http://vital.seals.ac.za:8080/vital/access/manager/Index en-us 5 http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4433 Wed 12 May 2021 23:33:07 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:1233 Wed 12 May 2021 20:12:32 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4186 Wed 12 May 2021 19:57:08 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4366 Wed 12 May 2021 19:11:57 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4999 Wed 12 May 2021 18:37:11 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:2188 Wed 12 May 2021 18:31:32 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4824 80%) of the bed material transport. Only large floods (termed 'reset'discharges) with average return periods of around 20 years generate sufficient stream power and shear stress to mobilise the entire bed. The macro-channel is thus maintained by the large ' reset' flood events, and the active channel is maintained both by the range of effective discharges and the ' reset 'discharges. These are the geomorphologically 'effective' flows. Results from the Mhlathuze River have indicated that the Goedertrouw Dam has had a considerable impact on the downstream channel morphology and bed material transport capacity and consequently the effective and dominant discharges. It has been suggested that the Mhlathuze River is now adjusting its channel geometry in sympathy with the regulated flow environment. Under present-day conditions it has been demonstrated that the total bed material load has been reduced by up to three times, but there has also been a clear change in the way in which the load has been distributed around the duration curve. Under present-day conditions, over 90% of the total bed material load is transported by the top 5% of the flows, whereas under virgin flow conditions 90% of the total bed material load was transported by the top 20% of the flows. For the Olifants River there appears to be no relationship between the estimated bankfull discharge and any hydrological statistic. The effective discharge flow class is in the 5-0.01% range on the 1-day daily flow duration curve. It has also been pointed out that even the highest flows simulated for the Olifants River do not generate sufficient energy to mobilise the entire bed. It is useful to consider the Olifants River as being adapted to a highly variable flow regime. It is erroneous to think of one ' effective' discharge, but rather a range of effective discharges are of significance. It has been argued that strong bed rock control and a highly variable flow regime in many southern African rivers accounts for the channel architecture, and that there is a need to develop an ' indigenous knowledge' in the management of southern African fluvial systems]]> Wed 12 May 2021 18:29:47 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4243 Wed 12 May 2021 17:46:07 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4308 Wed 12 May 2021 17:34:45 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:3050 Wed 12 May 2021 17:30:46 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:1698 Wed 12 May 2021 17:10:17 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4556 Wed 12 May 2021 16:21:23 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:4424 Wed 12 May 2021 16:06:45 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:5670 Thu 13 May 2021 14:21:17 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:5282 50%) comes from Lake Malawi. Bathyclarias nyasensis and other clariid catfish contribute up to > 20% of the total catches. Catches of Bathyclarias nyasensis in the inshore area of the south-east arm of Lake Malawi are declining and a management plan for the fishery is essentially lacking. There is paucity of biological data that precludes the use of any option to manage the species. The principal aim of the thesis was to define the ecological role B. nyasensis, the most abundant and common of the Bathyclarias species. By examining life history characteristics within a food web context, it was hypothesized that the study would provide an insight into the interrelationships between species, and, hence form the basis for the development of a rational exploitation strategy for the species. The study was undertaken in the south-east arm of Lake Malawi (9° 30'S, 14° 30'S). The principal objectives of the study were to investigate the feeding ecology of B. nyasensis by examining morphological characters and structures associated with feeding, diet of B. nyasensis, food assimilated in the species using carbon (∂¹³C) isotope analysis, daily food consumption rate for B. nyasensis; and to relate the feeding ecology to life history traits such as age, growth, and some aspects of the reproductive biology of B. nyasensis. The suitability of sectioned pectoral spines and sagittal otoliths to age B. nyasensis was assessed. Due to reabsorption of growth zones with increasing spine lumen diameter with fish size, and the relatively low number of spines that could be aged reliably, only otoliths were used. The maximum age for B. nyasensis was estimated at 14 vears. Growth was best was described by the four parameter Schnute mc: lt ={42+(81¹·⁸ - 42¹·⁸)x1-e⁻°·°⁵⁽t⁻¹⁾}¹/¹·⁸ over 1-e⁻⁽⁻°·°⁵⁾⁽¹¹⁾ for female, lt={41+(98¹·² - 41¹·²)x 1-e⁻°·°²⁽t⁻¹⁾}¹/¹·² over 1-e⁻⁽⁻°·°²⁾⁽¹³⁾ and for male fish. Age-at-50% maturity for females and males were estimated at 7 years and 4 years, respectively. Typically, fish grew rapidly in the first year, but slower during subsequent years. Smaller fish were found inshore while larger fish were found in offshore regions. It was hypothesised that the rapid growth in the first year and slower growth later is a consequence of change in diet