CHERVINSKI: Perhaps by doing this we are getting rid of some waterborne chemical
factor pmduced during reproduction which at high fry densities inhibits further repm-
duction?
MIRES: I don't believe so.
LOVSHIN: What is your source of water?MIRES: In the hatchery, it is the city water supply ; in outdoor ponds it is a mixture of
irrigation water, well water and water reclaimed from sewage effluent.
LOVSHIN: I was just thinking that even if you do keep wild tilapia out, they could be a
source of chemical factors in the water, but I do not really think this is important. What
is very significant in my view is that by draining the pond and removing the small fry you
are greatly reducing the scope for cannibalism and this is probably the basis of your
improved production. Partial selective harvesting or draining on a one- or two-month basis
is always a good thing. I would not discount the possibility of a chemical stimulatory
effect of water change on spawning or vice versa on an inhibitory effect by chemical
factors at very high fish densities, but I have experienced continuation of spawning at
very high densities (see Henderson-Arzapalo et al. 1980-Editors).ROBERTS: Much of our hatchery work is done in Edinburgh prison by long-term
prisoners. They take great pride in keeping the fish and preparing high pmtein diets, such
as earthworms, for them. We maintain there pure lines of S. niloticus, S. mossambicus and
S. spiluncs. We can get regular spawning virtually every month (say 28 to 35 days) fmm
given broodstock of all these species over an 18-month period. After this, they are less
effective as broodstock and we choose to replace them although they are still fecund.
These are fish which receive very careful attention and the best possible diet in small
aquaria. I realize that this would not be economical on a commercial scale and that our
situation is very different from Mr. Mires' hatchery. Our results suggest, however, that
you can smooth out the variability in spawning by optimizing all conditions.MIRES: Do your results apply to all your fish on an individual basis?ROBERTS: To virtually every fish.MIRES: Are they in separate tanks?ROBERTS: They are kept in tanks in groups of 3 or 4 females per male.HENDERSON: This suggests that we know very little about optimum husbandry methods
in commercial hatcheries.ROBERTS: I think that the main factor with our fish is their good husbandry and
diet, including live food.LOVSHIN: Regarding the age of broodstock and the length of time for which you can
use them, we found in Brazil that after 1 year of spawning activity, i.e., after 3 or 4 spawn-
ing~ (after which time our fish would be one to one-and-a-half years old) the number of
fry produced per unit body weight would fall by about 50%. We therefore began to
replace such fish, which were about 300 g in weight, with smaller fish, say 60 g, and we
found that such continuous replacement gave better production than carrying on with
the big fish. I cannot altogether explain this as of course larger fish produce larger total
numbers of fry per spawning than small fish. I suspect that it is a question of spawning
frequency. The smaller fish are more active and frequent spawners.NASH: Does the fecundity decrease in the larger older spawners, or does the viability of
the eggs decrease?