detritus and depth (and therefore, distance offshore), and consequently,
between depth and quality of diet. Thus, juveniles obtain a diet adequate in
protein for growth, but as they become larger and feed in deeper waters,
they progressively encounter protein deficiency, and as a result adults in the
lake are severely emaciated (Bowen 1979, this volume).
The smallest juveniles are released by female S. mossambicus in the
shallowest, inshore waters. Older (larger) fish are increasingly excluded from
the richer (shallow) feeding areas, and so growth declines progressively with
age, particularly in comparison with more typical S. mossambicus popula-
tions in other, nearby areas.
With water levels at or below normal, only the larger adult males are
reproductively active. With higher than normal water levels, successively
smaller (younger) males are recruited into the breeding stock, indicative of a
shift towards a more altricial mode (i.e., earlier age of first reproduction).
These smaller males attempt to breed in the most marginal waters around the
edge of the lake, including particularly any recently inundated by flooding.
This shift in life history style, together with the typical food-restricted
conditions in Lake Sibaya, is obviously the type of adaptive response we
would predict in such circumstances. The generally harsh (i.e., food-restricted)
environment, together with fluctuating lake levels, produces earlier matura-
tion (onset of the adult period) in Lake Sibaya. S. mossarnbicus in this lake
typically breed at about 0.43 (length at first breeding/maximum length),
compared to the more typical value of about 0.70 for African cichlids (Iles
1971; Table 3). Others (most notably Iles 1971; Fryer and Iles 1972; Lowe-
McConnell1975) have also mentioned similar cases of dwarfed tilapia stocks,
and have correctly interpreted them as representing adaptive responses to
particular environmental conditions. Our proposed mechanism, saltatory
development and heterochronous shifts in development (ontogeny), appears
to be the mechanism involved in these cases.
An earlier age of first reproduction (thus a shorter generation time) tends
to have the greatest effect on net reproductive effort, and to greatly accel-
erate population growth (Krebs 1978) (i.e.,it increases "r" very markedly). If
fish were to delay onset of sexual maturation, more of their available re-
sources would be channelled into somatic growth. Thus, they would be
larger (in size) at the time of their (later) maturation, and likely have higher
fecundity (more or less equivalent to fertility, but see Welcomme 1967b,
1970). However, increased fecundity and longevity generally have much less
significant effects on "r" than does an earlier age of first reproduction. Since
food (and therefore potential for future survival, growth and maturation) is
severely and progressively limited for S. mossambicus in Lake Sibaya,
it is clearly adaptive for these fish to begin breeding when resources are still
available to allow diversion into reproduction (and before adult mortality
rate begins to increase because of protein deficiency) (Horn 1978).
Lowe-McConnell (1975, this volume) summarized a number of studies of
growth and maturation in tilapia species throughout their range. She reports
that the same species delay their maturation when living in large, deep lakes,
whereas in smaller water bodies (floodplain pools, fish ponds, etc.) the fish
breed at smaller sizes and younger ages. The growth rates of fish in the latter
cases do not suggest physical "stunting", i.e. they are not small because they
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