The Biology and Culture of Tilapias

Discussion

NOAKES: A comment more than a question. One of your points is that temperature is

very significant for these fish. Their thermal preferanda should therefore change with

changing conditions, for example, fed vs. unfed. Is this so?

CAULTON: No, not as far as I know. If you feed the fish in a gradient tank, it makes

no difference where the food is, they still go to the same temperature.

NOAKES: Dr. Caulton, you were saying though that different physiological functions

have different optima. If this is the case, then it would be reasonable to assume that the

fish, when it is undergoing certain physiological functions, would choose the correspond-

ing optimum temperature.

CAULTON: Although this is a reasonable assumption, 1 do feel that certain physiological

optima are sacrificed for others. Certain physiological processes do certainly function

most efficiently at a prescribed temperature, yet metabolism, for example, is efficient, in

terms of energy saving, at low temperatures yet the overall physiological optimum for a

feeding tilapia is at a temperature of about 30°C.

NOAKES: It has been suggested in some temperate regions where there are thermoclines

in the summertime that some of the large predator species move above the thermocline in

the evenings to feed at a higher temperature where they can move around more efficiently.

Then they will go down into colder water where they can digest their food and grow very

efficiently.

BOWEN: The thermal preferendum of Sarotherodon mossambicus does change with age.

It gets lower as the animal gets larger. But even the thermal preferanda of the largest fish

are well above any temperatures normally achieved in the natural environment. So, in

many cases, they would be expected to migrate back and forth between the thermal

gradients along the shores.

HEPHER : Dr. Caulton, what was the average weight of your fish?

CAULTON: We worked with juvenile fish from about 20 to 50 g, but for a lot of the pre-

ferenda work, we used fish up to 300 or 400 g. For respirometry work we used a large

range of sizes, up to 200 or 300 g. Basically, however, we were concerned with modelling

on juvenile fish.

MIRES: Dr. Caulton, from what part of the fish did you take the fat content tissue
sample? From muscle?

CAULTON: We used the whole fish.

GUERRERO: Can tilapias get their essential amino acids from blue-green algae and
bacterial protein as they can from animal protein?

BOWEN: Yes, it is good quality protein.

MORIARTY: In Lake George, the diet of Sarotherodon niloticus consists of about 70%
Microcystis with a few other blue-greens and diatoms and a few percent bacteria. This was
obviously a good food.

GUERRERO: Dr. Bowen, have you studied the amino acid make-up?

BOWEN: Yes I have and it is not significantly different from the amino acid composition
of plant proteins in general, including those from macrophytes.

GUERRERO : How does it compare with fish meal?