of the fish. S.
variabilis spawned in shallower water than S. esculentus (spawning behavior
observed in a water lily swamp, Lowe (McConnell) 1956a), and juveniles
occurred in different nursery zones.
In an attempt to boost the tilapia fisheries, T. zillii from L. Albert was
introduced to Lake Victoria in 1954, and along with it came S. niloticus
and S. leucostictus. Tilapias in ponds draining the lake may also have gained
access to the lake around this time. T. zillii appeared in gillnet catches
by 1956, S. niloticus and S. leucostictus both appeared in commercial
records for the first time in 1960 (Welcomme 1967a). Some suspected hybrids
between (i) T. zillii and T. melanopleura (from pond escapes), and (ii) S.
variabilis and S. niloticus, were described by Welcomme (1967b). The spread
of the introduced species was described by Welcomme (1966): T. zillii
spread rapidly at the northern end of the lake, appeared in the southern