Prof. Roberts gave a very useful example of the confusion which can
arise if the last of these objectives is not fulfilled. At Stirling University,
experimental infections of S. mossambicus with a monogenean (Cichlidogyrus
sp.) would take only in a strain known as 'B' from a type location in Africa.
S. mossombicus strain 'A' which had come via Singapore and was slightly
contaminated was resistant to infection. A parasitologist working with 'A'
only would have drawn the erroneous conclusion that this particular Cich-
lidogyrus sp. was not a parasite of S. mossambicus.
The Concept of 'Stunting' and a 'Switch' Between
Somatic Growth and Gametogenesis/Spawning(The concept of a simple 'switch' is perhaps false, as somatic growth
demonstrably does not cease during gametogenesis. The greatest growth check
occurs from the onset of spawning through to mouthbrooding-Editors.)
The phenomenon of stunting was discussed in relation to the concept
of a switch from somatic growth to gametogenesis leading to first maturity.
The phenomenon of maturation at a small size is common to both guarders
and mouthbrooders and to riverine and endemic lacustrine species. For
example, S. esculentus normally take about three years to mature as 20cm
fish in Lake Victoria, but will mature after only five months as lOcm fish in
aquaria. This ability to shift the timing of the 'switch' has great adaptive
value particularly in lakes which dry up periodically, e.g., Lake Rukwa and
Lake Chilwa, where fish migrate to inflowing streams as the lakes dry, breed
at a small size, and then repopulate the lakes when they fill again.
Dr. Lowe-McConnell's data for S. niloticus suggest that breeding size is
low if condition factor is low in the population. This suggests that the
quantity and/or quality of available food may be a factor. Other possible
factors include water chemistry, temperature, salinity, dissolved oxygen and
parasite burdens.
There are further clues in results of field biology studies. For example, the
S. niloticus population in Lake George remained unfished from the 1920's to
1952 because of tsetse fly infestation of the area; their minimum breeding
size was about 28 cm. With increasing fishing pressure from 1952 to 1972,
the minimum breeding size fell to about 18 cm. Field studies have shown
that the majority of the fish have always concentrated around the lake
margins. Beyond 100 m offshore the bottom becomes a loose flocculent
ooze which is unusable for tilapia nests. This suggests that there is a severe
limitation on breeding sites and that the removal of large fish by the fisher-
men has allowed smaller individuals to come in and occupy these sites. The
mesh size of fishing (gill) nets has been reduced from 15 cm to about 10 cm.
In the early days of this fishery (1950's), very few females smaller than 28
cm had ripe ovaries, and therefore it was gametogenesis, not just spawning,
that was restricted to larger individuals.
In Lake Sibaya, it was stated that only the large males can find breeding
sites when the lake is at a low level: the smaller individuals take up sites