Brian R. Allanson

Unique qualities and special problems of the African Great Lakes

SynopsisThe African Great Lakes consist of large, deep rift valley lakes (e.g. Malawi & Tanganyika) and shallower lakes between the Eastern and Western Rifts (e.g. Victoria). They are a group comparable in size to the North American Great Lakes, but are old. Most are seasonally thermally stratified, and wind is the decisive factor that determines the annual cycle of cooling and mixing. Lakes Tanganyika, Malawi and Kivu are meromictic, with deep relict hypolimnia. Large magnitudes and time scales of periodic internal motion, where these have been measured, appear unique among lakes. These lakes harbour the worlds richest lacustrine fish faunas, and the family Cichlidae provides the supreme example of geographically circumscribed vertebrate evolution. The lakes provide a unique comparative series of natural laboratories for evolutionary studies. Primary production is generally high, but in the deeper lakes standing stocks of plankton and of small fish species are low. These pelagic populations are characterised by very high P:B ratios. The fisheries are productive and of socio-economic importance. Large-scale mechanised fishing is not compatible with the survival of the diverse fish communities. Cichlids appear especially vulnerable to unselective fishing. Aquatic reserves might offer a means of survival for at least some communities. Various pollution threats exist. Because water retention times are long, extremely long for some deep lakes, and flushing rates are low, the lakes are vulnerable to pollution which would be long-lasting. Introductions of alien fishes have mostly had undesirable or disastrous results. While the faunas are one of the significant natural heritages of mankind, their conservation must realistically be linked to the legitimate development of the lakes for the well-being of the people who live there. Scientific value alone will not protect the lakes. Just as survival of African terrestrial wildlife in extensive reserves depends heavily upon tourism, so also might the cichlid flocks in underwater reserves. Greater interest from the international scientific community is needed to further rational development and conservation of these great lakes.

Behavioral and trophic plasticity of juvenileTilapia mossambica in utilization of the unstable littoral habitat

SynopsisJuvenileTilapia mossambica Peters (2.5–10.0 cm standard length) in Lake Sibaya, South Africa, move daily from deep offshore waters to shallow (<0.5 m) littoral areas where they feed for several hours and then return to deep water. The timing of these movements varies in response to changing physical and biological features of the littoral environment. During this study (December 1973 – February 1976), lake level rose more than 1 m. At the start of the study, littoral areas visited by juvenileT. mossambica were free of vegetation and debris, but at higher lake levels trees and brush from the eroding shoreline were common in nearshore littoral waters. When the littoral zone was free of debris, juvenileT. mossambica visited nearshore waters only during daylight hours. After a 0.65 m rise in lake level and considerable accumulation of debris, these fish were abundant in the littoral zone only at night. This reversal can be attributed to efforts by juvenileT. mossambica to avoid their principal predator, the catfishClarias gariepinus. At low lake levelC. gariepinus ventured into the littoral only at night but at higher lake levels this predator was found within flooded vegetation during the day. When further increases in lake level flooded marginal grasslands, juvenileT. mossambica abandoned the littoral zone in favor of this newly created eulittoral habitat where potential fish predators did not occur. With the change in habitat, the diet changed from benthic detrital aggregate to periphyton. These observations suggest the importance of behavioral and throphic plasticity in the ability of tilapia to utilize unstable habitats.

The exploitation of Upogebia africana (Crustacea: Thalassinidae) for bait in the Knysna Estuary

The activities of people collecting Upogebia africana for bait at six popular collecting sites in the Knysna Estuary were monitored from February 1995 until April 1996. Three groups of bait harvesters were identified on the basis of their source of income: subsistence fishers who rely on bait collecting and fishing for their income; supplementary fishers who catch fish to supplement their income; leisure anglers who draw no income from fishing. Two groups of collectors were identified based on methods of collecting bait and fishing: leisure anglers who collect bait using a prawn pusher or pump and fish using a rod and tackle; non-leisure fishers who collect using tin cans and fish with hand or planted lines. The average harvest of bait per collecting trip by leisure anglers was 59 mud prawns, whereas non-leisure fishers took 101 animals, twice the legal limit. The numbers of bait collectors present per mud bank were found to be highest on public holidays ([xbar] = 43.5) and higher during the summer holidays ([xbar] = 16.5) than during the spring/summer ([xbar] = 8.6) and autumn/winter ([xbar] = 4.6) periods. Most collectors spent 11–30 minutes on the mud banks. It was estimated that 1.86 x 106 U. africana or about 740 kg (dry mass) was removed by bait collectors annually from the six bait-collecting sites studied. This represented about 8.5% of the mud prawn stocks at these sites and about 0.9% of the entire estuary stock. 85% of the mud prawns taken as bait was removed by 77% of the bait collectors who were the non-leisure fishers.

