The Biology and Culture of Tilapias

contains very significant amounts of these nitrogenous compounds in addition
to microbial protein. Dr. Moriarty questioned the methodology used to
estimate these compounds and also Dr. Bowen's conclusion that they were
not protein. The extraction methods used to estimate these compounds-6N
HC1 for 24 hr at 100°C--would in fact hydrolyze all peptidic material to its
constituent amino acids. It was also considered significant that while these
compounds in Dr. Bowen's work were not extracted by 0.1 N KOH and were
therefore not considered proteins, this method does not always give complete
protein extraction. Their exact nature must await further study with im-
proved methodology. Dr. Bowen accepted that methodology needed improve-
ment but emphasized that even with the existing crude methodology, his
work had demonstrated that microbial protein could account for only a
portion of the N-content of detritus. The food value of the refractory
nitrogenous compound requires further study.
Detritus and detritivory are very important in pond aquaculture. It was sug-
gested that agricultural wastes could stimulate microbial production in ponds
by providing large surface areas for bacteria. The use of animal manures to
give high concentrations of suspended colloidal particles for bacterial colo-
nization was considered very important as well as the development of
benthic detrital aggregates. This discussion centered on Israeli experience
with cow and chicken manure in which the main thesis was that as yields
from anaerobic bacterial production are only about 10% of those from
aerobes, a system of suspended colloidal material for aerobic bacteria to
work on is the best. Frequency of manuring is very critical in the main-
tenance of sufficient dissolved oxygen for aerobic bacteria and for normal
fish growth. Pond aeration can help here. Daily manuring is recommended
and rates as high as 180 kg dry matter/ha/day have been used. Therefore,
pond management techniques are already being used to maximize bacterial
production. Tilapias are the most responsive of all fish to such techniques.
Dr. Lovshin recalled that pretreatment of fish ponds with manure was
a long established technique designed to build up plankton populations.
It was recognized that this technique must also encourage detrital production
as well and that perhaps the recommended time lags between manuring and
stocking should be re-examined. This requires a full investigation, i.e.,
not only how to pretreat to get maximum bacterial and detrital production
but also how to continue treating to maintain these through growout. The
food value of such sources was also debated. Obviously, before culturists
are encouraged to develop detrital systems to feed their fish, it must be
demonstrated that these will supply all the required dietary components
especially essential amino acids, essential fatty acids, vitamins, etc. The
consensus of opinion was, however, that high bacterial diversity and the
chemical complexity of detritus made it unlikely that deficiencies in essen-
tial nutrients would arise in well-managed detrital-based culture: for example,
in manured ponds. Experience with marine fish larval rearing has shown that
some long-chain polyunsaturated fatty acids present only in some phyto-
plankton species are essential for growth and survival, but this situation was
not considered comparable to detritivorous adult fish feeding in a pond
where the diversity of food items is very high.