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The influence of contrasting freshwater inflows on the feeding ecology and food resources of zooplankton in two eastern Cape estuaries, South Africa

  • Authors: Grange, Neil
  • Date: 1993
  • Subjects: Freshwater zooplankton -- Ecology -- South Africa Aquatic ecology -- Research -- South Africa
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5714 , http://hdl.handle.net/10962/d1005400
  • Description: The trophodynamic implications of reduced freshwater inflow on the zooplankton of eastern Cape estuaries was investigated by a comparison of the community composition and standing stocks, grazing rates and food resources of zooplankton in two systems, the Kariega and the Great Fish estuaries, which are subject to contrasting freshwater inflow. The climate of South Africa is semi-arid, and the low rainfall, coupled with high evaporative loss, result in the region having one of the lowest conversions of rainfall to run-off in the world. In addition, many of the major rivers are extensively impounded, consequently, estuaries often experience prolonged periods of zero or reduced freshwater inflow. The amount of freshwater available for estuarine management in the future is expected to decline as the demand for domestic, agricultural and industrial use increases. The influence of climate, tidal amplitude and wave action are essentially constant, consequently, the individual characteristics of an estuary are determined largely by the indirect influences of catchment size and regulation. Estuaries along the eastern Cape coast range from negative hypersaline systems, to positive systems in which a salinity gradient is well established. The Kariega estuary is a homogeneous marine estuary as a result of minimal freshwater inflow, whereas the Great Fish estuary receives sustained freshwater inflow and is partially-stratified. The quality and quantity of particulate food resources for suspension-feeders depended to a large extent on the allochthonous import of material associated with freshwater inflow. Nutrients, rather than light penetration of the watercolumn are the major factor limiting phytoplankton standing stocks. In the Kariega estuary, phytoplankton standing stocks were low (up to 1.0 μg l⁻¹) and the estuary is classified as oligotrophic. Correlation analyses indicated that lower quality detritus, originating from fringing macrophytes, may contribute substantially to suspended particulate organic material. Phytoplankton food resources were considerably higher in the Great Fish estuary (up to 21.8 μg ⁻¹) which is classified as mesotrophic/eutrophic, and correlation analyses indicated that phytoplankton comprised the dominant fraction of the particulate organic material. Although this suggested that the organic material was of a higher quality, the seston was "masked" by a higher suspended inorganic load. Consequently, the organic fraction comprised between 13 and 22% of total particulate material in the Great Fish estuary, compared with between 20 and 39% in the Kariega estuary. Food resources demonstrated a fairly uniform distribution along the length of the Kariega estuary and exhibited a slight seasonal increase during warmer months. By contrast, the point source influence of freshwater inflow resulted in a spatial gradient of food resources in the Great Fish estuary with higher values recorded towards the upper reaches. There was evidence that higher concentrations of particulate material in the upper reaches are also a consequence of hydrodynamic trapping. There was no marked seasonal pattern in the availability of food resources which were generally elevated in response to sporadic pulses of freshwater inflow. Tidal currents were responsible for elevating suspended food resources by re-suspension of material from the sediments. This effect was probably of greater importance in the Kariega estuary where food resources were generally limiting. In the Kariega estuary, the zooplankton community was dominated by calanoid copepods of the genera Acartia and Pseudodiaptomus. However, in the Great Fish estuary, the community was dominated by the mysid Mesopodopsis siabberi, and the calanoid P. hessei. Community biomass generally reflected the trophic status of the estuary. The mean seasonal biomass recorded in the Kariega estuary was 38 mg m⁻³ compared with 1597 mg m⁻³ in the Great Fish estuary. Greater spatial variability in community biomass was evident in the Great Fish estuary, partly in response to the food resource gradient, but also due to the inability of the mysid shrimps, which dominated the community biomass, to penetrate the lower salinity water of the upper reaches. Zooplankton in the Kariega estuary demonstrated a seasonal pattern of abundance whereas in the Great Fish estuary, community biomass was elevated in response to sporadic pulses of freshwater inflow. Grazing rates, measured in situ using a modified Gliwicz-Haney chamber, indicated that the zooplankton communities were capable of "clearing" substantial proportions of the watercolumn at certain times of the year. The pattern of grazing pressure over a diel cycle was examined in relation to the diel vertical migration movements of the zooplankton. Higher nighttime grazing rates were generally associated with the greater abundance of zooplankton resulting from the movement of zooplankton into the watercolumn after dusk, and their return to the sediments at dawn. Seasonal estimates of diel grazing pressure, extrapolated from daytime and nighttime feeding rates, indicated that the zooplankton "cleared" up to 40% of the watercolumn in a day in the Kariega estuary, and up to 120% d⁻¹ in the Great Fish estuary. However, values of around 25% d⁻¹ in the Kariega estuary, and 50 to 80% d⁻¹ in the Great Fish estuary, were not uncommon. Multiple regression analyses were used in an attempt to explain the influence of environmental factors on the variation in in situ grazing rates. These attempts were largely unsuccessful and the possible reasons, as well as recommended improvements to the methods used, are discussed. Seston concentration in the estuaries was highly variable as a result of the effects of tidal re-suspension and freshwater inflow. Consequently, further laboratory-based experiments were carried out to examine the influence of seston concentration on the filtration rates of the dominant calanoid copepods. Results indicated that some of the unexplained variability in the community filtration rates may be attributed to differences in species-specific response to changes in seston concentration.
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  • Date Issued: 1993

Aspects of the biology of the infaunal bivalve Mollusc Solen cylindraceus (Hanley) in the Kariega estuary