An introduction to the chemistry of the water column of the Knysna Estuary with particular reference to nutrients and suspended solids

The Knysna Estuary is the largest clear-water estuary along the coast of South Africa, with a spring-tide prism volume of 19 x l06 m3 and a tidal range of 1.8 m. The tidal flow covers an area of 1827 hectares at HWST. The tidal system is divided into an upper estuary which receives the variable flow of the Knysna River, a lagoon sector and a large marine embayment. The position and magnitude of the salinity gradient varies according to river flow and, in view of this variability, a linear nutrient concentration-salinity relationship (conservatism) is only weakly established, if at all. Urbanisation of the littoral has encroached upon intertidal wetlands and both stormwater and sewage treatment plant flows introduce nutrients into the water column. While these provide localised inputs, their contribution to the nutrient budget of the system compared with that introduced by the tidal prism is small. The overall effect is to sustain the oligotrophy (chlorophyll-a = 2.16 pg/I) of the water column. Total suspended solids introduced via stormwater and stream inflows decrease transparency during summer, and are considered a serious threat to the long-term survival of the extensive eelgrass meadows upon which so much of the nutrient mobilisation depends. Among the heavy metals, lead and cadmium show elevated concentrations in the soft tissues of oysters, but not as yet to dangerous levels. Polynuclear aromatic hydrocarbons are undetectable in the water column and polychlorinated biphenyls are rarely recorded. It is argued that the magnitude of the tidal prism overrides, at present, the impact of nutrients or toxic materials of anthropogenic origin on the water quality of the estuary.

An estimation of the standing stock and population structure of Upogebia africana (Crustacea : Thalassinidae) in the Knysna Estuary

Within the Knysna Estuary Upogebia africana has an extensive distribution from north of Leisure Isle to the Red Bridge, occupying 62% of the available intertidal zone. Six sites were surveyed within the above distributional range. U. africana was found from the high-water level (Spartina zone) to the shallow subtidal zone (to about 1.5 m depth). The density, biomass and population structure of the mud prawns were found to be highly variable within the estuary. Only small animals (carapace length [xbar] = 10.9 mm) were found in the upper estuary at the Red Bridge. Although large animals were present, the Invertebrate Reserve had a low density and biomass of U. africana ([xbar] = 11.7 m−2; 3.9 g.m−2 dry weight). By contrast a relatively inaccessible centre mud bank (Oyster Bank) in the middle reaches of the estuary had much larger populations ([xbar] = 176.5 m−2; 65.3 g.m−2 dry weight). This site is, therefore, a natural mud prawn reserve. Within the intertidal zone, mud prawn density (74–76 m−2) and biomass (26–27 g.m−2 dry weight) was usually greatest in the Spartina and lower Zostera zones. Sex ratios throughout the estuary did not deviate significantly from 1:1.

Ecology of the sundays river Part I. Water chemistry

SummaryWater analyses were done on samples taken at a series of points along the Sundays River in the Eastern Cape, South Africa. The chemical characteristics of the water were then correlated with the geological and geographical nature of the catchment.ZusammenfassungWasserproben von einer Reihe von Stellen des Sundays Rivers in der Östlichen Kapprovinz, Süd Africa, sind analysiert. Die chemischen Eigenschaften sind bezogen auf der geologischen and geographischen Natur des Sammelgebietes.