from high quality and abundant food source to a more dilute food and that this may be associated with a shift in habitat. Morphological characters associated with feeding were used to predict the food and feeding behaviour of B. nyasensis. The size of premaxillary, vomerine, pharyngeal dental and palatine teeth and premaxillary and vomerine tooth plates suggested the capability of B. nyasensis to handle both large and small prey, with a propensity towards smaller prey in composition to C. gariepinus. The molariform teeth on the vomerine tooth plate suggested that molluscs form part of the diet. The relative gut length (1.27±0.24) suggested omnivory, with an ability to switch between planktivory and piscivory. Buccal cavity volume and filtering area changed with fish size at 500-600 mm TL upon which it was hypothesised that the fish diet changed to planktivory at this size. Detailed diet analysis provided information upon which the above hypotheses could be accepted. Percent Index of Relative Importance (%IRI) and a multi-way contingency table analysis based on log-linear models were used to analyse diet data. Results showed that B. nyasensis is omnivorous, but with a distinct ontogenetic dietary shift from piscivory to zooplanktivory at 500 - 600 mm TL. The increased buccal cavity volume at the same fish size therefore, suggests that B. nyasensis is well adapted to filter the dilute zooplankton resource. Increased foraging costs of feeding on zooplankton explained the slower growth of larger fish. The dietary shift was finally corroborated by results of the ∂¹³C isotope analysis. A polynomial equation described the change in carbon ratios with fish size: ∂¹³C = - 33.188 + 0.4997L - 0.0045 (total length)² (r² = 0.598, n = 12, p=0.022). The ontogenetic shift in diet was synchronised with a habitat shift postulated in life history studies. In the inshore region, B. nyasensis were predominantly piscivorous (apex predators), and were zooplanktivorous in the offshore region, thereby forming part of the pelagic food web in the latter region. After examining "bottom-up" and trophic cascade theories, it was postulated that perturbations of the B. nyasensis stock would be discernible both at the top and lower trophic levels. As a piscivore and therefore apex predator, effects of overfishing B. nyasensis in the inshore region could cascade to unpredictable ecological changes in inshore areas and, due to the ontogenetic habitat shift, in the offshore regions. Examples of trophic cascade phenomena are provided. On the basis of the feeding study, it was possible to reconstruct the pelagic food web of Lake Malawi. Apart from the lakefly Chaoborus edulis, B. nyasensis is the other predator that preys heavily on zooplankton in the pelagic zone. Perturbations of the B. nyasensis stock could affect size composition of zooplankton which in tum, could affect production of C. edulis, a resource for the top predators in the food web. The findings of the present study contributed to the ongoing debate of introducing a zooplanktivore into the pelagic zone of Lake Malawi. Proponents for the introductions have argued that zooplankton predation by fish is inferior to that of C. edulis. Introduction of a clupeid zooplankton was proposed as a strategy to boost fish production in the lake. The zooplanktivore would either out-compete or prey on C. edulis to extinction. Opponents to this view argued that zooplankton biomass in the pelagic region was too low to support introductions and that the fish biomass in the pelagic region may have been underestimated. Results from the present study suggest that planktivorous fish (including B. nyasensis) might not be inferior to C. edulis in utilising the zooplankton resource; B. nyasensis is well adapted to utilise the dilute zooplankton resource, and by omitting B. nyasensis from previous studies, overall zooplankton predation by fish may have been underestimated by between 7 - 33%. On the basis of the theoretical migratory life history cycle of B. nyasensis, it is recommended that the current interest in increasing fishing effort in offshore areas should proceed with caution. Ecological changes that may have occurred in the inshore areas due to overfishing have probably not been noticed: as the offshore zone has never been fished. The latter zone may have acted as a stock refuge area. Higher fishing intensity in the offshore areas could lead to serious ecological imbalances and instability. The study has shown that life history characteristics studied in the context of the food web, and in the absence of other fisheries information and/or data, strongly advocates the precautionary principle to managing changes in exploitation patterns.]]> Thu 13 May 2021 12:45:47 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:5676 Thu 13 May 2021 07:14:39 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:1866 Thu 13 May 2021 06:48:29 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:2956 Thu 13 May 2021 05:59:13 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:5730 Thu 13 May 2021 05:52:49 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:10814 Thu 13 May 2021 05:44:38 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:1579 Thu 13 May 2021 05:30:49 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:1557 Thu 13 May 2021 03:49:30 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:2563 Thu 13 May 2021 03:16:42 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:21065 Thu 13 May 2021 03:03:44 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:2766 Thu 13 May 2021 02:40:21 SAST ]]> http://vital.seals.ac.za:8080/vital/access/manager/Repository/vital:5347 Thu 13 May 2021 01:31:33 SAST ]]>