successively use (a) small cylindrical 0.5 m3 cages made entirely of 4 mm
mesh plastic netting hanging from a small, rigid, floating frame; (b) 1 m3
cubic cages of the same design with 8 mm mesh plastic netting; for 200 to
300 g growout, use medium size cages of about 20 m3 with either nylon
fibre netting (20 mm mesh; R470 tex twine) or plastic netting (18 to 25 mm
mesh) as in Table 2.Cage Culture: General ConsiderationsGood water circulation and adequate protection against floating debris
and wave action are normally the two essential requirements for a culture
site. Other factors, e.g., water quality, site accessibility, security and distance
to markets, are also important. A water depth sufficient to place the cages at
least 2 to 5 m above the bottom sediments is preferred.
In lake environments, wind-induced surface currents and fish movements
should provide a continuous exchange of the water in the cages, keeping the
DO high and removing wastes. Knowledge of seasonal limnological cycles
will help to identify any critical periods during which cage culture might
have to be discontinued. Such periods correspond to the seasonal turnover
of the water mass, when the thermal stratification breaks down. Deep
deoxygenated water is suddenly brought to the surface where it may cause
heavy fish mortalities in the cages. A period follows during which very low
DO'S may persist for several days.
Tilapias are relatively tolerant of low DO. Caged S. niloticus in the Ivory
Coast have survived concentrations as low as 0.7 mg/l or 9% saturation for
several days (Coche 1977). In 1976, however, 64% of the adult fish and all
the fingerlings suddenly died within 3 days, when the DO dropped below 0.5
mg/l (Traore and Campbell 1976). From recent observations during similar
periods, it would appear that a DO of 3 mg/l should be considered the limit
in cage culture below which adverse effects begin to appear: feed digestion
stops, the growth rate decreases sharply and stress intensifies.
In the presence of such critical periods, it might be advisable to suspend
culture if no adequate artificial aeration can be supplied to the caged fish. If
such a suspension is not feasible, the following practices are suggested to
minimize the mortality risks: space the cages further apart, several meters
away from each other; use larger cages providing a relatively larger air/water
interface; use lower fish stocking rates, never exceeding 15 to 20 kg/m3 and
low densities not exceeding 200 to 250 fish/m3; stop feeding the fish (Coche
1977, Traore and Campbell 1976).
The management of caged fish stocks encompasses the stocking of juvenile
fish, feeding during growout and cage maintenance. During a cage production