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- ItemEffect of salinity and environmental changes on the fish community of the St Lucia Estuarine System with focus on the salinity tolerance of Oreochromis mossambicus (Mozambique Tilapia)(University of Zululand, 2014) Schutte, Quintin; Vivier, L.; Cyrus, D.P.St Lucia, the largest estuarine lake system in Africa, has been closed since 2002, except for a brief opening in 2007 caused by Cyclone Gamede. Mouth opening in 2007 was preceded by five years of low lake levels and hypersaline conditions which greatly reduced marine fish species diversity. Regardless of this, the freshwater tilapia, Oreochromis mossambicus has been found in abundance in all parts of the system. This indicates that O. mossambicus is capable of surviving hypersaline conditions when no other fish species can. Mouth opening in 2007 caused the system to partially fill marine salinities were restored and also allowed for recruitment of marine species back into the system. Following re-closure in August 2007, salinities in North Lake became hypersaline during 2009-2010. In 2012, a “beach channel” was dug to re-connect the St Lucia Narrows to the Mfolozi-Msunduzi Estuary, allowing water to enter the system and provide access to the ocean for recruitment of marine species. The study investigated the effect of salinity and environmental changes on the fish community structure of the St Lucia system during periods of mouth closure as well as after opening of the new beach channels connecting St Lucia to the Mfolozi system.Sampling was conducted from May 2008-Nov 2014 at six sites in the St Lucia system using small (10m) and large (70m) seine nets and CPUE’s were calculated for each site as the number of fish caught per meter of net, per haul. Fish were identified measured and returned to the system. The salinity tolerance of O. mossambicus from St. Lucia was also investigated under laboratory conditions. Acclimation of O. mossambicus was done at salinity intervals of five with a 96 hour exposure trial to test for survival. Spatial changes in the fish community were driven by the salinity gradient across the system, while temporal changes coincided with major changes in the salinity state of the system. Marine species dominated in terms of species number but freshwater and estuarine species dominated the fish abundance, notably O.mossambicus and Ambassis ambassis. Prior to opening of the beach channel, the number of species decreased from 51 in 2008 to 37 in 2011. The abundance of marine species decreased by >75%as the system became progressively more hypersaline and the closed mouth limited recruitment. Opening of the beach channel caused a decline in salinities and allowed for 12 marine species top re-enter the system in limited numbers. Oreochromis mossambicus was the most abundant species in the system, accounting for 48.8% of the catch between 200-2014 and dominating the fish assemblage in salinities between 30-90 (highest recorded salinity). Estuarine species were dominant in salinities between 0-20, while marine species dominated the fish assemblage in salinities between 20-30. In the laboratory, the salinity tolerance of O.mossambicus was found to be 0-105, with >70% survival in 105. Freshwater deprivation and mouth closure had the biggest impact on the fish assemblage and a regular more extensive connection to the ocean is therefore, needed to maintain healthy fish populations in Lake St Lucia and off-shore marine populations.