THE BENTHOS OF SOME SOUTHERN AFRICAN LAKES PART III: THE BENTHIC FAUNA OF LAKE NHLANGE, KWAZULU, SOUTH AFRICA

Summary Lake Nhlange (KwaZulu, South Africa) is connected to the sea through the Kosi estuary and has a salinity between 36%0 and 5%0. The benthos of the lake was sampled at approximately six-monthly intervals from July 1965 to January 1969. The substrates are mainly sandy in shallow water and also on steeply sloping profiles even in the deepest water of 30 m. Flat areas are mainly silty. In December 1965, following the closure of the estuary mouth and severe rains, the lake level rose to about 2 m above its normal level. Although the salinity changed little (5%0, to 3%0), quantities of peat-stained water and detritus were brought into the lake. A waning pattern of low oxygen conditions below about 12 m in the hot season, followed by reasonably well-oxygenated conditions in the cold season, was found. Particularly severe oxygen depletions were discovered at the mud water interface, even where well-oxygenated conditions pertained in the open water column at equivalent depths. Evidence that the fauna was dr...

Ecology of the Sundays River Part II . Osmoregulation in some Mayfly Nymphs (Ephemeroptera: Baetidae)

Summary1.Nymphs of the mayfly Cloeon crassi can tolerate total dissolved solids concentration up to 10,500 ppm; these results agree with field observation.2.Juvenile nymphs are more sensitive than adults to high concentration of dissolved solids.3.No difference was detected in tolerance experiments on C. crassi between media made up with sea water or media of the same ionic composition as Sundays River water.4.Measurements of haemolymph freezing point depression indicate that C. crassi can both hyper-regulate and hypo-regulate, depending upon the concentration of the media.5.Haemolymph of nymphs of C. africanum in freshwater have the same osmotic concentration as that of C. crassi but no indication of any hyporegulatory ability was found in other media.6.Nymphs of Centroptilum excisum cannot tolerate concentrations of total dissolved solids greater than 7,000 ppm.7.The results are discussed in relation to the distribution of the three species in the Sundays River.Zusammenfassung1.Die Nymphen der Eintagsfliege Cloeon crassi sind imstande eine Lösung von einer 10.500 ppm hohen Konzentration fester Stoffe zu ertragen.2.Kleine Nymphen sind empfindlicher gegen hohe Konzentrationen solcher Lüsungen als gro\e.3.In Toleranzversuchen mit C. crassi wurde kein Unterschied zwischen mit Meerwasser zubereiteten Media and Media von gleicher ionischer Zusammensetzung, wie das Wasser vom Sundays River, festgestellt.4.Messungen der Haemolymph-Gefrierpunkt-Depression zeigen da\ C. crassi je nach der Konzentration des Mediums zu Hyperund Hyporegulierungen fahig ist.5.Haemolymph der Nymphen der Eintagsfliege C. africanum in Sü\wasser ist von gleicher osmotischer Konzentration wie C. crassi; es wurde jedoch keine Andeutung irgendeiner Hyporegulationsfähigkeit in anderen Media gefunden.6.Nymphen der Eintagsfliege Centroptilum excisum sind unfähig eine Lösung von einer Konzentration höher als 7.000 ppm fester Stoffe zu ertragen.7.Diese Ergebnisse sind mit Bezug auf die Verbreitung der drei Arten im Gebiet des Sundays River diskutiert.

The Knysna Basin Project reviewed—research findings and implications for management

The reasons for establishing the Knysna Basin Project (KBP) are given, and a synthesis is provided of those research findings from the investigation array which have immediate management implications. Acknowledgement for the support given to the Project is made.

Efficiency of particle retention in Solen cylindraceus (Hanley) (mollusca: bivalvia)

Abstract Solen cylindraceus retains particles down to 2·5-3 μm with great efficiency. Below this particle size, retention efficiency decreases rapidly and a net production of particles is recorded below 1·5 μm. Particle retention is independent of temperature (15 °C and 25 °C) and salinity (15‰ and 35‰). Filtration rate is shown to vary minimally at the different temperature and salinity combinations tested: 19·4 ml min−1 recorded at 25 °C and 35‰, and 18·2 ml min−1 at 15 °C and 15‰. Despite being a dominant member of the estuarine macrobenthos, it is estimated that the Solen population filters only 3–4% of the tidal volume per tide and is likely to play a minor role in any resource depletion.