  • Authors: De Villiers, Casper Johannes
  • Date: 1990
  • Subjects: Mollusks -- South Africa -- Eastern Cape Bivalves -- South Africa -- Eastern Cape
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5639 , http://hdl.handle.net/10962/d1005321
  • Description: Solen cylindraceus is an infaunal filter-feeding bivalve inhabiting the intertidal mud banks of many southern African estuaries. It is particularly abundant in the Kariega estuary (33°41'S; 26°42'E) where it reaches densities of 400m⁻² (192g shell-free dry wt. m⁻²). The Kariega is a permanently open, marine dominated estuary about 18km in length, and S. cylindraceus is most abundant in its middle and upper reaches. Some physical characteristics of the estuary (temperature, salinity, sediment and water turbidity) are described, and the possible role of these factors in determining the density and distribution of S. cylindraceus within the Kariega estuary, is discussed. The structure of the alimentary system, gills and labial palps of S. cylintfraceus is described, all of which showed no major variation from the "typical" eulamellibranchiate form. Solen cylintfraceus was found to be a euryhaline osmoconformer with a salinity tolerance range of 15-65%. When animals were removed from their burrows, osmotic equilibration of the haemolymph was rapid (1-2 hours). By contrast, in animals left undisturbed in their burrows, osmotic equilibration was retarded (72-204 hours). It is suggested that the observed decrease in the rate of change of haemolymph osmolarity for animals in their burrows is linked to the stability of the interstitial salinity. A temperature tolerance range of 5-44°C was determined for S. cylintfraceus (in situ), in which prolonged exposure to 5°C and 40-45°C (12-36 hours respectively) resulted in a decreased burrowing ability, coma and death. Animal burrowing responses were not affected by temperatures in the range 15-35°C. Field experiments were carried out over several tidal cycles, in which the measurement of crystalline style volume was used as a means of assessing extracellular digestive activity. No major variation in style volume was recorded and it appeared that S. cylindraceus did not exhibit any cyclical pattern of style dissolution and regeneration. It is suggested that S. cylindraceus feeds continuously from the water column during high tide and possibly within its burrow, at or below the water table, during low tide. At a suspensoid concentration of 5Omg l⁻¹, S. cylindraceus was found to filter water almost continuously (90-95% of the time). Time spent filtering dropped to 68% at 100mg l⁻¹ and 32% at 500mg l⁻¹. Filtration rates for summer collected animals (25°C) were 22.86 ± 4.36ml min.⁻¹, some 3ml min.⁻¹ greater than that recorded for winter (16°C) collected animals. Filtration rate may be expressed as a function of shell length by the equations: y=0.247x¹̇⁰⁶⁶ (winter) and y=0.758x⁰̇⁸²⁶ (summer). Solen cylindraceus was capable of acclimating its filtration rate to both high and low temperatures under laboratory conditions. Filtration rate exhibited a thermal optimum in the range 15-35°C, declining at higher and lower temperatures. Q₁₀ values of filtration decreased rapidly from greater than 4 to less than 2, when the thermal optimum was reached. Maximum rates generally occurred at approximately 5°C above the temperature to which the animal had been acclimated. Optimal filtration rates (19-23ml min.⁻¹) were recorded in the salinity range 15-45%. When subjected to abrupt changes in salinity, filtration rates were immediately depressed. The extent and duration of these decreased filtration rates were dependent upon the magnitude and direction of salinity change, and were always less in animals exposed to hyper- than hyposaline conditions. Animals exposed to increased temperature and simultaneous elevated or unchanged salinity, showed a slight increase in filtration rate followed by rapid acclimation. A decrease in both temperature and salinity resulted in an initial decrease in filtration rate and a longer acclimation period. The ability of S. cylindraceus to acclimate fully within a wide temperature and salinity range, and to filter maximally in hypersaline conditions may, in part, explain its unusually high abundance in the Kariega estuary, despite it being close to the southernmost limit of the animal's geographical distribution. No significant difference in flItration rate was recorded at suspensoid concentrations of 5-100mg 1⁻¹. However, at 250 and 500mg l⁻¹ filtration rates decreased significantly, and coincided with increased levels of pseudofaecal production. Solen cylindraceus retained particles down to 2.5-3.0µm with great efficiency (ca. 60-90% efficiency). Below this particle size, retention efficiency decreased rapidly and a net production of particles was recorded below 1.51µm. Particle retention was independent of temperature (15 and 25°C) and salinity (15 and 35%). Use was made of stable carbon isotope analyses (¹³C/¹²C ratios) in an attempt to determine the important food sources of S. cylindraceus within the Kariega estuary. The results obtained demonstrated an enrichment in δ¹³ values for S. cylindraceus from the upper (-27.9%) to the middle (-25%c) and lower (-21.6%o ) reaches of the estuary, with no seasonal variation apparent. The bivalve was substantially more depleted in ¹³C relative to the dominant aquatic macrophytes Zostera capensis (-9.1 to -15.6%o) and Spartina maritima (-12.5%o). The use of δ¹³ alone, however, to unequivocally "pin point" specific food sources of a filter feeder in a predominantly detritus based food web, is limited. It is suggested that in the Kariega estuary, riparian litter and other terrestrially derived vegetation contribute to the carbon pool. A possible contribution of ¹³C depleted food sources via chemoautotrophic and/or anaerobic pathways, to the diet of S. cylindraceus, is suggested.
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  • Date Issued: 1990

Tidal exchanges of carbon, nitrogen and phosphorus between a Sarcocornia salt-marsh and the Kariega estuary, and the role of salt-marsh brachyura in this transfer

  • Authors: Taylor, David Ian
  • Date: 1988
  • Subjects: Salt marsh ecology -- South Africa -- Eastern Cape Salt marsh animals -- South Africa -- Eastern Cape Kariega River
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5623 , http://hdl.handle.net/10962/d1004524
  • Description: Tidal exchanges of organic carbon, nitrogen and phosphorus between a south temperate Sarcocornia marsh and its associated estuary are examined. Subterranean water flow was small, and the hydraulic exchange between the two systems largely surficial. The dominant tidal signal was semi-diurnal, and the extent of inundation of the marsh varied considerably as a consequence of interactions of semi-lunar tidal cycles with changes in daily mean sea level. Annual net fluxes of organic carbon were directed from the marsh to the estuary, but amounted to less than 2% of marsh aerial net primary productivity. This indicates the incompatibility of E.P. Odum's outwelling hypothesis to this marsh-estuarine system. The direction of net flux of organic carbon switched on a time-scale of days. These directions were largely correlated with mesoscale oceanic events, which materially altered the extent of marsh inundation, and which provided evidence of the mutual exclusivity of outwelling of DOC from the marsh and oceanic upwelling. Laboratory mesocosm experiments using intact marsh blocks of sediment from the marsh were conducted to identify the proximate processes and interactions at the marsh-water interface responsible for the variability of marsh-estuarine exchanges. Patterns of fluxes of organic carbon, total nitrogen and phosphorus were markedly different in the structurally contrasted tidal creek and Sarcocornia Zone regions of the marsh. Both regions exported these components, but the fluxes of organic carbon and total phosphorus were significantly larger from the tidal creek than from the Sarcocornia zone, and the opposite applied to nitrogen. The presence of brachyuran crabs . the most numerous macrofauna on the marsh enhanced the flux of carbon, nitrogen and phosphorus from the marsh biocoenosis, largely as a result of the effect of their bioturbation. Evidence is examined which suggests that differential mobilization of nutrients in the two zones by crabs is responsible for biogeochemical coupling of these two regions , which may account for the elevated productivity of salt- marsh systems
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  • Date Issued: 1988