- ItemEffect of dietary protein level, feeding frequency and amount of food offered on growth and gastric evacuation of Oreochromis mossambicus fry(University of Zululand, 2012) Luthada, Rendani Winnie; Jerling, H.L.Excess protein in fish diet may be wasteful and unnecessarily expensive. Furthermore, when fish are fed insufficiently or excessively, their growth or feed efficiency may decrease, resulting in increasing production costs and water quality deterioration. Therefore this study was conducted to determine the optimum dietary protein level, feeding frequency and feeding rate on growth, gastric and intestinal evacuation of O. mossambicus fry under hatchery conditions, in order to reduce the production costs while optimizing growth rate. Diets contained 20%, 30%, 35%, 40% and 45% protein levels, feeding frequency of once, twice, thrice, four times and five times per day and feeding rates of 10%, 15%, 20%, 25% and 30% body weight per day were tested in separated experiments conducted in a temperature and light cycle controlled environmental room. The results indicated that both dietary protein level and feeding frequency had a significant effect on weight gain, specific growth rate and gross food conversion ratio (ANOVA, P<0.05) but not survival rate (ANOVA, P>0.05). Feeding rate had a significant effect on weight gain and gross food conversion ratio (ANOVA, P<0.05) but not on specific growth rate and survival rate (ANOVA, P>0.05). A diet containing 30% protein level, feeding frequency of four times per day and 15% of the fry body weight per day were the optimal levels obtained from the growth experiments. Dietary protein level had a significant effect on gastric and intestinal evacuation (ANOVA, P<0.05); feeding frequency had a significant effect on intestinal evacuation rate and time (ANOVA, p<0.05) only but not on gastric evacuation rate and time (ANOVA, P>0.05), while feeding rate had no significant effect on both gastric and intestinal evacuation rate and time (ANOVA, P>0.05). The optimum levels obtained in gastric and intestinal evacuation are 40% dietary protein level, feeding frequency of twice per day and a feeding rate of 15% body weight per day.
- ItemSalinity tolerance and osmoregulation in several subtropical decapods(University of Zululand, 2012) Khanyile, Sithembele Nikeziwe; Newman, B.K.This study investigated salinity tolerances and osmoregulatory strategies of several subtropical brachyuran mangrove crabs and an anomuran prawn, with particular reference to Uca vocans, Uca urvillei, Uca chlorophthalmus, Uca annulipes, Dotilla fenestrata, Macrophthalmus depressus, Macrophthalmus grandidieri, Metopograpsus thukuhar, Chiromantes eulimene and Callianassa kraussi. All species investigated were either directly exposed or acclimated to salinities between 0-75 and their tolerance to these salinities and osmoregulatory strategies monitored over a 4 day period. Other experiments conducted included an investigation of the time dependant responses of species following direct transfer to various salinities, and for one species also the influence of temperature on salinity tolerance and osmoregulatory strategy. All the species were shown to be euryhaline, as would be expected for species inhabiting an estuarine environment. However, the degree of euryhalinity varied between species. The general salinity range they could tolerate was between 0-55, but species like U. annulipes, D. fenestrata, C. eulimene and C. kraussi tolerated salinity as high as 65. Direct exposure was shown to be more stressful than acclimation, especially in low and high salinities. Out of seven species that were directly exposed and acclimated, C. eulimene was the only species able to tolerate freshwater (salinity 0) following direct exposure. All crab species followed an osmoregulation strategy by hyper-regulating at low salinities and hypo-regulating at higher salinities. The hyper-regulatory ability of most species was stronger than the hypo-regulatory ability, as this was shown by the hemolymph osmolality line which was much closer to the isosmotic line at salinities above the isosmotic point and also by the lower osmotic capacity (OC) at comparable salinity differences below and above the isosmotic point. Callianass kraussi osmoregulated at salinities lower than 25 and osmoconformed at salinities above 25. All Uca species investigated were able to tolerate direct transfer to freshwater for up to eight hours without experiencing any mortality. All specimens of U. vocans, which occurs lowest in the intertidal zone, died within 24 hours of exposure. All U. urvillei died within two days of exposure. Uca annulipes, which lives in the highest region of the intertidal zone, was the most tolerant to rapid freshwater exposure, with 70% of crabs surviving up to 72 hours. Uca annulipes regulated its hemolymph osmolality more efficiently than U. urvillei and U. vocans, which live on the lower level of the intertidal zone. The ability of Uca crabs to survive as well as regulate their hemolymph osmolality when directly transferred to freshwater was closely linked to the level they occupy in the intertidal zone. The third part of this study looked at the influence of temperature on the salinity tolerance and osmoregulation of C. eulimene. Temperatures between 14-22oC had no effect on salinity tolerance or osmoregulatory capability of C. eulimene at salinities between 0-45. Exposure of C. eulimene to lowered temperatures had no effect on the salinity tolerance and osmoregulation capacity of this species. Lower temperatures do not inhibit the distribution of this species from South and West Coast of Africa.
