The production of tilapia fry in spawning ponds depends on three main
factors: the spawning frequency of each female in the parental stock, the
number of eggs produced in each spawn, and the survival of the fry. These
aspects will be considered for intraspecific and interspecific crosses.Egg and Fry Production: General ConsiderationsIt is important to define the term 'fecundity'. Lowe (McConnell) (195513)
defined fecundity as "the number of young produced during the life time of an
individual". This definition is perfectly acceptable for natural conditions,
but it is very hard to use in culture or in experimental conditions. In this
paper it is proposed to change the time limit from "life time" to 12 months
starting from the first spawning.
It has been stressed that the number of eggs in each spawn depends
on the length and/or weight of the spawner (Fryer and Iles 1972) but
Peters (1959) and Lee (1979) found different results. Lee (1979) found
that for some individual spawners there was a trend of increasing clutch size
through successive spawns, but this was not uniform (see also Tables 1 and
3). Babiker and Ibrahim (1979) showed correlations between the length or
the weight of S. niloticus and the number of oocytes in the ovaries, but there
are doubts as to whether these correlations can forecast the actual quantities
of eggs or fry obtainable from a given female during a given period under
various conditions.
Tables 1, 2, 3, and 5 show a big variability not only in the spawning
frequency between spawners of different species but also in the total number
of eggs produced by the females within the groups. Moreover Lee (1979)
showed a 20 to 30% difference in the number of eggs produced within
female groups of two different species, stocking three females per aquarium.
Peters (1959) showed an exact correlation between the weight of the
clutches and the body weight of the females; he also stressed that "fish
either produce relatively few large eggs or relatively many small eggs". It
seems possible therefore that on an annual basis small but more fecund
females can and will probably produce more eggs than big but poor spawners.
Spawning procedures and conditions can also influence the production of
fry in the spawning ponds: for example the rate of water changes. There is
no substantial data on the influence of this factor on fry yields from spawn-
ing ponds, but for aquaria most of the hatchery operators in Israel believe
firmly that periodical water change will usually cause a renewal of spawning
activity. Commercial fry producers also believe that the change of water in
spawning ponds has a beneficial effect on spawning frequency. This is prob-
ably due to the flushing out of metabolites. It seems that overcrowding in
ponds, tanks, etc., may induce earlier maturity than in nature and a larger
production of smaller sized eggs (Fryer and Iles 1972).
Feeding conditions also have an influence on egg and fry production
(Miranova 1977). For example, S. mossambicus females spawned more
frequently when they were underfed, and the total amount of eggs was
higher than when they were fed abundantly. This response to unfavorable
conditions has probably permitted many tilapia species to adapt to harsh and