The Biology and Culture of Tilapias

Discussion

LOVSHIN: I found the paper very interesting and stimulating as a culturist. I have

thought along the same lines for quite a while, and I think the points you raise have

importance. For example, why does a fish dwarf in a pond? What are the environmental,

behavioral or density-related factors involved? However, I have a very strong resistance to

your basic conclusion. You said that restrictive food and stable conditions would mean

that the fish would not reproduce at such an early age, whereas Dr. Lowe-McConnell has

said that tilapia in poor condition will switch to reproduction at a smaller size than those

in better condition. I have also found that to be true in culture ponds. You also said there

is some evidence that at high densities, reproduction is restricted. There is probably no

doubt about that, but you have to understand that, at these high densities, if you do not

feed fish they are not going to grow. You have to feed fish to get them to a large size

particularly at high densities. This conflicts with your idea of restricting food. Your basic

theory is very interesting, but I think we have to determine whether we can manipulate

the environment, or manipulate something in some way so that we can eliminate the

reproduction of tilapias in their first year. If so, we would in fact have solved many of our

culture problems. This would be a tremendous advancement to tilapia culture, especially

in developing countries. The high stocking densities to which you refer will reduce

reproduction but these are only achievable with high cost, intensive, aerated circulation

systems and they require much feeding.

NOAKES: I don't disagree with anything you have said. Obviously, fish are not going to

' grow unless you feed them. I will try to clarify that position. The question is not so much
whether you feed, but given that you feed a certain amount, what leads the fish to
channel that food into reproduction or into somatic growth? The problem is that we
don't really know to what signal the animal is responding when making such a physiol-
ogical shift.

LOVSHIN: But you make a very strong point in your paperC'We feel that our approach

is not only more rational than a great variety of attempts to produce monosex culture of

tilapias fishes for pond culture.. .to chemically or surgically castrate fish to inhibit

reproduction... prospect for ultimate success." The way it is stated in this paper, it will

not work at those high densities; not for culturing large fish.

HEPHER: I would like to support Len Lovshin. You said that you think that at a suffi-

ciently high density, reproduction will stop. I would like to quote an experiment done at

Auburn University by Allison who stocked 50,000, 100,000 and 200,000, densities you

would suggest will make reproduction stop. Reproduction did decrease, but even at

200,00O/ha, he got 38,380 fry/ha produced, which means there was still reproduction

enough to affect the whole system and to cause stunting in the population. I would also

like to quote the work of Miranova (1969). She held S. mossambicus in aquaria and the

food was very limited. Fish of 6 g spawned in her aquarium. Of course, they didn't grow

much because of lack of food, but they did spawn.

NOAKES: The particular density value we gave is simply a value we took from the

literature. I am sure you are well aware that it is difficult to find data to test our ideas.

I do not disagree with what you are saying. What I would like to say is that our hypothesis

predicts that there should be a trend: if you decrease stocking density, it should tend to

increase reproduction. The important thing is not so much to what extent stocking

density is a factor, but that we find out what the fish are responding to--whether they

spawn or whether they grow. It may be density; it may be any of a number of things.

BOWEN: I think we can divide the factors that are likely to be important into two

categories: physicochemical types of cues, and cues related to food. When I need fry for

experiments, I can always put juvenile Sarotherodon mossambicus of about 4 cm stand-

ard length in aquaria and raise the temperatures to 25'~ and they will begin to reproduce,

even in the presence of abundant food, at this very small size.

Some groups of fishes, such as cyprinids, are known to secrete pheromones when

in densely packed communities. These have a negative feedback effect on growth and