- ItemThe Mzingazi gauging weir and its effects on the fish and macrocrustacean communities of Lake Mzingazi(University of Zululand, 2012) Moloi, Puleng Cathrine; Vivier, L.; Cyrus, D.P.Lake Mzingazi is a freshwater coastal lake in Richards Bay, KwaZulu-Natal. Because of ongoing industrial and residential developments in the Richards Bay area, the need for potable water increased and led to the construction of a weir across the outlet of Lake Mzingazi. The first weir on this historically estuarine lake was built in 1955, after which it was raised several times to satisfy the increasing demand for water. Due to potential salt-water intrusion via groundwater from the saline Mzingazi River into the lake, another weir, namely the Saltwater Barrier, was built 1.4 km downstream of Lake Mzingazi to limit saltwater intrusion into the lake. The Mzingazi weir largely prevented the migration of fish and crustaceans between the marine/estuarine environment and the freshwater environment. In 2005-2006, the DWA designed and constructed a new and improved gauging crump weir across the lake outlet, which included a pool and weir fishway to allow the migration of fauna in and out of the lake. The fishway was aimed at allowing euryhaline fish species to migrate out to sea to spawn and back to the lake to complete their life cycle since Lake Mzingazi historically served as an important nursery habitat for juveniles of a number of euryhaline fish species. Monitoring of the fishway was required to assess the efficiency of the design, and if necessary, to modify flow conditions to meet the requirements of migrating fauna. This study aimed firstly at monitoring the operation and efficiency of the Mzingazi fishway and secondly, at monitoring its effect on the fish community of Lake Mzingazi. In order to assess the operation and efficiency of the Mzingazi fishway, the fish and macroinvertebrate fauna were sampled at six locations in the Mzingazi system, i.e at the Saltwater Barrier, in the upper Mzingazi River, at the entrance to the fishway, in the fishway pools, at the fishway exit and in the lake immediately above the fishway. The fishway was sampled over two days on a monthly basis over the period August 2007 to July 2008, using funnel traps, dip nets and a small seine net. In order to monitor the effect of the fishway on the lake fish community, seasonal sampling in the lake was conducted for a period of two years, from August 2007 to June 2009, using seine and gill nets. A total of 3 288 fish, representing 29 species, were recorded throughout the study. Of these 29 species, 20 species were able to locate and enter the fishway. The fishway was found to allow passage for 17 fish species between the estuary and the lake. The dominant species that managed successfully to migrate through the fishway were Myxus capensis, Glossogobius callidus and Eleotris fusca, making up 77% of the total catch. The fish migrating through the fishway included five marine species, five estuarine resident species, six freshwater species and one unclassified Gobiid larvae sp. Abstract The Mzingazi gauging weir and its effects on the fish and macrocrustacean communities of Lake Mzingazi ii A total of 8 188 macrocrustacea representing eleven species were also recorded in the Mzingazi system, of which seven prawn and one crab species were able to locate and enter the fishway. The fishway provided migratory passage for seven prawn species, these being dominated by Macrobrachium equidens, Caridina nilotica and Caridina indistincta respectively, together making up 97.6% to the total catch. This study proved that the fishway was effective and efficient for upstream migration of the target species. The design of the fishway was found to create suitable hydraulic conditions for migration of juvenile and sub-adult fish (10 to < 100 mm SL) and for macrocrustacea (3 to < 26 mm CL). Peak upstream migration of fish through the fishway occurred in August, September, October and December, which coincides with the peak recruitment period of most estuarine and marine spawning species. Peak migration and species abundance of both fish and macrocrustaceans were found to be a natural effect of seasonal recruitment and breeding, which was indirectly driven by seasonal temperature variations. The fish community of Lake Mzingazi during the study comprised 16 species, which included four marine species, five estuarine resident species and seven freshwater species. The community was dominated by freshwater species such as Oreochromis mossambicus and Tilapia rendalli and estuarine species such as Glossogobius callidus and Gilchristella aestuaria. The operation of the Mzingazi fishway was found to be causing a gradual change in the lake fish community by re-introducing euryhaline fish species into the freshwater coastal lake. The following important recommendations were made. Firstly, further long-term monitoring of the fishway and the lake fish community is required to validate the absence of previously recorded euryhaline species and to determine the long-term effect of the fishway on the fish and macrocrustacean community. Secondly, the operation of the fishway is entirely dependent on water levels in the lake and ongoing water abstraction for domestic and industrial purposes often results in the water level in the lake being drawn below the overflow level of 3.0 masml. For this reason, the Department of Water Affairs should limit abstraction from the lake so that there is sufficient water allocated to allow the fishway to operate. The water level should never be drawn below the minimum level of 3.0 mamsl, at which the fishway can still operate.
