pines where people are working a great deal in brackishwater. At salinities between 15
and 30%0, S. niloticus do not reproduce. They seem to grow better at 15%0.
HEPHER: But S. mossambicus will spawn in seawater.CHERVINSKI: Tilapia zillii, S. aureus and S. niloticus are all very salt tolerant.TREWAVAS: It is interesting that T. zillii can tolerate high salinities in, for example.
Lake Qarun (Egypt), but appears unable to do so in West Africa where it is replaced by
T. guineensis.MIRES: North of Eilat in Israel, hybridization between S. niloticus females and S. aureus
males is very difficult in salinities of 20%0and above.AVTALION: Is there any evidence that environmental parameters can affect the sex
ratio of the progeny?MIRES: Not to my knowledge. Temperature, at least, appears to have no influence.HENDERSON: I would be suspicious of drawing any general conclusions from the
effects of varying salinity on reproduction. There are so many physiological factors
involved.PULLIN: I was interested to hear you talk about oxygen tolerance. I believe you said
that when a culturist who subjected his fish to low oxygen conditions such as early
morning dissolved oxygen (DO) of around 0.1 ppm the fish suffered growth depression,
and could not make this up even if the oxygen levels went back to normal during the day.
We sometimes get DO'S as low or lower than this for several hours in manured ponds in
the Philippines.CHERVINSKI: With experiments conducted with Sorotherodon aureus it has been
shown that there is a correlation between low oxygen in the early morning and growth.PULLIN: Twenty-four hour records of oxygen levels are essential in experimental
work.MORIARTY: The important point to consider here is, I think, the interaction between
temperature and oxygen. As the temperature goes up, the fish are going to have a higher
oxygen demand and be much more stressed. We did have a fish kill some time ago in Lake
George, Uganda, but it was in the center of the lake where the plankton densities were
highesf, not around the edges. It happened at night. The tilapias could have come to the
surface and breathed air. None of the air breathing species died. In Lake George, the
temperatures are very high. The lethal stress was therefore the high temperature and
the high oxygen demand when the oxygen levels are already low.CAULTON: I would like to make one point which is not often considered. The oxygen
stored in the swimbladder can, in S. mossambicus, sustain metabolism for some time. My
estimates are that a fish of 400 g may survive for as long as 30 minutes using this oxygen
at 25'~. Obviously, the greater the depth that the fish is found in, the greater the volume
of oxygen available. Similarly, the lower the temperature and the larger the fish, the
longer the supply could last.LOVSHIN: I used to think that tilapias utilized atmospheric oxygen a lot but now I
doubt it. I think rather that they come up to the surface film where the dissolved oxygen
is high and flush their gills with this water to get enough oxygen to survive. If they do not
have access to the surface or are crowded in cages, they die very quickly at low DO'S.
I think I have read somewhere that tilapia have the ability to lower their metabolism