A contribution to the oceanology of the Prince Edward Islands

  • Authors: Parker, Llewellyn Derek
  • Date: 1985
  • Subjects: Oceanography -- Prince Edward Islands
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5605 , http://hdl.handle.net/10962/d1002055
  • Description: While the terrestrial ecosystem of the Prince Edward Island Group has been subject to intensive research, the marine ecosystem has to a large extent been neglected. This together with the possible existence of an "island effect" at these islands, as was first proposed after the visit to these islands of the French vessel Marion Dufresne, led to the initiation of a programme to determine the distribution of standing stocks and the productivity of phytoplankton and zooplankton in the neritic seas of these islands. To do this and before a detailed biological survey could be attempted, it was first necessary to define the physical and chemical properties of the circuminsular waters. This dissertation discusses the results of several surveys to these islands in the light of a possible "island effect" and comments upon processes likely to influence such an effect
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  • Date Issued: 1985

An evaluation of Mimorista pulchellalis (Dyar) (Lepidoptera : Pyraustidae) as a biocontrol agent against jointed cactus in South Africa

  • Authors: Nieman, Erik
  • Date: 1984
  • Subjects: Cactus -- South Africa , Weeds -- Biological control -- South Africa , Pyralidae -- South Africa , Moths -- South Africa
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5812 , http://hdl.handle.net/10962/d1007052 , Cactus -- South Africa , Weeds -- Biological control -- South Africa , Pyralidae -- South Africa , Moths -- South Africa
  • Description: From the introduction: The work on Mimorista is divided into two sections: a laboratory orientated study which describes the biology and rearing techniques and a field orientated study where the establishment of the insect and its impact on jointed cactus populations are examined. In the final chapters the integration of this insect in the current control program are discussed and recommendations regarding its future in South Africa are given.
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  • Date Issued: 1984

Investigations into the ecology of a population of Citrus Red Scale (Aonidiella aurantii Mask.) in the Swaziland lowlands

  • Authors: Atkinson, P R
  • Date: 1982
  • Subjects: Citrus -- Diseases and pests -- Africa, Southern
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5619 , http://hdl.handle.net/10962/d1003733
  • Description: In the Swaziland lowlands, biological control of the citrus pest red scale, Aonidiella aurantii (Mask), has been consistently unsuccessful although it is successful in middle altitude regions. However, recent cases of resistance to organophosphate insecticides elsewhere in southern Africa, make it important to reconsider biological control of red scale in the lowlands. Should similar resistance be evolved in this region, the pest could be controlled only by natural means together with oils. This account attempts to answer questions about the natural control of a red scale population in the lowlands, by examining data collected between 1972 and 1975. The population was continuously breeding with overlapped generations. It consisted of three sections, on leaves, twigs and fruit, having different rates of reproduction, development and mortality. These sections are considered separately but that on leaves, being relatively unimportant, is largely ignored. The generation mortality is estimated in the sections on twigs and fruit and related to climatic and biotic indices. The causes of seasonal and between-years variation in the sections on twigs and fruit are elucidated. The effectiveness of each kind of natural enemy is examined and the factors affecting natural enemy numbers are investigated. The question of population regulation is discussed. Differences in mean infestation levels in middle altitude and lowland regions are explained in terms of different rates of reproduction and mortality.
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  • Date Issued: 1982

The factors which influence algal growth in the P.K. le Roux impoundment, Orange River, South Africa

  • Authors: Selkirk, Wayne Thomas
  • Date: 1982
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5798 , http://hdl.handle.net/10962/d1005900
  • Description: Introduction: In South Africa, much of the work on algal limitation in man-made lakes has centered on nutrients rather than the possible effects of physical factors such as turbidity.In order to determine which is the most important factor in algal limitation in the turbid Orange River system, a comprehensive study of spatial and seasonal changes of the following factors was undertaken in the P.K. Ie Roux impoundment: trace elements , silicon, nitrate, phosphate, total phosphate, phytoplankton standing stock, total suspended solids, secthi disc transparency, light attenuation and the spectral quality of the light. None of the chemical factors appeared to be in short supply but the shallowness of the euphotic zone (0.8 m) had two effects. Firstly. the quantity of light in the upper 1.5 metres was extremely small due to the rapid attenuation of the light by silt. Secondly. the quality of the light was poor due to the selective screening of the shorter wavelengths of light. Once light limitation had been implicated as an important factor in the control of phytoplankton stocks a series of column experiments were used to demonstrate that increased circulation rate. and therefore a higher light dosage. would elevate the algal standing stocks without the addition of nutrients. It was demonstrated that light. as a result of the negative effects of turbidity was the principal cause of low algal standing stocks within the impoundment. and not nutrients as has previously been suggested by other workers.
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  • Date Issued: 1982

The social and spatial organisation of the Greater kudu (Tragelaphus strepsiceros Pallas 1766) in the Andries Vosloo Kudu Reserve, Eastern Cape

  • Authors: Allen-Rowlandson, T S
  • Date: 1981
  • Subjects: Greater kudu -- South Africa -- Eastern Cape
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5614 , http://hdl.handle.net/10962/d1003120 , Greater kudu -- South Africa -- Eastern Cape
  • Description: Methods of capture, marking and age determination in the field, were investigated. The density and structure of the population, individual movements and social relationships were determined. Aspects of reproduction examined included age at puberty, agespecific fecundity, calving intervals and seasonality. Temporal changes in the spatial and social organisation are discussed in relation to environmental conditions, reproduction and population structure. Factors influencing the fecundity, dynamics and grouping patterns within the population are considered.
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  • Date Issued: 1981

Community metabolism and phosphorus dynamics in a seasonally closed South African estuary

