ately too high, or where overfeeding takes place.
Regarding weather constraints, storm damage may occur if the farm site
is not sufficiently protected. Such losses have occurred on Lake nopango
in El Salvador (Hughes 1977) and in the Ebrie Lagoon in the Ivory Coast
(Campbell pers. comm.).
Jordan and Pagh (1973) reported incursions and residence of wild
S. mossambicus in cages for growing S. aureus. This greatly reduced feeding
efficiency and production. The wild fish could also be vectors of parasites
and diseases.
The shortage of tilapia seed is considered as one of the major present
constraints to the development of the culture industry (Coche 1977, Guerrero
1979% Anon. 1979a). A new methodology for the mass production of tilapia
fry and fingerlings should be developed. Moreover, caged tilapias have
practically no access to natural feed (apart from filter feeders in eutrophic
water) and require essentially complete supplemental feed. The two main
constraints here are the formulation of such feeds, e.g., in El Salvador
(Hughes 1977) and Africa (ADCP 1980), and their availability and cost to
the farmers.The design and construction of adequate cages is still considered as a
major problem in the Philippines (Anon. 1979a) and large cages used in El
Salvador have proved difficult to harvest (Hughes 1977). In the Ivory Coast,
the utilization of weak fibre netting material has resulted in great fish
losses (Campbell 1978a). S. niloticus grazing on the settled algae on R250
tex nylon twine damaged it within 8 months sufficiently to necessitate a
complete replacement by stronger netting (R470 tex). Following serious
damage to fibre netting by crabs in brackishwater, imported plastic netting
will replace this for future commercial production in the Ivory Coast (Camp-
bell pers. comm.).
Damage to underwater structures by predators may also result in important
losses, including the escape of the cultured fish. Such problems have been
encountered in Indonesia with monitor lizards and turtles (Pedini pen.
comm.) and in the Ivory Coast with iguana and large Nile perch (Lates
niloticus). In brackish water, crabs have heavily damaged cage nets (Campbell
pers. comm.). In such cases, either a predator net barrier should be used or
the cage construction should be reinforced.- DISEASE ASPECTS OF CAGE CULTURE
Caged fish living in confined conditions are probably under greater stress
than pond fish and are more susceptible to attack by parasites and diases.
Fortunately, the various tilapias cultured in cages have generally demonstrated
high disease resistance. Only a few cases of health problems have been
documented, mostly following increased stress due to elevation of salinity or
fish biomass. In Puerto Rico, caged S. aureus reared in sea water(359100
salinity) suffered chronic mortalities with open body lesions, fin rot and