out that a given species usually spawns at a fixed time within a 24-hour
cycle in aquarium conditions. There could be scope for further investi-
gation in this, especially as gametogenesis in trout has been suppressed
experimentally by continuous illumination. The effect of temperature
may be different for fish of different ages or sizes and more important
than the effects of feed quality and quantity. If so, then S. mossambicus,
first spawning at four months in a relatively cool location, could spawn
at, say, two months at a hotter location, irrespective of the feeding regimes.
Nutrition- THE PHYSIOLOGY OF DIGESTION IN TILAPIAS
Dr. Moriarty summarized the digestive physiology of microphagous
tilapias, i.e., those feeding on phytoplankton, blue-green algae (cyano-
bacteria) and detritus (including bacterial proteins). Previous studies have
shown that although the stomach in these fish is structured such that some
food can pass from esophagus to intestine through the anterior end only, it is
a true stomach with a pyloric sphincter and a very important cycle of
acid secretion.
The sequence of events observed in large individuals having a relatively
large stomach is as follows. In the early morning (6:OO AM.), when feeding
commences, the stomach is collapsed (contracted) and secretes little or no
acid. For the fit few hours of feeding, therefore, some of the food passes
through the anterior end of the stomach straight from esophagus to intestine.
As feeding increases, however, the stomach becomes progressively distended
and takes in food. It also begins to secrete acid. By 10:OO AM., acid secretion
is maximal and over the next 1 or 2 hours the pH in the ventral portion of
the stomach falls below 1.8, at which point acid lysis of bacterial and algal
cell walls commences. Over the 6-hour feeding period from 10:OO AM. to
4:00 P.M. ingested food can either pass into this very acidic portion of the
stomach for complete lysis or pass'through only the acidic portions (pH >
2.0) or bypass the stomach more or less completely. After feeding stops,
the rate of movement of food through the stomach slows down, thus allow-
ing all the food material to be exposed to a low pH. Complete lysis and
digestion of all food occurs, whereas bacterial or blue-green algal cells that
pass rapidly through the stomach and thus are not exposed to a high acid
concentration are incompletely digested. The assimilation efficiency may be
as high as 80% for blue-green algae ingested during the period of peak
acidity, but it is near zero at the commencement of feeding and variable at
other times. The mean assimilation efficiency over the whole feeding period
is probably 40 to 50%. For small tilapias and for Haplochromis nigripinnis,
which has a similar digestive physiology, the overall assimilation efficiency is
higher, probably because the acid concentration is high throughout their
small stomachs.