T. sparrmanii is basically a small species, S. andersonii a large one), in all the
tilapias investigated growth varies with the conditions. Certain 'stress factors'
evidently switch the physiological state of the fish from growth to reproduc-
tion at a smaller size ('dwarfing', 'nanism') and at a younger age than in
normal populations, irrespective of %he rate of growth to achieve this size.
The mechanisms for this are not yet known, but are reflected in the condition
(weight for length) of the fish. Pond experiments at Malacca (Chen and
Prowse 1964) demonstrated that the size of the inhabited water body
influences maturation size and final size in some as yet unexplained way.
Is this ability more marked in species living in habitats likely to dry up?
Even species endemic to the Great Lakes of Africa, such as S. esculentus,
evidently have this ability. In Lake Victoria, S. esculentus only matures
when two or three years old and at least 20 cm TL, but it can mature at five
months old when only 10 cm TL in aquaria (Cridland 1961). The mechanisms
for such a switch need investigation. In aquaria fast growth and precocious
spawning in T. zillii was stimulated by keeping the fish at a high temperature
(up to 31°C); changes in illumination had little effect (Cridland 1962).
Field studies have presented a number of clues about the switch: (i)
tilapia in poor condition (low weight for length) switch to reproduction
at a smaller size than those in better condition (see Figure 2). (ii) In Lake
George, Uganda, marked declines in maturation sizes both of S. niloticus and
of S. leucostictus were recorded after twenty years intensive fishing had
reduced the general size of fish caught. (iii) In the Kafue, Dudley (1979)
found some evidence that S. andersonii matures at a smaller size in dry years
than in years when the water level remains high at the end of the dry season.
(iv) In Lake Sibaya precocious breeding in S. mossambicus is associated with
decline in food value of the food available to the fishes as they grow and
move into deeper water (as discussed by Bowen, this volume). But quite
possibly the tilapias are responding to some factors that we are not yet
considering.
In addition to (1) the size and age at which the fish mature, tilapia popu-
lations are also affected by: (2) the length of the breeding season and
number of broods produced a year. This may vary from only one (rarely
two) as in Malawi Sarotherodon of the S. squamipinnis flock (Lowe 1952;
Bems et al. 1978) and S. andersonii (according to Mortimer 1960), to a
succession of broods, as seems more usual. These may be produced either
through a defined breeding season, where environmental conditions are more ,
seasonal, or throughout the year as in some equatorial waters, (3) The
number of eggs produced at a time ('fecundity') which increases with the
size (weight) of the female fish; (4) The egg size which is species-specific,
substrate-spawners producing smaller and more numerous eggs than mouth-
brooders (Lowe 195513 listed increasing egg size in T. zillii, S. galilaeus,
S. leucostictus, S. niloticus, S. variabilis, S. esculentus, S. karomo, S. lidole
and S. saka). Egg size shows some latitude, however, as females of dwarf
populations produce somewhat smaller eggs than normal sized populations
(Lowe (McConnell) 1957, Peters 1963), which helps to counteract the
effect of fewer eggs per female in populations with dwarf females; (5) The
number of young that can be mouthbrooded, which tends to increase to a
sean pound
(Sean Pound)
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