Invasive Nile tilapia Oreochromis niloticus (Linnaeus, 1758) in the Limpopo River system, South Africa : conservation implications

Abstract

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.