know if anyone has any information on tilapias in low pH waters. My observations
suggest that the sarotherodons are alkaline water fish and if we attempt to raise them in
acid waters they will not respond well, quite apart from the low productivity. Are there
tilapias which do well and spawn in low pH situations? [Tilapias are generally absent from
most of the central basin of the Congo, the forested part (Trewavas, pers. comm.), but
Dubois (1959) reported T. congica living in Lake Tumba, Zake, where the pH is 4.5 to 5.0
(see Philippart, this volume)].
LOWE-McCONNELL: I do not think they get into very low pH, but I have pH's for S.
niloticus. Some of the lagoons have pH's around 9.
ROBERTS: We keep our experimental populations in Scotland in tap water at pH's of
6.4 to 6.8.
HEPHER: What about high pH's? For example, 10.2 was mentioned. What is the source
of this high pH? Is it caused by a fluctuation in the bi-carbonate cycle or by deposits of
sodium carbonate? A pH as high as 10.2 may affect tilapias.
CHERVINSKI: In fact we are talking about fluctuations of pH.
HEPHER: That is all right. Most fish will tolerate those levels. The main question is,
can you construct tilapia ponds in a swampy area which has acid soils?
LOWE-McCONNELL: Dr. Prowse at Malacca had tilapia in ponds at very low pH's; I
think around 2 to 4 or something of that order. If you look in the Malacca reports, you
will find all the data. These were acid lands which were of littlc use for anything else but
to grow fish. He was liming them. I don't know what the pH was after he limed them.
CAULTON: I would like to comment on an unusual aspect of environmental adaptation
shown by S. mossambicus. I have been informed by a reliable source of three reports of
live S. mossambicus being dug out of apparently dry river beds in Zimbabwe. These
reports have never been scientifically verified, but I am quite happy with their reliability.
In all instances, no surface water was present yet the fish survived in the damp subsurface
moisture as deep as 50 cm below the surface. The fish have obviously adapted to a
severely modified microenvironment and appear to remain alive under extremely adverse
conditions. All attempts at recreating these conditions in the laboratory have, however,
resulted in total mortality.CHERVINSKI: It is known by fish culturists that when you drain ponds, tilapias can
withstand very low oxygen levels in the mud. After one or two days, when you ppt the
water back again, they will revive.HEPHER : But that is not dry.CHERVINSKI: No, but still the oxygen is still very low, and it may be that tilapias
are able to use atmospheric oxygen. We see them in ponds going up to the surface and
swallowing air. We have done some small experiments and showed that when tilapias
cannot reach the surface, there is a lot of mortality.ROBERTS: I would like to comment on the pH tolerances. In Mombasa, Haller has
stocks of Sarotherodon alcalicus grahami. They will not survive in any pH below 9 and
live at pH 10 or 12 without any difficulty. Also, I cannot agree with Dr. Chervinski that
levels of C02 are unimportant. We have a very considerable problem in several places
where high C02 water supplies cause nephrocalcinosis.CHERVINSKI: I agree that C02 can be very important, but in the work I was discussing,
lowered O2 is more important. The fish can avoid high C02 waters.GUERRERO: I would like to give some information about tilapia culture in the Philip-