In most tropical river systems there has been a lack of integrated ecological research to
investigate the dynamics and impacts of invasive species on recipient river systems. This is in
sharp contrast to temperate river systems. This thesis investigated the nature, extent, and impact
of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758), on indigenous congenerics within the
Limpopo River basin in northern South Africa. An integrated approach was adopted to gain a
better understanding of factors that allow Nile tilapia to be a successful invader and also to gain
an insight into its invasion rate and conservation implications within South Africa.
Morphometric and genetic variation between Nile tilapia, indigenous congenerics and
their associated hybrids were determined. Intermediate meristic characters obscured the
identification of hybrid specimens from pure morpho-specimens and species identity was only
confirmed through mtDNA analysis. Preliminary evidence points to unidirectional hybridization
among Oreochromis congeners in the Limpopo River system. The hypothesis that bigger Nile
tilapia males may have a competitive advantage over spawning grounds and in female mate
choice is proposed.
The trophic ecology of Nile tilapia was investigated using both stomach contents and
stable isotope analysis. A high similarity in stomach contents was observed but interspecific
differences were revealed in the isotopic composition of diets that suggest fine scale patterns of
resource partitioning that could be achieved by the ability of fish to selectively feed on what is
immediately available and the ability to perceive the dynamics that determine food resource
availability. Ecological niche models were used to determine the potential invasive range of Nile
tilapia and revealed broad invasive potential over most river systems in southern Africa that
overlapped the natural range of endemic congenerics. It was noted that model performance and
the degree of niche conservatism varied significantly with variable selection and spatial extent of
study area. This implied that the spatial distribution of suitable and unsuitable environmental
variables varied between the native and introduced ranges of Nile tilapia and also indicated the
ability of Nile tilapia to survive in conditions incongruent with its native range. The extreme
hardiness and adaptive life history characteristics of Nile tilapia have probably predisposed it to
be a successful invader in novel systems within southern Africa.
Lastly, a qualitative risk assessment method was developed as a potential application to
determine the risk of establishment and spread of the invasive Nile tilapia. Results showed that in
the absence of quantitative data on ecosystem structure and functioning, habitat suitability
analysis in terms of known physiological tolerance limits to minimum water temperature,
presence or absence of dams, seasonality of river flows and the presence of indigenous fish
species of concern could be adequate for identifying vulnerable river systems. The model
developed also provides an objective method that is easy to implement, modify and improve on
as new data become available. Furthermore, the model can be applied to highlight areas of
uncertainty where future research should be directed. / Thesis (PhD)--University of Pretoria, 2012. / Zoology and Entomology / Unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/31246 |
| Date | 03 September 2012 |
| Creators | Zengeya, Tsungai Alfred |
| Contributors | Chimimba, Christian Timothy, Bastos, Armanda D.S., Booth, Anthony J. |
| Publisher | University of Pretoria |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
| Rights | © 2012 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. |
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