Zoology

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Browsing Zoology by Subject "Estuaries -- South Africa -- KwaZulu-Natal"

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    The benthos of the Siyaya estuary : species composition, density and distribution
    (1996) Mackay, Christine Fiona; Cyrus, D.P.
    This study describing the zoobenthos of the Siyaya Estuary is the result of data collection spanning three consecutive years (1992 - 1994). The investigation of the zoobenthic component of the estuary, forms part of a multidisciplinary study to monitor the effects of catchment rehabilitation. The abiotic and biotic characteristics of the estuary have been used as a tool to determine the effectiveness of improved management practices in the catchment. The response of the zoobenthos was first examined in 1983, and this showed 37 zoobenthic taxa with a strong marine/estuarine component. From 1992 to 1994, the zoobenthos was sampled on a seasonal basis at each of five sites along the length of the estuary. A total of 88 taxa representing four phyla were collected over three years. During 1992, 50 taxa were recorded in samples and the impression was of a wide faunal assemblage, with representatives from Nematoda, Annelida, Crustacea, Insecta, and Gastropoda. Five less taxa were recorded during 1993, although the faunal assemblage was still fairly wide with the inclusion of several more insect taxa. The greatest number of taxa were recorded during 1994. Of the 59 zoobenthic taxa, 40 represented the freshwater component. Over the three year period, few decapod invertebrates were sampled, the majority were primarily post prawn larvae. Results of this study have therefore shown that the number of taxa have increased. However, the majority of new taxa added to the total species list belong to the freshwater component of the zoobenthos. This is due to prolonged closure of the estuary mouth as a result of the prevailing drought conditions. A comparison of zoobenthic densities from the estuarine and freshwater components revealed that the estuarine component still dominated the benthos, and that this dominance decreased from 1992 to 1994. During 1992, the estuarine component constituted 97.2% of the total zoobenthos, while this decreased to 84.6% and 54.4% in 1993 and 1994, respectively. Of the overall mean density of 29 623 invertebrates m"2 calculated from the sum of five sites, on a seasonal basis over three years, 48.7% (J4 433 m"2) of the total was from 1992. Zoobenthic densities declined in 1993 and 1994, to 28.2% (8 340 m"2) and 23.1% (6 850 m"2) of the total for the three years. Zoobenthic densities were generally highest in winter, and lowest in autumn. However, a multifactor ANOVA showed that season alone was not a significant factor governing the increase or decrease of zoobenthic densities. The relict estuarine amphipods Granclidierella lignorum and Corophium triaenonyx, and the tanaid Apseudes digitalis were among the taxa dominating the benthos in all seasons. Various parameters were used to determine the physico-chemical condition of the estuary, and several were used as an input matrix to determine their effects on the distribution and abundance of the zoobenthos. Salinity, temperature, dissolved oxygen concentration, depth and turbidity were measured each year. In 1994, a more detailed water quality and sediment analysis was performed. A gradient of turbidity, salinity and oxygen existed from the upper to lower reaches of the estuary, and the substratum constituted medium sands in the lower reaches to detrital muds with silt patches in the upper reaches. The estuary became increasingly fresh over the study period, as salinity declined in the upper reaches from 6%o in 1992 to 0%o at the end of 1994. A suite of multivariate techniques involving classification and ordination methods revealed that sediment particle size was not the most important environmental factor determining species distributions. A combination of turbidity, dissolved oxygen, pH and the percentage organic content proved to be the most important effect of the environment, accounting for most of the variability in the distribution and abundance of the zoobenthos. On the basis of the results obtained, it is concluded that the effects of improved catchment management practices have had a positive effect on the ecology of the estuary, and particularly on the state of the zoobenthos. However, the prevailing drought conditions also had an effect on the benthos, in terms of a change in species composition from 1992 to 1994. Despite this, the estuarine taxa still dominate the estuarine benthic fauna.
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    The 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.
University of Zululand