  • Authors: Liptrot, Mark Robert Michael
  • Date: 1978
  • Subjects: Estuarine ecology -- Research -- South Africa -- Swartvlei , Eelgrass -- Research -- South Africa , Enteromorpha -- Research -- South Africa
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5810 , http://hdl.handle.net/10962/d1006966 , Estuarine ecology -- Research -- South Africa -- Swartvlei , Eelgrass -- Research -- South Africa , Enteromorpha -- Research -- South Africa
  • Description: The effects of seawater inflows and macrophyte beds on community metabolism and phosphorus dynamics in the seasonally-closed Swartvlei estuary were investigated. Metabolic rates were determined by diurnal oxygen curve analysis. Gross primary production ranged from 0,7 to 14,9 g 0₂ m⁻² day⁻¹, and respiration from 0,9 to 25,2 g 0₂ m⁻² day⁻¹. The highest production rates were recorded inside the dense beds of Zostera capens is Setchell (x- = 7,8 g 0₂ m⁻² day⁻¹ ). Metabolism was positively correlated to submersed macrophyte cover, and decreased upstream of the mouth region. No seasonal variation in gross production could be detected, and mouth closure had no detectable effect on metabolic rates. Growth of the green alga Enteromorpha sp. in winter, and the effect of decay of this alga on dissolved oxygen, total inorganic carbon and total dissolved phosphorus in the water column is discussed. High day-time respiration values, measured in a darkened polythene enclosure, indicate that photorespiration occurs in Zostera. Apparently the annual amount of oxygen respired throughout the estuary exceeded that produced by 2 7100 tonnes, i.e. the estuary was heterotrophic. This is discussed in relation to the effectiveness of the diurnal curve method as a way of measuring metabolic rates, and to the possibility of organic matter input via river inflows. There is a net gain of up to 17,4 kg of particulate phosphorus over mean tides. Diurnal phosphorus studies indicate that Zostera releases phosphate into the water column in the light. Enteromorpha takes up phosphate in the morning, which it appears to release over the rest of the day. The cycling of phosphorus between seawater, sediments, macrophytes and the water column is discussed.
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  • Date Issued: 1978

The anatomy of the triassic theropod Syntarsus rhodesiensis (Saurischia : Podokesauridae) and a consideration of its biology

  • Authors: Raath, M A
  • Date: 1978
  • Subjects: Reptiles, Fossil Dinosaurs Paleontology -- Triassic , Dinosaurs , Fossils , Reptiles , Palaeontology , Triassic period
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5601 , http://hdl.handle.net/10962/d1002051
  • Description: The osteology of the Upper Triassic podokesaurid Syntarsus rhodesiensis is described, based on a series of 30+ individuals representing all skeletal elements, recovered since the description of the holotype (Raath, 1969). A brief account of the geology of the finds is given, with an attempt at a reconstruction of the palaaoenvironment. The excellence of preservation of the bones has permitted an attempt at the restoration of soft tissues including the brain, cranial nerves, main cranial blood vessels and the musculature of the jaws, neck and limbs. Histological sections of limb bones have shown that the compact bone was highly vascular, and this, together with the structure of the brain, palaeoenvironmental considerations, social behaviour and group structure, leads to the conclusion that Syntarsus was an endothermic homeotherm inhabiting a hot arid region at the end of the Triassic, with a social organisation into "flocks" in which females predominated numerically. Clear evidence of sexual dimorphism is presented. Syntarsus is reconstructed as a bipedal, saltatorial predator which differs in subtle, but probably generically significant, characteristics from the closely related North American genus, Coelophysis. Its anatomy characterises it as a medium-sized agile animal with a highly kinetic skull; incipiently opposable pollex in the raptorial manus; highly cursorial hindlimb; and with features in the dentition and hallux which suggest a grooming function. It is concluded that the Triassic coelurosaurian stock provided an advanced and well adapted base from which the successful coelurosaur radiation into the later Mesozoic sprang, and that this stock was physiologically pre-adapted for the emergence of the avian (and possibly the pterosaur) lineages in the Jurassic
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  • Date Issued: 1978

Colour vision of the citrus psylla Trioza erytreae (Del Guercio) (Homoptera: Psyllidae) in relation to alightment colour preferences

  • Authors: Urban, Alan Joseph
  • Date: 1977
  • Subjects: Trioza , Homoptera , Jumping plant-lice , Color vision
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5885 , http://hdl.handle.net/10962/d1013286
  • Description: The colour vision of adult citrus psylla, Trioza erytreae, was investigated in the laboratory using the behavioural parameters: alightment and walking. Light green flushing leaves (under which the nymphs develop) were significantly preferred, visually, to dark green mature leaves for alightment. Diffuse reflectance spectroscopy showed (when expressed in the parameters of human colour vision) that flush has a very slightly longer dominant wavelength, and roughly double the reflectance and purity. Alightrnent frequency correlated almost equally well with "purity" (as noted by Moericke, 1952 et seq., in "yellow-sensitive" aphids) as with the aphidological colour parameter "long/short ratio" developed by Kennedy et al. (1961). Elucidation of the mechanism underlying the citrus psylla's alightment colour preference was initially attempted with a printed spectrum and several paint series of measured spectral characteristics. It was clear that T.erytreae belongs to the "yellow-sensitive" group of Homoptera, but it was impossible to distinguish which pararneter(s) of colour the psyllids were responding to. Phototactic (walking) response to the individual parameters of colour was therefore measured using a monochromator. The phototactic action spectrum (against wavelength) was tri-modal, with peaks in the yellow-green (YG), blue (B), and ultra= violet (UV). Rate of phototaxis was not influenced by bandwidth (roughly equivalent to purity), but was proportional to intensity (roughly equivalent to reflectance). To investigate the influence of the above three wavelength regions on alightment, use was made of a very simple flight chamber incorporating a target of coloured light. Yellow-green and UV light both independently stimulated alightment . Their effect was additive. Different thresholds indicated distinct YG and UV receptor systems. Blue light alone did not stimulate alightment, and was strongly alightment-inhibitory in combination both with YG and with UV light. On the basis of the above physiological/behavioural findings, a new alightment formula was drawn up for describing the hamopteran's apparent manner of alightment determining integration of surface reflectance. The flush preference and alightment distributions on the series of artificial surfaces were found to correlate slightly more accurately, on average, as well as more consistently, with the new formula than with previously-available colour parameters. These findings are placed in perspective to the literature, and their possible economic relevance is discussed.
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  • Date Issued: 1977

Feeding ecology of the cichlid fish Sarotherodon Mossambicus in Lake Sibaya, KwaZulu

