exophthalmus (Miller and Pa* 1973;Miller and Ballantine 1974). Pathogenic
bacteria (Aeromonas and Vibrio spp.) were thought to be implicated. There
is heavy pollution from raw sewage in the area.
In the Ivory Coast, a great decrease ins. niloticus production was observed
in several cages where mycosis occurred (Coche 1977). Later, Campbell
(pers. comm.) also observed cages with a high proportion of blind fish
(Diplostomum ?). Susceptibility to all infections was higher in 1 m3 cages
when the biomass exceeded 73 kg/m3. S. niloticus reared in shallow brack-
ishwater (4 to 20% salinity) and relatively close to the lagoon sediments
(0.5 m) suffered high parasitic burdens and mortalities (Campbell pers.
comm.).
A high rate of mortality has been recorded in Alabama following outbreaks
of bacterial diseases in pond cages with S. aureus (Pagh 1973). In Tanzania,
a generalized Saprolegnia infection spread through a caged T. zillii population
(Ibrahim et al. 1975).
Poaching constitutes a major problem in cage culture. To ensure the
security of commercial operations, Street (1978) has recommended in El
Salvador the employment of full-time watchmen. Locating the cages close
by the farmer's residence may also help solve the problem. In the Philippines,
poaching is also mentioned among the main problems facing cage farmers
(Guerrero 1979a).Research support and rational planning of the development for tilapia
cage culture are essential. Development should be supported by an adequate
Extension Service, through which a continuous two-way flow of information
can be established between farmers and researchers. Dramatic examples of
the results of a lack of careful planning are available from the Philippines.
In the 50 ha Lake Calibato, 170 fish cages containing about 10 million
tilapias were present in 1978 (Anon. 1978). Within only a few years, the
profit expected from tilapia cage culture fell by 50% because of such over-
crowding. A similar situation has arisen in Lake Sampaloc (Sollows pers.
cornm.): see above. To bring such wild development under control, several
planning steps are required: a survey of the lake's capacity for cage culture;
setting guidelines for the sitingloperation of the cages and licensing legitimate
operators (Anon. 1978). A competent technical body is needed to oversee
the development.The research needs for tilapia cage culture in the Philippines have been
recently reviewed (Anon. 1979a) and the following identified: the design