Dr. Lovshin reported that in his experience predator control was the
most reliable method of controlling recruitment. He also voiced the opinion
that there was a very great potential for managed fish production from the
thousands of livestock wateringlirrigation ponds in tropical developing
countries by using predator-prey systems. Guidelines for stocking and
harvesting would have to be worked out as for the bass-bluegill systems
described by Swingle for the southern U.S.A.
- INTEGRATED FARMING AND WASTEWATER REUSE
It was agreed that tilapias were ideal species for integrated agriculture-
aquaculture farming systems, particularly in tropical developing countries. It
was recognized that the public health aspects of such systems needed fuller
investigation.
The use of wastewater (sewage) to fertilize tilapia ponds was considered
a very controversial topic. Dr. Hepher described Israeli work in which
human bacterial pathogens had been shown to penetrate the muscle and
internal organs of fish above certain threshold levels of bacterial populations
in the water. The thresholds vary with species: silver carp are the most
susceptible to contamination, common carp are intermediate and tilapias
the most resistant. Tilapias have particularly high thresholds for Salmo-
nella and Shigella penetration. The thresholds also vary greatly with stress.
They become lowered in stressful conditions such as low dissolved oxygen
and high ammonia. Fish which have become contaminated with pathogens in
muscle and internal organs take a very long time to depurate in clean water:
usually over one month. Fish which have taken pathogenic bacteria into
their guts only can be depurated in 2 to 3 days.
Mr. Mires voiced great concern at the use of wastewater in aquaculture
and drew attention to the adverse publicity that it could give to fish culture
in general. It was his opinion that public attitudes would be strongly against
this method of growing fish even if the health hazards could be controlled. It
was pointed out, however, that fertilization of fish culture systems with
human wastes has a long history in Asia and that human waste recycling
through aquaculture should not be abandoned because of public attitudes in
the developed countries.
The buildup of heavy metals in manured ponds was also discussed. Dr.
Hepher stated that in Israel, both common carp and tilapias remained
largely unaffected by heavy metal buildup. The levels of heavy metals
in the pond sediments increase with heavy manuring but the fish are present
in the ponds for such a short time that heavy metal levels in their tissues
always remain well below World Health Organization recommended safe
limits. In particular, common carp take up virtually no heavy metals at all,
presumably because they lack the highly acidic stomach of the tilapias.
It was agreed that much more information was required on the public
health aspects of the use of organic manures and wastewater in aquaculture.