  • Authors: Bowen, Stephen H
  • Date: 1977
  • Subjects: Cichlids -- Ecology , Fishes -- Ecology -- South Africa -- KwaZulu-Natal -- Sibaya, Lake
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5837 , http://hdl.handle.net/10962/d1009688 , Cichlids -- Ecology , Fishes -- Ecology -- South Africa -- KwaZulu-Natal -- Sibaya, Lake
  • Description: The feeding of the cichlid fish Sarotherodon mossambicus in Lake Sibaya, KwaZulu, was studied in relation to various biotic and abiotic features of the l ake environment from January, 1973 to July, 1975. The primary goal of the study was to determine the cause of stunting and poor condition of adults in this population. Juvenile and adult S. mossambicus feed on a mixture of detritus, bacteria and diatoms that occurs as a flocculent layer on sand substrates throughout the lake. Concentrated gastric acid, commonly at pH values of 1.5 and lower, lyses diatoms and bacteria which are subsequently digested in the intestine. This is the first report of digestion of bacteria by a fish, and the nutritional significance of the finding is discussed. Juveniles feed predominantly on shallow sand terraces found along the margin of much of the lake, while adults usually feed in offshore waters at depths of 3 m and greater. Benthic floc from feeding areas of juveniles and adults have similar concentrations of organic matter, total carbohydrate, soluble carbohydrate and calories, but differ markedly in respect to diatom and protein concentration. Diatom concentrations are generally high in terrace floc but are consistently low in floc from deep water. Protein concentration of benthic floc decreases with increasing depth from 0 - 5 m. Partial correlation analysis shows that protein and diatom concentrations have no correlation independent. of their common relationship to depth, and the evidence implicates detrital bacteria as the primary source of protein in benthic floc. Probable causes of the observed distributions of diatoms and protein are discussed. The protein content of benthic floc profoundly influences its nutritional value. A comparison with available data on animal nutrition shows that the ratio of digestible protein to digestible energy in floc in the shallows is high enough that this food resource would be expected to support good growth, but the ratio for deep water floc is so low that it would be expected to result in conspicuous malnutrition. Thus, inadequate dietary protein is identified as the principal cause of stunting and poor condition of adult S· mossambicus in Lake Sibaya; It is argued that the ratio of digestible protein to digestible calories can be expected to determine the food value of particulate organic matter that includes amorphous detritus in other ecosystems. Despite stunting and poor condition of adults, S. mossambicus are abundant in Lake Sibaya. The precocious breeding of this population is discussed as an adaptation that allows it to maximize reproductive output given the limited resources available to adults. Juvenile S. mossambicus perform daily movements from deep water onto the terraces where they feed and then back into deep water. During periods of relatively low lake level when terrace waters were usually less than 1.5 m deep (1-73 to 1-74), juveniles were present on the terrace throughout daylight hours. At relatively high lake levels (7-74 to 8-75), juveniles were present on the terraces for only about five hours following sunset. Periods of heavy wave action interrupt feeding activity and result in reduced numbers of fish in terrace waters. The possible roles of predator avoidance and temperature in determination of daily feeding behavior are discussed. Daily energy assimilation by juveniles, estimated by a field technique, is approximately 115 cal per g fish dry weight.
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  • Date Issued: 1977

The biology of Clarias Gariepinus (Burchell, 1822) in Lake Sibaya, Kwazulu, with emphasis on its role as a predator

  • Authors: Bruton, Michael N
  • Date: 1977
  • Subjects: Catfishes -- South Africa -- KwaZulu-Natal -- Lake Sibaya. , Clarias , Clariidae
  • Language: English
  • Type: text , Thesis , Doctoral , PhD
  • Identifier: vital:5886 , http://hdl.handle.net/10962/d1013313
  • Description: Aspects of the biology of the sharptooth catfish, Clarias gariepinus (Burchell, 1822) were studied in a clear coastal lake (Lake Sibaya, KwaZulu.). Catfish reach maturity towards the end of the first year (total length 200-250mm) and breed in summer. Gonadal maturation is cyclical, and most fishes are ripe between September. and January e ach year. Maturation stages and fecundity are described. Twenty-two catfish spawning runs were witnessed. There were no large aggregations of catfish before the Summer rains, as reported in some impoundments and rivers, but catfish accumulated on the shallow terrace immediately before a spawning run. Spawning usually took place in flooded marginal areas after heavy rain on dark calm nights. Peak activity occurred between 20h00 and 02h30. Pre-nuptial aggression, courtship and mating are described and illustrated in detail for the first time from field observations. Gonadal products are released while in amplexus and fertilized externally. The eggs have an adhesive disc for attachment to plants, and develop rapidly. Early development and feeding are described. The growth rate of catfish was determined using rings on the pectoral spines, and validated using recent techniques. C.gariepinus in Lake Sibaya grow rapidly and remain in good condition to a length of 500mm, but larger catfish have a poor growth rate and condition, and few exceed 650mm. The modal length and weight of the Sibaya population is intermediate between that of other populations but the number and condition of larger catfish in Lake Sibaya is lower. Other studies on C.gariepinus growth are reviewed. Very high and low first year increments obtained previously may be due to methodological errors, but catfish do appear to have variable growth rates. The size reached by C.gariepinus is reviewed. Catfish are equipped morphologically and behaviourally to feed on a wide variety of·prey in different situations, but they usually feed at night on exposed, active benthic organisms. Social hunting facilitates the capture of prey which is too fast or manoevrable for a single predator. Food preferences are scored by three different methods and presented for the total catfish population and for different length groups, seasons, and habitats. Juveniles feed more often and on a greater variety of prey, and their condition is better than that of adults. The proportions of several food items in the diet changed at different lake levels as marginal areas became inundated. In particular, increasing lake levels resulted in a change in relative density of three cichlid prey in the shallow water feeding area of catfish. This situation provided the opportunity to study the effect of changing cichlid density on predation by catfish. The world literature on factors influencing the predator-prey relationship in fish is reviewed and several key factors are identified. The interaction of fish predators and prey in Africa, and particularly Lake Sibaya, is examined in detail, and prey density, modified by accessibility, is found to be an important factor. The effect of cichlid prey density on catfish predation was therefore investigated using field collections and experimental observations. Catfish prey on the cichlid species with greatest relative abundance, with some deviations which can be explained in terms of accessibility. The experimental results also describe feeding periodicity, imitative hunting and the effect of water depth , day-night light intensity changes and predator number on predation efficiency on cichlids. Catfish predation is discussed in terms of recent predation theory, and it is concluded that their ability to switch from one prey to another as prey availability (= density+ accessibility) changes , is an an important aspect of their success as predators. Catfish predate more efficiently on invertebrates, but feed largely on fishes after the first year. This preference for fish prey is probably related to food quality, as fishes contributed over 75% of the energy content of all ingested food, as well as abundant protein and the correct amino acid balance. Finally, I conclude that the role of C.gariepinus as a predator on S.mossambicus is three-fold: (a) to enhance species' fitness by removing enfeebled individuals i.e. to act as a 'cleanser' (b) to restrict the distribution of prey under certain conditions i.e. act as a 'restricter' (c) to dampen fluctuations in cichlid abundance i.e. act as a 'regulator'. The relative importance of these roles may alter at different lake levels in Lake Sibaya in response to changes in the vulnerability of fish prey.
  • Full Text:
  • Date Issued: 1977

