press), from Zambezi tilapias by Duerre (1969), Kapetsky (1974), Dudley
(1974), for S. mossambicus by Bruton and Allanson (1974), Hodgkiss and
Man (197713) and from the Great Lakes from Lowe (1952) and as summarized
by Fryer and Iles Q972, Table 14). Total lengths were calculated from
standard length data for Lakes Chad and Sibaya.
Growth data for the nilotic tilapias all indicate that within a water body S.
niloticus grows to a larger size than S. galilaeus (though S. galilaeus is deeper
bodied so weight differences are not so marked): both grow faster and larger
than T. zillii. S. aureus also grows faster and larger than T. zillii, but not as
large as S. galilaeus. Payne and Collinson (in press) have suggested that the
extended breeding season of S. aureus in Lake Mariout (Egypt) slows somatic
growth, for here S. niloticus and S. aureus grow at comparable rates until
they mature at one year old, after which S. niloticus with its more restricted
breeding season grows faster and larger than S. aureus.
In riverine populations S. niloticus males grow larger than females (Banks
et al. 1965). This also occurs in lagoons and ponds as we have already seen
(Figure I), but in large lakes, where growth is good, males ahd females may
mature at, and grow to, comparable sizes. This phenomenon has also been
noted in S. esculentus, where growth of the two sexes was not significantly
different in lake populations (Lowe (McConnell) 1956b; Garrod 1959). Again,
in T. zillii in Lake Kinneret, Israel, males and females were of comparable
sizes (maturing at 13 to 14 cm) though in the Dor aquaculture station ponds,
where they mature at a smaller size than in the lake, males grow faster than
females, even though the pond fish grow faster than the lake fish, achieving
18 to 20 cm in one year instead of two years as in the lake (Chervinski
1971).
All three Zambezi tilapias (S. andersonii, S. macrochir and T. rendalli)
grow faster and to a larger size on the Barotse floodplain than on the Kafue
floodplain (Figure 4). But on both floodplains S. andersonii grows fastest
and to the largest size, followed by T. rendalli, while S. macrochir does not
grow as fast or as large. It is interesting that in this case the Tilapia (T.
rendalli) grows better than one of the accompanying Sarotherodon (S.
macrochir), unlike the soudanian region where all the Sarotherodon grow
better than the Tilapia (T. zillii). The Zambezi region does, however, have an
additional smaller Tilapia (T. sparrmanii).
In natural waters, S. andersonii rarely matures when less than three
years old (at 27 cm TL); a few S. macrochir spawn at the end of their
first year (when 13 to 16 cm TL). In T. rendalli some fish start to spawn
in their second year (when 17 cm TL). In the Zambezi, Sarotherodon
sexual differences in size become marked after maturation, female growth
slowing down more than male growth, and all the large fish are males.
T. rendalli showed no sex differentiation in growth for the first two years.
After the sixth year these tilapias grew very little.
From the Kafue, tilapia scale rings permit growth to be backcalculated
enabling variations in growth from year to year in the different age groups
to be examined in relation to environmental factors such as water temper-
atures and flood levels. Such data for fifteen years examined by Dudley
(1974, 1979) indicated that: a) there are significant variations in growth
rate from year to year for most age groups of all three species and b) growth
sean pound
(Sean Pound)
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