Fish culturists were advised to take note of the very slow passage of food
through the tilapia intestine during non-feeding periods (4:OO P.M. to 6:00
AM.) in systems where natural or ad libitum feeding is available. The assi-
milation efficiency is very high during non-feeding periods. This suggests that
the most efficient utilization of food in cultured fish could probably beu'
obtained by a series of feedinglnon-feeding cycles, i.e., a feeding period
sufficient to distend the stomach and stimulate maximum acidity followed
by a non-feeding period to allow maximum lysis of the ingested food and
slow passage through the intestine. With controlled feeding, perhaps two or
three such cycles could be applied each day. Further work is needed to
determine the effect of rate of movement through the intestine on assimila-
tion rate and efficiency, and thus whether such feeding cycles are advan-
tageous.
Dr. Bowen's work on S. mossambicus in Lake Valencia was discussed. It
was noted that highquality proteinaceous material is usually completely
assimilated within the first quarter of the intestine whereas the peptidic
fractions of detrital agregates (see below) are digested and assimilated all
along the intestine. It was agreed that although such tilapias do produce a
pepsin (pepsinogen has been demonstrated) there is probably no proteo-
lytic digestion in the stomach. The stomach function in S. niloticus has
become specialized and restricted to acid lysis: proteolytic digestion is con-
fined to the intestine.
Dr. Bowen pointed out that S. mossambicus differs from the above
pattern (which was largely taken from studies on S. niloticus in Lake George,. feeding on lake plankton) in that it can fill its stomach within 10 minutes to
one hour, depending on the size of the fish. Observations on trawled speci-
mens of S. mossambicus (size range 4 to 25 cm) from Lake Sibaya and Lake
Valencia show that the degree of stomach acidity depends on stomach
fullness. The pH of an empty (resting) stomach is from 4 to 7 whereas that
of a full stomach is around 1.5. Therefore, with S. mossambicus, there is no
long lag period to reach maximum efficiency. The first batch of diatoms
ingested can still pass through the gut without lysis, but the very rapid
stomach filling and acid secretion mean that the system can work at close to
100% assimilation efficiency thereafter.
There are perhaps some lessons here for culturists. Phytoplankton feeding
S. niloticus in ponds would largely resemble the Lake George situation and
operate well below maximum assimilation efficiency. Any species that can
fill its stomach rapidly, however, such as fish receiving supplementary feed
or S. mossambicus ingesting large quantities of detritus will be much more
efficient. Of course economics will dictate what is possible in a commercial
situation: phytoplankton is cheaper than supplemental feed!
There was further discussion on the functional anatomy of the tilapia
gut. S. mossambicus was said to have a relatively muscular stomach which
maintains its shape and size when empty better than S. niloticus in which the
stomach shrinks to a very small size. Dr. Moriarty showed, however, that the
stomach of S. niloticus at maximum distension holds about 50% of the total
ingested material: the other 50% is spread out along the very long intestine.
In S. mossambicus, the distended stomach holds only about 10% of the total
ingested food, the remainder being in the intestine.