The influence of abiotic factors on the distribution of Biomphalaria pfeifferi (Krauss, 1848) (Planorbidae: Mollusca) and its lifecycle in South-Eastern Africa

  • Authors: Appleton, Christopher Charles
  • Date: 1976
  • Subjects: Schistosomiasis , Mollusks as carriers of disease
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5835 , http://hdl.handle.net/10962/d1009495 , Schistosomiasis , Mollusks as carriers of disease
  • Description: Previous studies on the influence of abiotic factors on the distribution of bilharzia intermediate host snails of the family Planorbidae are reviewed. Much of this work is basically descriptive and relatively few attempts have been made to examine the effects of these factors on snails in their natural habitats - to identify the factors precisely and to measure their critical levels. An account is given of recent studies on Eiomphalaria pfeifferi (Krauss) in two climatically different regions of southeastern Africa, at 6S5m altitude on the eastern Transvaal escarpment and 19m on the coastal peneplain of northern Zululand. These have shown the species to undergo a similar annual life-cycle of three overlapping generations in both areas. Further, in these areas where the snail has a discontinuous distribution, two abiotic factors, stream geology and water temperature, were found to be important in limiting its distribution. The longitudinal distribution of B. pfeifferi and another host snail Bulinus (Physopsis) globosus (Morelat), extended upstream in a perennial watercourse, the Gladdespruit, on the escarpment only as far as permanently lentic habitats were available. These habitats, usually detached from the channel, characterized the stream's lowest physical zone with its substratum of exposed granite, as opposed to the sandy-bottomed lotic zones upstream. The limit of the snails' occurrence lay close to the junction of these two substratum types. Granite is resistant to erosion but contains soft inclusions which erode more quickly than the hard matrix causing uneven weathering. This results in the formation of depressions in the stream bed (i.e. pools, potholes and backwaters) which provide the permanently lentic conditions necessary for the snails' survival. Temperature recordings made on this plain, upstream of the snails' limit, suggest that the water temperature here exceeded the critical lower levels required for a positive intrinsic rate of natural increase sufficiently often to permit the species to survive here, though not optimally. Above this point however, current velocities are continually in excess of the maximum tolerated (0.3 m/sec.) so that in this situation at least, current velocity is a dominant factor. A contrasting situation exists on the coastal peneplain of northern Zululand where B. pfeifferi occurs in some lentic habitats and not in others. This discontinuous distribution is shown to be related to the prolonged duration of temperatures above the level for optimal increase. The apparently suitable habitats from which B. pfeifferi was absent were found to be both very shallow and to experience these prolonged high temperatures during spring and early summer. This corresponds to the maturation period of the B. pfeifferi 1st generation as defined by Appleton (1974) and causes reduced fecundity probably through impaired gonad development. The density of its filial (2nd) generation is correspondingly reduced. A statistically significant partial-correlation (at a 1% level) was found between the fecundity of the 1st generation and increasing periods of temperatures above the species' optimal limit in the habitats involved during its maturation period (spring). It is interesting to nate here that B. (Ph.) globosus, which is known to be better adapted to high temperatures than B. pfeifferi (Shiff & Husting, 1966) occurred in all the waterbodies concerned. Both these factors therefore play important roles in determining the country-wide distribution pattern of the host snails. This distribution is closely correlated with the availability of permanently lentic habitats. In river systems, which form the main environment for these snails, such stable habitats occur almost entirely in low-gradient reaches over hard bed-rock which is resistant to erosion. Current velocities above 0.3 m/sec. render steeper reaches and those over soft, non-resistant rock and sand (unstable substrata) unsuitable and therefore constitute a most important limiting factor. Temperature however becomes a dominant factor in permanently lentic waterbodies. Biomphalaria pfeifferi is adversely affected by high temperature regimes and where such regimes occur in waterbodies which are too shallow to permit temperature gradient to develop, the species cannot survive whereas B. (Ph.) globosus can do so. This intolerance of high temperature regimes on the part of B. pfeifferi accounts for the species' adoption of a discontinuous distribution over the coastal plain of south-eastern Africa. A comparable pattern has been reported for the species over this plain northwards to approximately 16°S latitude in Mozambique and is probably due to a similar high-temperature effect. Further northwards in equatorial latitudes B. pfeifferi is restricted to the more elevated and cooler areas above an altitude of 500 - 600 m, probably because the very high temperature regimes prevailing in the lowlands there exclude the species altogether.
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  • Date Issued: 1976

A contribution to the understanding of the ethology of the cichlids of Southern Africa

  • Authors: Ribbink, Anthony J
  • Date: 1975
  • Subjects: Cichlids -- Africa, Southern , Fishes -- Africa, Southern
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5889 , http://hdl.handle.net/10962/d1013369
  • Description: Allopatric populations of Pseudocrenilabrus philander were found to differ in male breeding coloration. These colours are described and attention is drawn to the possibility that male coloration might prevent population hybridisation. Behaviour of the allopatric populations was studied in the laboratory. The execution of behaviour patterns of the various populations was found to be indistinguishable, and very similar to that of Haplochromis. Behaviour of P. philander was recorded in three morphologically different natural water bodies. Diurnal activities were found to follow a set rhythm. Though P. philander was shown to have adapted to a variety of habitats they retained their behavioural Components in an unchanged form. Field and laboratory observations are used to propose a behavioural mechanism for the natural regulation of population density. Although aggression between conspecific territorial Sarot herodon mossambicus habituated so that neighbours could live close to one another, it was found that P.philander are intolerant of their neighbours and are consequently forced apart. Though the execution of behaviour patterns was apparently identical for all populations of P. philander, field observations indicated that the frequency of performance differed. An experimental procedure was established to quantitatively compare the behaviour of various populations. As a result of comparative and choice chamber investigations, it is suggested that one of the four populations of P. philander would be unlikely to hybridise with the others if they were to become sympatric. These popUlations would be kept apart because of colour and behavioural differences. Differences of egg-size, fry-size, developmental and interbrood periods were also found, providing further evidence of incipient speciation. Evolutionary divergence of the allopatric populations of P. philander is discussed against a background of geological and geographical evidence. Consideration is given to the role of male coloration, ethological barriers and the importance of ethological differences to taxonomy. Behavioural evidence suggests that P. philander has closer affinities with the haplochromids than was initially realised. It is concluded that the clinal populations of P. philander should not be taxonomically separated, and it is argued that it would be premature to give the Kuruman population an elevated taxonomic status.
  • Full Text:
  • Date Issued: 1975

