cycle, several interacting factors have to be optimized to give maximum
production efficiency.
The supply of seed (fry and/or fingerlings), must be adequate b~th in
quantity and in quality and available as and when required. There should
be as little as possible variation in size. This remains the major limitation to
the further development of tilapia cage culture. Existing breeding practices
are inadequate and a new technology should be developed for selected
tilapias (Coche 1977,1979).
Adequate feeding is essential for growth and survival. In some cases,
natural food (plankton, aufwuchs or benthos) may be sufficient and no
supplementary feeding is required. When natural food is insufficient but still
an important part of the diet, supplementary feeding is practiced with
relatively low-cost ingredients. Under intensive rearing conditions at high
stocking densities, the natural food available for the caged fish becomes
insignificant and a complete artificial food is required.
The choice of the daily feed ration which will optimize the utilization
efficiency particularly for costly food is of crucial importance: much more
so in cages than in earth pond culture. Very little information exists on the
relationship between feeding rate and FCR for a particular type of feed as a
function of interacting factors such as species, fish size, fish density and
water quality.
Cage maintenance should be regular and geared towards increasing the
working life of equipment, maintaining the water quality by fouling control
and reducing fish losses from escapes, mortalities and predation. Both the
environment and the fish stock should regularly be monitored.- GROWTH AND PRODUCTION
The biomass of the caged fish per unit volume (B kg/m3) is related to:
- the individual growth rate which tends to decrease as the biomass
increases; 2. the average monthly production (MP kg/m3) which increases as
the biomass increases until the carrying capacity of the cage is reached and
decreases thereafter and 3. the FCR which decreases as the biomass'increases
until an optimum biomass is reached, above which FCR increases. The average
individual weight of the caged fish (Pm) is related to: - the individual growth rate which decreases as Pm increases and
- the FCR which, for a constant daily feed ration, increases as Pm
increases.
The mmaxxirmumcarrying capacity of a cage (MCC in kg/m3) is mainly
determined by the: ~tbughout the cage* This varies witijrthe me&-&%
&d cage size. Campbell (1978a) has defined the MCC's (in kg/m3) for
S. niloticus reared in well-oxygenated water with good circulation (at least 2
cmlsec) as follows: with plastic netting of 25 rnm mesh, 90 for 1 m3, 70 for
6 m3 and 40 for 20 m3; with nylon fibre netting of 14 mm mesh, 40 for 20
m3. Therefore, as cage size increases, MCC decreases. In practice,% is always
safer to stock below the MCC, as the risks of diseases and mortalities greatly
increase as the MCC is approached. For 1 m3 cages, the re~m~4~ee&mfe
limit is about 73 kg (FA0 1976).
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