The phenotypic plasticity of the cichlid Oreochromis mossambicus was examined in fresh and brackish water populations living in the eastern Cape near the southern extremity of the natural range. The primary goal of the study was to examine the wide variety of life-history styles which were expressed in relation to the varying environmental conditions. Four natural populations were studied during 1986 and 1987, and some experimental work under artificial conditions was carried out at the Rhodes University fishfarm during 1988. This comparative approach gave insights into the complex interactions between biotic and abiotic factors which are exploited by this hardy species. O. mossambicus displayed a wide variety of life-history tactics under harsh environmental conditions. The growth of juvenile fish was rapid and mortality of the young-of-the-year was high. Age at first onset of sexual maturity ranged from one to three years. Under harsh conditions growth rate was markedly reduced after maturity, which occurred during the second or third summer and growth disparity between the sexes was more pronounced. Longevity was found to vary from five to ten years for different populations. Under more favourable conditions (resembling the typical "floodplain" habitats of this species at lower latitudes) very rapid juvenile growth, and a high maximum mass of over 2300 g for both sexes was iii achieved. Sexual maturity under these conditions could be delayed until the third or fourth summer of life. O. mossambicus demonstrated an ability to withstand and even benefit from fluctuating water levels, and adjusted its life style according to the prevailing conditions: at times of high water level recruitment was low and sexual maturity delayed. During low water periods reproductive rates increased. Q. mossambicus showed a distinct preference for gentle gradients and well-vegetated habitats for feeding, nesting and refuge. During the study winter temperatures were near and at some times below the generally accepted lower limits for this species. This implies that these indigenous populations have a degree of cold tolerance. A further implication is that Q. mossambicus has the ability to thrive under adverse conditions which may partially explain its wide and successful colonisation of water bodies far from its natural range, and its obvious fisheries potential.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:rhodes/vital:5187 |
| Date | January 1990 |
| Creators | James, Nick, 1955- |
| Publisher | Rhodes University, Faculty of Science, Ichthyology and Fisheries Science |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Masters, MSc |
| Format | 213 leaves, pdf |
| Rights | James, Nick, 1955- |
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