Trophic interrelationships between the exotic Nile tilapia, Oreochromis niloticus and indigenous tilapiine cichlids

Get Complete Project Material File(s) Now! »

General introduction

Globally, fish are among the most introduced vertebrates because of their long association with human movements (Pillay 1977). The primary reason for fish introductions has been to increase food production through aquaculture and the augmentation of inland fisheries. As a consequence, commercially important species such as the tilapias, carps and catfishes have been widely distributed worldwide for aquaculture (De Silva et al. 2006). Food security has also been achieved through augmentation of existing fisheries by: 1) introducing new fish species such as the introduction of the Nile perch Lates niloticus into Lake Victoria (Witte et al. 1992; Gouswaard et al. 2002); 2) enhancing existing fisheries such as the introduction of various tilapiine species into Lakes Kariba and Victoria to improve local subsistence fisheries (Balon 1974; Oguthu-Ohwayo and Hecky 1991); and 3) filling vacant ecological niches such as the introduction of the freshwater sardine Limnothrissa miodon into Lakes Kariba and Kivu to fill an empty pelagic niche (Marshall 1993). Recreational and sport fishing have also developed rapidly to become a multi-billion dollar industry. As a result, the global introduction of sport fishes such as largemouth bass Micropterus salmoides, rainbow trout Oncorhynchus mykiss, and brown trout Salmo trutta has also been eagerly promoted (Cambrary 2003). Several fish species have also been introduced for the biological control of aquatic weeds and pests.

Rationale

Cichlid fishes, belonging to the order Perciformes, is one of the largest and most widely distributed families of fresh and brackish water fishes and occur in tropical areas of North and South America, Asia, the Levant and Africa (Fryer and Iles 1972; Skelton 2001). Over 70% of known cichlids are found in Africa, especially in the ancient lakes of East and Central Africa (Tanganyika, Victoria and Malawi), where about 2000 endemic species are known to occur and represent the most spectacular radiations of vertebrates in the last 10 million years (Fryer and Iles 1972; Kocher 2004). The family also represents the most species-rich vertebrate family with about 3000 species exhibiting variability in body shape, colour pattern, behavioural diversity and a high degree of trophic and ecological specialisation (Fryer and Iles 1972; Meyer 1993; Kornfield and Smith 2000; Kocher 2004; Salzburger and Meyer 2004).

Nile tilapia introductions: Species description

Nile tilapia is native to the Nile River basin, south-western Middle East and the Niger, Benue, Volta and Senegal Rivers, Lakes Chad, Tanganyika, Albert, Edward, and Kivu (Trewavas 1983; Daget et al. 1991). Nile tilapia has been widely introduced for aquaculture, augmentation of capture fisheries, and sport fishing (Trewavas 1983; Welcomme 1988). It is well-suited for aquaculture because of its wide range of trophic and ecological adaptations, and its adaptive life history characteristics enable it to occupy many different tropical and sub-tropical freshwater niches (Trewavas 1983). Nile tilapia exhibits an opportunistic and versatile feeding strategy that reflect the abundance and composition of food sources in different environments, seasons and either the presence or absence of competing fish species and predators (Gophen et al. 1993; Balirwa 1998; Njiru et al. 2004; Njiru et al. 2007; Zengeya and Marshall 2007; Zengeya et al.

Description of study area

The study area was located in the central sub-catchment of the Limpopo River basin in northern Limpopo Province, South Africa. This area comprises of the Limpopo River, from the confluence of the Crocodile and Marico rivers to Crooks corner on the western boarder of the Kruger National Park and their associated tributaries (ca. 22ºS – 24ºS; 26ºE –31º E; Fig. 1). The region has an arid to semi-arid climate with a mean annual rainfall of less than 400 mm in the Limpopo valley to over 1 500 mm along the Drakensberg escarpment in the east (WSM/DWAF 1995). The rainy season (October-April) is short, erratic and unreliable and this often results in frequent droughts (FAO 2004).

Thesis approach

The research approach adopted for this thesis was multidisciplinary and included molecular, morphometric, and ecological analyses. Firstly, the realised distribution of Nile tilapia was confirmed through a fish survey of the Limpopo River and its associated tributaries within the Limpopo Province of South Africa. One major constraint in the conservation of Oreochromis spp. in river systems in southern Africa is that their morphological identification is often difficult because of considerable variation and broad interspecific overlaps in meristic and morphometric characters that are used in species descriptions. Therefore, the presence or absence of Nile tilapia within a given river segment was verified through genetic and morphological identification of sampled populations. Morphometric and molecular techniques were used in conjunction to assess levels of gene transfer among conspecific Oreochromis species, species diversity, their distribution, translocation/introduction into novel areas and hence identification of areas and/or species at risk of hybridisation.

Thesis outline

This thesis consists of two sections. The first contains two chapters (Chapters 2 and 3) and explores those factors that make Nile tilapia such a highly successful invasive species by investigating some aspects such as hybridisation and trophic interrelationships with indigenous Oreochromis species in the Limpopo River Basin. The second (Chapters 4 and 5) develop predictive models to evaluate the invasive potential of Nile tilapia to establish into novel systems in southern Africa by identifying river system(s) that are vulnerable and at serious risk of Nile tilapia invasions.

Table of Contents :

• Abstract
• Acknowledgments
• Disclaimer
• List of Figures
• List of Tables
• Chapter 1: General Introduction
  • Rationale
  • Objectives
  • Thesis approach
  • Thesis outline
  • References
• Chapter 2: Morphometric variability and hybridization of invasive and endemic tilapia in a subtropical African river system (Limpopo River, South Africa) obscures the identification of the three Oreochromis species
  • Abstract
  • Introduction
  • Methods
  • Results
  • Discussion
  • References
• Chapter 3: Trophic interrelationships between the exotic Nile tilapia, Oreochromis niloticus and indigenous tilapiine cichlids in a subtropical African river system (Limpopo River, South Africa)
  • Abstract
  • Introduction
  • Methods
  • Results
  • Discussion
  • References
• Chapter 4: Ecological niche modeling of the invasive potential of Nile tilapia, Oreochromis niloticus in African river systems: concerns and implications for the conservation of indigenous congenerics?
  • Abstract
  • Introduction
  • Methods
  • Results
  • Discussion
  • References
• Chapter 5: A qualitative ecological risk assessment of the invasive Nile tilapia, Oreochromis niloticus in a sub-tropical African river system (Limpopo River, South Africa)
  • Abstract
  • Introduction
  • Methods
  • Results
  • Discussion
  • References
• Chapter 6: General discussion
  • References

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