Lake Victoria Wetlands and the Ecology of the Nile Tilapia

John Stephen Balirwa

• Publisher: CRC Press
• Year: 2017
• eBook ISBN:
  9781351435970
• Topic: Physical Sciences
• Subtopic: Ecology

This volume is taken from an ecological study of wetlands undertaken in northern Lake Victoria (East Africa) between 1993 and 1996 with the major aim of characterizing shallow vegetation-dominated interface habitats, and evaluating their importance for fish, in particular, for the Nile tilapia.

Chapter 1
Introduction

Lake Victoria in East Africa covers an open water surface of about 68,500 km² making it the second largest lake in the world. Major determinants of its morphometry, hydrological balance and geographical features (Table 1) include a 3,440 km long shoreline characterised by a variety of vegetated littoral habitats frequently referred to as swamps. The plant communities of these swamps are closely linked to the ecology of the lake (Beadle, 1981), particularly that of the littoral zone.

Table 1. Morphometry, hydrological balance and geographical features of Lake Victoria

Characteristic	measure
Position: Latitude	0° 20’ - 3° 00’ S
Longitude	31° 39’ - 34° 53’ E
Altitude (m above sea level)	1134
Catchment area (km^2)	184,000
Lake basin area (km^2)	68,500
Lake area as % catchment	37
Shore line (km)	3,440
Max. length (km)	400
Max. width (km)	240
Mean width (km)	172
Max. depth (m)	84
Mean depth (m)	40
Volume (km^3)	2,760
Inflow (km^3.yr^-1)	20
Outflow (km^3.yr^-1)	20
Precipitation (km^3.yr^-1)	114
Annual lake level fluctuations (m)	0.4 - 1.5
Max. rise in lake level (m)	2.4
Flushing time (years)	138
Residence time (years)	21

Sources: (Crul, 1995; Scheren, 1995; Hecky & Bugenyi, 1992)

Among the more common wetland plant communities at the shore are those dominated by the emergent species: Cyperus papyrus L. (papyrus), Vossia cuspidata (Roxb.) (hippo grass), Typha domingensis Pers. (bulrush) and Phragmites mauritianus Kunth (reeds). Their areal extent is large but not exactly known. These wetlands have undergone considerable hydrological and geographical changes; initially following a rise in lake level in 1960/62 (Welcomme, 1965), and, over the last decades, an increase in the human population in the catchment. Reclamation of the remaining swamps could lead to a reduced buffering capacity resulting in a further eutrophication of the lake (Crul, 1995). The relatively recent (1989) appearance but rapid spread of the exotic floating aquatic weed, Eichhornia crassipes Mart. Solms-Laubach (water hyacinth) in the lake (Twongo, 1991) especially along the shores seems to have become a permanent structural part of the lake’s shoreline ecology.

In general, large stretches of the Lake Victoria shoreline lack a characteristic and well developed underwater flora (e.g., of Ceratophyllum and Potamogeton) at the shallow interface between the shoreline and open water. In addition to the eutrophication (Hecky, 1993), extensive water hyacinth mats may suppress development of natural euhydrophytes in general. Therefore, it is difficult to define the lakeward extent of the littoral zone solely in terms of these plants, even though they are often used (Odum, 1 971) to classify such areas. Interface habitats can be classified according to their dominant structural features at the lake shore. These may be macrophytes, rocky or sandy areas.

This study mainly concerns aquatic habitats backing on emergent macrophyte dominated shorelines. Other structural elements in these shallow habitats include nutrients, plankton, macrofauna, bottom sediments and fish. The relative qualitative and quantitative importance of the structural elements is strongly influenced by physical factors (temperature, oxygen, pH, conductivity, depth, slope and fetch) as well as space (distance from the shore) and time (diurnal and seasonal) scales. By use of secchi transparency, it is possible to define the width of the littoral zone off the respective dominant macrophytes at the shore, and to study the ecology of shallow aquatic ecosystems. For Lake Victoria, this is a new approach as most studies on this lake have been based either within the swamps (e.g., Carter, 1955; Lind & Visser, 1962) or further away from the edges i.e., from the sub-littoral zone out into the open water (the majority of studies). In the latter, habitats have frequently been referred to as inshore, offshore, open water, etc without much indication of the defining structural elements.