A contribution to the biology of Pseudodiaptomus hessei (Mrázek) (Copepoda : Calanoida) in Lake Sibaya, South Africa

  • Authors: Hart, Robert Clynton
  • Date: 1974
  • Subjects: Copepoda -- South Africa Freshwater biology -- South Africa
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5854 , http://hdl.handle.net/10962/d1012210
  • Description: Aspects of the biology of the calanoid copepod Pseudodiaptomus hessei are described, with particular reference to its vertical migratory behaviour. The present investigations were carried out largely by means of Nansen-type plankton nets, but several new pieces of apparatus were developed and. are described herein. The daytime vertical distribution of P. hessei varies according to developmental stage and depth of water. In shallow areas of the lake the entire population is benthic or in very close association with the lake bed. In the deepest part of the lake (40m) the naupliar and early (i.e. C I - C III) copepodite stages are essentially pelagic, but the adult and late (i.e. C IV - C V) copepodite stages are predominantly benthic and may be quiescent or infaunal. During the hours of darkness, the calanoids are distributed through the water column. The nauplii are consistently abundant in the surface waters but the distribution of the other stages is not regular. The distribution can be related to lunar intensity in many cases, with the post-naupliar stages frequently occurring deeper in the water column on bright moonlight nights and in the surface waters on overcast moolnless nights. The dusk ascent and dawn descent of the calenoids is clearly related to changes in light penetration in most individuals. The movements of a fraction of the adults occur in the apparent absence of adequate light cues. This behaviour is shown more extensively by the adult females. An endogenous activity rhythm has been shown in the species under laboratory conditions and it is suggested that this may play a part in the migratory movements. A basic examination of the feeding methods, feeding appendages and food sources of adult and late copepodite stages has been made. Changes in feeding intensity through twenty-four have been examined in the field and under laboratory conditions. Using as an index of feeding intensity, the proportion of animals with food in their guts, it has been shown that a pronounced diel difference occurs in adult calanoids in the lake. Feeding is almost entirely restricted to the nocturnal presence of the calanoids in the water coloumn. This difference exists in the pre-adult copepodite stages, but is not nearly as striking. The absence of feeding during daylight is not readily accounted for in terms of food availability and it is attributed to the quiescent or possible infaunal existence of the adults. A periodicity in egg hatching has been shown Naupliar release from the parental egg sac is predominantly a nocturnal phenomenon in the hot and cool seasons. It is suggested that this may be important in attaining a favourable vertical distritution for the nauplii, and may be important in the distribution of the species. A preliminary acccunt of the seasonal cycle and population dynamics of P. hessei is given, based on data collected over two years at a single station. Seasonal changes in calanoid abundance are intermediate between those recorded in truly tropical areas and in temperate latitudes. Potential food sources show relatively little change. The vertical migration of P. hessei is considered in relation to its apparent ecological significance and comparisons are drawn with observations made on the same species or other pseudodiaptomids in estuaries and lagoons elsewhere.
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  • Date Issued: 1974

A contribution to the limnology of Swartvlei: the effect of physico-chemical factors upon primary and secondary production in the pelagic zone

  • Authors: Robarts, R D (Richard D)
  • Date: 1974
  • Subjects: Limnology -- South Africa -- Swartvlei
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5822 , http://hdl.handle.net/10962/d1007474
  • Description: From Resumé: The effect of physico-chemical factors on the biology of the upper reaches of Swartvlei has been investigated during 1971-1972. Physico-chemical data have shown that Swartvlei was characterized by an extremely labile ectogenic meromixis. This instability was due to three factors : (1) the inflow of freshwater, (2) the inflow of sea water, and (3) wind stress. The magnitude of the effects of these factors upon the physics, chemistry and biology of the upper reaches was dependent upon whether or not the estuary mouth was open or closed. The phytoplankton of the pelagic zone of Swartvlei was dominated by nannoplankton. Three major categories were recorded: dinoflagellates, flagellates and diatoms. The major factor regulating their productivity in the upper reaches was light. As a result of humate staining and suspended detrital matter light conditions in Swartvlei were comparable to those in extremely eutrophic northern hemisphere lakes. A maximum integral primary productivity of 39.66 mg C m⁻² h⁻¹ was recorded in November 1972. The aerobic heterotrophic bacterial population in Swartvlei was usually less than 300 col. ml⁻¹ (plate counts). The activity of the total microbial population was measured with ¹⁴C techniques. Uptake of acetate was dominated by flagellates and one species of dinoflagellate when they were present. Glucose uptake was due to bacteria as was acetate uptake in the absence of heterotrophic phytoplankton. Glucose uptake was usually greatest in the anaerobic zone. This, and the presence of H₂S, suggested that a large active photosynthetic and chemosynthetic bacterial population may have been present in the monimolimnion. The possible importance of these bacterial processes in the total productivity of the pelagic zone of Swartvlei was discussed. Daytime zooplankton population size was statistically correlated with the size of the flagellate population. The zooplankton was dominated by Acartia and Halicyclops. These two animals were found in the anaerobic bottom of Swartvlei after October 1971. This corresponded to the disappearance of the flagellate population from the water column. The dominance of these animals in the zooplankton population may have been related to their ability to live in anaerobic water where the bacterial population appeared to be considerably more abundant than in the aerobic zone. Further implications of the results are discussed in reference to phytoplankton cell size and heterotrophy indicating a possible adaptive significance of these factors in Swartvlei.
  • Full Text:
  • Date Issued: 1974

The ecology of juvenile Rhabdosargus holubi (Steinachner) (Teleostei : Sparidae)

