During the intensive International Biological Programme studies on
the ecology and production of S. niloticus in the equatorial Lake George
(Uganda), Gwahaba (1973) found that the maturation size of these fish had
diminished since earlier studies (Figure 3). This he attributed to the effects
of intensive fishing. Early in the history of the fishery, 50% of the females
were mature at 27.5 cm TL, by 1960 this had fallen to 24.5 cm, and in 1972
was 20 cm. Data on S. leucostictus in Lakes GeorgeIEdward and Albert,
where it is indigenous, and from numerous lakes, dams and ponds where it
has been introduced, summarized by Lowe (McConnell) (1957), show that,
in this species too, maturation size varies greatly with the conditions (from
26 cm TL in Lake George to 7 cm TL in ponds). And about twenty years
later Gwahaba (1973) found the maturation size in Lake George had declined
to 14 cm TL, though the reason for this decline was not clear. In certain
waters male S. leucostictus matured at, and grew to a larger size than the
females; in ponds this species bred when only three months old. In some
ponds immature males were known to have grown faster than the females.18 20 22 24 26 28 30 32 34Total length (cm)
Figure 3. The decline in maturation size of female S. niloticus in Lake George, Uganda.
Gwahaba's 1971-72 maturity curve (0) compared with Fry and Kimsey's 1960 curve
(e) and Lowe (McConnell's) (1958) mean maturation size (x) (after Gwahaba 1973).
In natural waters S. leucostictus occupies an inshore zone and is common
in lagoons. In equatorial lakes, as in ponds, breeding occurs throughout the
year, without well-defined seasons. Egg numbers increased about 100 in a
7 cm female to roughly 1,000 in a 26 cm female, a relationship later studied by
Welcomme (1967a). Small females tend to produce smaller, less robust fry
than large females. Eggs hatch in about six days after fertilization and young
are brooded for a further three weeks, though for the last week only at
night. Lake George individuals caught in midlake showed a preponderance of