COCHE: There is one factor which has not been mentioned from the fish culture point
of view. Is the amount of food available affecting growth efficiency? It is well known in
fish culture, for instance, that if you feed tilapia ad libitum their assimilation efficiency
will decrease. That is why the ration becomes so important for a culturist.
HEPHER: There is another point which we should stress more and that is the effect
of temperature, particularly at night, on growth rate. I do not know if you hold a fish for
long periods at 18'~ whether it will grow as well as if you hold it at higher temperatures.
BOWEN: With regard to Dr. Coche's comment, unlike carnivorous and omnivorous
animals, the herbivores and detritivores that I am aware of do not in the natural environ-
ment ever confront a shortage of food. So quantity to my knowledge is never a limiting
factor. It is quality and if I understand Dr. Caulton's contribution correctly, what he has
done is to look at how the fish interact with the variables in their environment and try to
maximize the amount of food that they are capable of consuming in the absence of any
limit on availability. So this does not apply immediately to the culture pond situation
where you are trying to maximize the efficiency of food conversion.LOVSHIN: This is just a comment on the migrations to different temperatures. At
Auburn we have a lot of small pool units from which we pick small fry. There are very
definite migrations towards the surface when the fish are in small schools. The fry are
much easier to collect between 11:OO AM and 2:00 PM when the temperatures are high.
There is a very definite time of day when the schools are much more visible and higher up
in the water.
CAULTON: Thermal preferanda could be responsible for both a horizontal and a vertical
migrations-see text of my paper. In natural tilapia systems, juveniles invariably show a
horizontal migration since they are predominantly marginal in habit while adults, present
over deeper water, often show a vertical movement. Both environments are affected by
daily temperature inversions, hence, the movement of fish.MORIARTY: Dr. Caulton, did you do any temperature preference studies at night as
well as by day?CAULTON: Yes. The results of these experiments demonstrated that there was no
significant difference between daylnight thermal preferenda. It also made no difference if
the fish were fed or unfed males or females or introduced singly or in shoals.PULLIN: I would like to comment on the questions raised by Dr. Coche. The amount of
fool: given in a culture situation is obviously very important. I find Dr. Bowen's view that
herbivores and detritivores in their natural environment are never likely to encounter
shortages of food quantity very significant as some of the detritivores we are trying to
grow in fertilized ponds may be encountering a shortage of food. There may well be ways
which we can manipulate and improve this kind of culture environment. This may not be
as easy as regulating supplemental food, but has anyone any ideas on how detrital feeding
can be improved in pond culture?HENDERSON: The quality of the food is most important because ingestion by the fish
is partly related to how much time they spend for the process. If they are processing poor
detritus and using a lot of energy and time in the process, they are going to get a lot less
benefit than from feeding on blue-greens. But then again, a high quality detritus system
may be an improvement.PULLIN: Yes, and a fertilized pond culture system using organic manures would have a
head start over one using inorganic fertilizer.MORIARTY: The form in which detritus occurs is also important. We have heard mainly
about macrophyte feeders and detritus feeders and not very much on the phytoplankton
feeders which are also used in pond systems. For the detritus feeders their food must be