  • Authors: Blaber, Stephen J M
  • Date: 1974
  • Subjects: Osteichthyes Fishes -- Breeding Fish populations Fish culture -- South Africa Estuaries -- South Africa -- Eastern Cape Aquatic ecology
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5672 , http://hdl.handle.net/10962/d1005357
  • Description: INTRODUCTION: Estuaries have received considerable attention from biologists in southern Africa in the past thirty years. Professor J.H. Day and the Zoology Department of the University of Cape Town have undertaken ecological surveys of a large number of estuaries of a variety of types, laying the important groundwork for more detailed studies of single species or problems. Their studies showed that the fish populations of southern African estuaries consist mainly of marine species. Why and when these species enter or leave the estuaries and whether they grow or breed in them were not established. The only relatively detailed study of a fish in southern African estuaries was that of Talbot (1955) who investigated aspects of the growth, feeding and reproduction of Rhabdosargus globiceps (Cuvier). No quantitative information has been published on any of the fish species, perhaps due to the difficulty of obtaining such data; results such as population estimates, mortality rates and growth rates are particularly unreliable when the area being investigated is in direct contact with the sea. Studies relating to even a single species of fish are hampered by immigration and emigration between estuary and sea, which make the population continuously variable. The larger estuaries are also difficult to sample adequately, especially with regard to netting, except perhaps on a very large scale using expensive commercial equipment. Along the southern African coast many of the smaller estuaries with a limited catchment area are cut off from the sea for most of the year due to erratic rainfall and longshore drift of sand. These closed or 'blind' estuaries of south eastern Africa, of which there are at least thirty in the eastern Cape Province, provide excellent areas for studying estuarine fish populations which enter when the estuary is open to the sea, but become isolated once it closes. These fish which become cut off in closed estuaries are subject to a new series of conditions. They are exposed to the wider temperature and salinity fluctuations of an estuary and they are forced to utilise the food resources of the estuary. Additionally they may be subjected to predation from piscivorous birds, and those predatory fish which are also cut off in the estuary. These factors will affect the size, mortality, and growth rates of a fish population. Since the fish are isolated no recruitment from the sea can take place, and any increase in numbers would have to come from breeding within the estuary. It is inevitable that the most numerous species should receive attention first. Rhabdosargus holubi (Steindachner) (Synonomy : Sargus holubi Stndr, Austrosparus tricuspidens Smith, Rhabdosargus tricuspidens (Smith))(Plate 1) is one of the most abundant fish in the estuaries of the eastern Cape Province. According to Smith (1965) it is endemic to southern Africa, occurring from the Cape to Zululand, being most common between Mossel Bay and East London. It is largely replaced by Rhabdosargus sarba (Forskal) in the north of its range and by Rhabdosargus globiceps (Cuvier) in the south. Commonly called the 'flatty' or silver bream it is considered only as being of nuisance value by fishermen, seldom exceeding 30 cm in length. In this study the ecology of R.holubi was examined in relation to the closed West Kleinemond estuary which is a small 'blind' estuary typical of those found along the eastern Cape coast. The growth, mortality and population size of R.holubi in this estuary were investigated and linked where possible with predation and detailed studies into the tolerances, food, and feeding of the species. It was not known at the commencement of the study whether breeding could occur in estuaries, although Smith (1965) stated that R.holubi does breed in estuaries. During the course of the sampling programme in the West Kleinemond estuary, measurements of the growth and population structure of two other species of fish were recorded for comparison with R.holubi: the sparid Lithognathus lithognathus (Cuvier) (White Steenbras) and the piscivorous carangid Hypacanthas amia (L.) (Leervis or garrick). Laboratory studies on R.holubi were possible due to the fact that large numbers of this species could be caught in eastern Cape estuaries and transported to Grahamstown, where they could be maintained in captivity for up to four months in recirculating seawater aquaria. During the course of the study over 2000 fish were kept in captivity at different times for a variety of experiments. Another 2500 were preserved for gut analyses and lipid determinations, while over 5000 were captured, examined and released in the field.
  • Full Text:
  • Date Issued: 1974

A contribution to the biology of Tilapia mossambica Peters in Lake Sibaya, South Africa

  • Authors: Bruton, Michael N
  • Date: 1973
  • Subjects: Cichlids , Tilapia , Mozambique tilapia
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5839 , http://hdl.handle.net/10962/d1009950 , Cichlids , Tilapia , Mozambique tilapia
  • Description: An account is given of some aspects of the biology of Tilapia mossambica Peters in Lake Sibaya, South Africa. Previous work on Tilapia in Lake Sibaya is reviewed. Apart from brief gillnet surveys, which recorded the species' presence, no research on T. mossambica had been performed at Lake Sibaya before the two-year study of Minshull who collected data on food preferences, depth distribution of juveniles and breeding biology of adults. The main physiographical features of the lake are outlined. Lake Sibaya is a warm shallow coastal lake with extensive shallow terraces in the littoral which shelve abruptly into underwater valleys. The substrate is predominantly sandy. Aquatic macrophytes are generally restricted to water 1-7 m deep. Adult fishes (over 8 cm SL) are usually absent from water deeper than 12 m and shallower than 0.5 m, whereas juveniles may occur at all depths, and fry only in very shallow water. T. mossambica inhabits the littoral in the warm and transition periods (August to April) but moves into deep water in the cool season (May to July). Exposed and sheltered shallow areas are utilised for different purposes by adult fishes, the former for nesting, and the latter for feeding and mouth-brooding. Habitat selection by males was governed by nest site selection. Habitat Nests were most common in sheltered, sparsely vegetated littoral and sublittoral areas, but also present in well-vegetated sheltered areas. Breeding females preferred sheltered littoral areas but ventured onto the terrace to release the young. Juvenile and fry T. mossambica inhabited shallow exposed shores with a temperature gradient which reversed diurnally. The breeding, shoaling and feeding behaviour of T. mossambica is described, and integrated with data on T. mossambica from other systems. The breeding season spans seven months. Shoaling takes place in shallow water probably as a means of protection. T. mossambica is an omnivorous feeder relying largely on diatoms. The main predator is probably the barbel Clarias gariepinus, but avian predators may be more important. A method whereby the time of formation of rings on the scales of T. mossambica is described. The fishes were found to reach maturity after one year at a length of about 8 cm in females, and after two years at 10 cm in males. The breeding population had a standard length mode of 14 cm (females) and 17 cm (males). The maximum final size was about 24 cm SL. An estimate of the standing crop for fishes in the littoral and subiittoral areas of the eastern and southern shores of the south basin is given. The data were derived from a mark and recapture The biology of T. mossambica in Lake Sibaya as revealed by the present study is discussed with reference to data on the same and similar species in other systems. The utilis ation of the available resources in the lake by T. mossambica is commented upon, and reference is made to the significance of stunting, and the import ance of the retention of generalised characters for the successful habitation of the cyclically-renewed habitat of the littoral. Precocious breeding in T. mossambica is regarded as a functional adaptation which increases the proportion of fishes small enough to utilise the rich food resources in shallow water.
  • Full Text:
  • Date Issued: 1973

A study of the ecology, behaviour and systematics of Tockus Hornbills

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