- ItemThe diet of selected benthic feeding fish in the Mfolozi-Msunduzi system, KwaZulu-Natal(University of Zululand, 2011) Nhleko, Jabulile Blessed Busisiwe; Cyrus, D.P.; Vivier, L.The St Lucia estuarine system is considered to be the largest nursery area for estuarine-associated marine fish in southern Africa. The prolonged mouth closure due to regional droughts has resulted in large declines in its fish and prawn communities with deleterious consequences for regional marine populations, especially those that require estuaries for completion of their life cycle. Mouth closure of the St Lucia system contributed to a renewed interest in the St Lucia-Mfolozi System link, particularly the importance of the Mfolozi system in terms of its role as a refuge and alternative nursery area for species which cannot recruit into the closed St Lucia system when the Mfolozi remains open. The Mfolozi-Msunduzi estuarine system is the closest open estuary to St Lucia but there is a paucity of data on the ecological functioning of the system. Studies on the benthos of the Mfolozi-Msunduzi estuarine system have found it to be impoverished but large numbers of benthic feeding fish are present in the system at times. This raised the question: if the zoobenthic community is impoverished and high numbers of benthic feeding fish are nevertheless present in the system, then what are these fish feeding on? The stomach contents of 961 fish of 15 different species collected between August 2008 and March 2010 using seine and gill nets were examined. Of these 15 species, only six were caught in sufficient numbers for gut analysis. The six species that were used in the analysis were Leiognathus equula, Acanthopagrus berda, Glossogobius giurus, Ambassis ambassis, Ambassis gymnocephalus and Ambassis natalensis. Four methods were used for stomach content analysis, namely Frequency of occurrence, Numerical occurrence, Points (Proportional Volumetric) analysis and Index of Relative Importance (IRI). It was found that the diet of L. equula was dominated by the copepod, P. stuhlmanni, with the macrobenthos contributing <0.5% to the overall diet of the species. Similarly, the diet of A. berda was dominated by P. stuhlmanni, with the benthic fauna contributing <1.5% to the diet. The dominant prey in the diet of G. giurus was Glossogobius sp, while the only benthic species that was fed on was the crab P. blephariskios, which contributed not more than 0.3% to the overall diet of the species. A large proportion of the Ambassids (A. ambassis, A. gymnocephalus and A. natalensis) had empty stomachs, whilst most of those that contained food had stomachs < 30% full, based on the Points determination method. The diet of all three Ambassid species was dominated by P. stuhlmanni, with A. gymnocephalus feeding only on the copepod. The total contribution of the benthic fauna found in the diet of A. ambassis was <30% and <1% in A. natalensis. Copepods dominated the diets of the majority of the species examined, indicating that in the Mfolozi system, zooplankton rather than benthic organisms are being fed on. This tends to tie in with the impoverished state of the benthic fauna. The results of this study indicate that only those species capable of adapting their diets to whatever is available and abundant to feed on will be able to survive in the Mfolozi-Msunduzi system.
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