Overview of the research problem

Specific concern for the health of the Lake Victoria ecosystem is directly related to human impacts on fisheries, wetland buffers and water quality. As a result of these, fish species changes, wetland degradation, water quality changes (eutrophication) and invasion by the exotic water hyacinth, E. crassipes, currently threaten Lake Victoria’s littoral zone. These impacts probably account for part of the ecological changes in Lake Victoria as a whole about which much has been reported. However, in comparison to the open lake, much less is known about the ecology of the shoreline wetland belt in spite of its assumed ecological and biodiversity importance. This lack of detailed knowledge adds to uncertainties caused by the rapid human population expansion (including urbanisation), leading to more pressure on wetland and aquatic resources, and thus fuelling further ecological changes in the whole Lake Victoria ecosystem but especially in the littoral zone. In addition, the previously important endemic fish species (e.g., Oreochromis esculentus Graham, O. variabilis Boulenger) in the littoral zone and the high fish species diversity there, have been replaced by stocked species, especially O. niloticus Linné (the Nile tilapia) which now dominate the socio-economically important fisheries. Because of the rapid and relatively recent changes in the lake ecosystem, many of which are due to external factors, and are more quickly manifested in the littoral zone, there is a need to establish to what extent the associated wetland dominated habitats are playing a significant role in the ecology of the lake, in order to sustain multiple uses of the aquatic resources.

Fish as an important socio-economic tool could be used to improve aquatic resources’ management and conservation strategies, for instance for wetlands. Therefore, the main interest of the present study was to identify and characterise major shoreline wetlands in Lake Victoria’s littoral zone in relation to the ecology of the commercially important fish, the Nile tilapia (O. niloticus).

Based on literature and general assumptions, it appeared reasonable to assume that wetlands, especially macrophyte dominated interface habitats, are important in the ecology of fish. The main goal of the study was thus to identify and characterise shoreline vegetation dominated habitats in Lake Victoria, and to show which of the structural elements in these habitats were important in the ecology of the Nile tilapia.

As these habitats are assumed to be important fishery areas, it was also expected that there would be significant differences in fish populations between areas adjacent to the lake edges and those further away towards open water habitats of the lake. To prove whether or not this is the case, two null hypotheses for the study had to be formulated thus:

(1) There are no significant structural differences among or within shoreline vegetation dominated habitats, (2) there are no significant effects of the habitats on the ecology of fish.

To establish influence or non-influence of wetland type, the experimental design was based on in situ structural features of the littoral zone rather than on hypothetical populations as in controlled experiments. Hence, the sites sampled were selected with the aim of maximising the range of littoral vegetated habitat types which were considered to be representative of the large scale shoreline landscape. C. papyrus is the most dominant fringing community type at the shore and was therefore more represented in the sampling regime. E. crassipes which is a new wetland feature of the shoreline, was included in this study, and was considered to be a form of modification of the natural wetland landscape.

Study approach and structure of the thesis

The study was planned as an integrated ecological investigation. The Jinja area of Lake Victoria was selected as the base for the studies because of the logistical and laboratory facilities it could provide for lake sampling. Since it was assumed that these wetlands have undergone considerable areal geographical changes, the area was first examined for evidence of cover change which could be ascribed to human activity. Therefore, an initial survey of the major wetland vegetation types in the Jinja area was undertaken to evaluate whether the area indeed represented the larger scale landscape of the lake’s shoreline. For each habitat type, replicate sites were identified and within each site, two zones were categorised: the first being nearest to the vegetation fringe and another representing areas perpendicular to but further away from the fringe vegetation towards open water. In addition, an evaluation of climatic data indicated that four seasons could be recognised. These seasons together with the periods to which they approximately corresponded (in parentheses) were: the main dry season (15 December to 15 March), the main rainy season (16 March to end of June), the short dry but cool season (July to 15 September) and the short rainy season (16 September to end of November). Therefore, sampling of most sites was carried out in specific seasons, usually during the middle of each season. However, there was unexpected rain (February, 1996) during a normally dry period. Subsequently, apart from characterising habitats of assumed importance for fish populations, exploratory data analyses were aimed at detecting possible influences and relationships which could lea...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Dedication
• Contents
• Acknowledgements
• Abstract
• Chapter 1. Introduction
• Chapter 2.1. The Lake Victoria environment: its fisheries and its wetlands - a review
• Chapter 2.2. The evolution of the fishery of Oreochromis niloticus (Pisces: Cichlidae) in Lake Victoria
• Chapter 3. Structural characterisation of shoreline wetlands bordering Lake Victoria (Uganda)
• Chapter 4. Species composition and biomass of fish in the shallow (wetland) habitats of northern Lake Victoria, East Africa
• Chapter 5. Patterns in the abundance and size structure of Oreochromis niloticus Linné (The Nile tilapia) among different vegetation-dominated habitats of the littoral zone in northern Lake Victoria (Uganda)
• Chapter 6. Reproductive biology of the Nile tilapia, Oreochromis niloticus Linné in some East African lakes with emphasis on littoral habitats in northern Lake Victoria (Uganda)
• Chapter 7. Trophic characterisation in the Nile tilapia, Oreochromis niloticus L. in the shallow habitats of northern Lake Victoria, Uganda
• Chapter 8. The growing influence of water hyacinth Eichhornia crassipes (Martius) Solms-Laubach on the ecology of the shoreline wetlands of Lake Victoria, East Africa
• Chapter 9. Summary and conclusions
• Samenvatting en conclusies
• Curriculum Vitae