ment lasts for 35 days. This effect, and the steroid residues, disappear within
5 days of cessation of treatment when the fry are transferred to growout
tanks. The use of virginiamycin (e.g., Biazon) was also discussed. This
has been used as a growth promotor in grouper culture in Malaysia, but
not apparently with tilapias. Despite its name, it is not an antibiotic.
- FOOD PRESENTATION
Dr. Coche noted that some pond culture work, for example, Miller's
work with tilapias in the Central African Republic, has shown that powdered
supplemental feed could give yields as high as pelleted feeds. This suggests
that the high cost of pelleting can be avoided. Dr. Lovshin supported this
observation from studies in experimental ponds at Auburn University,
Alabama, in which meal diets gave comparable yields to pelleted diets. It was
agreed, however, that pelleted feeds were desirable for cage culture.
Dr. Coche reported that very good results have also been obtained in the
Ivory Coast with mixed feeds presented as a "mash" shaped into large balls,
placed in the middle of cage covering nets and lowered underwater. For
mariculture, unpelleted wet feeds are generally used.
The problem of food wastage in cage culture was discussed. Although
a variety of chemical binders is available for pellets, these can cause prob-
lems, for example, the kidney disorders observed in experimental marine
flatfish culture in the U.K. Prof. Roberts mentioned that freshwater-moist-
end diets are preferable for tilapias cultured in seawater as these supply
dietary water and reduce osmotic work below that required for salt regulation
following seawater drinking. Dr. Caulton commented that in the tropical
developing countries, 2% dried green banana powder is the best and cheapest
binder available and is used in commercial feeds for Macrobrachiurn culture.
However, other materials such as boiled plantains, potatoes, manioc or any
other starch source can be used as cheap binders at 10 to 20% of the diet.
They would be cheaper and more available than 2% dried green banana
powder in many developing countries.
The extent of food wastage from commercial tilapia cages does not
appear to have been quantified. All commercial growers use sinking as
opposed to the more expensive floating pellets. The ability to manufacture
floating pellets may be absent in many of the developing countries. Food
wastage seems to depend on the degree of powdering of the pellets during
storage and handling (this powder is easily washed from the cage), the
pattern of water currents and the rate of feeding. Water currents are partic-
ularly important. Dr. Nash pointed out that elongated cages which set
parallel to the current can be useful in maximizing food retention if suitable
end baffles can be fitted: there are, however, structural limitations to this
in strong tidal streams.
It was agreed that tilapias in cages can be very voracious feeders similar to
salmonids. Feeding at a very slow continuous rate seems, however, to be very
advantageous. Dr. Coche reported that Campbell in the Ivory Coast has used
a very slow feeder in which a rope hanging down into the water below the
hopper is moved by gentle water currents to allow a trickle of pellets to