Habitat utilisation by juvenile fishes in Mhlathuze Estuary and Richards Bay Habour
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Date
2002
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Abstract
Estuaries in South Africa are important nursery areas for many fish species, but this role has been threatened by habitat degradation due to anthropogenic impacts, especially in KwaZulu-Natal systems. This has emphasised the importance of harbours in the province for juvenile estuarine fishes, but as areas of ongoing development the future ecological value of harbours is not guaranteed. Descriptions of shallow water estuarine habitats and an assessment of their utilisation by juvenile fishes is important if fish stocks are to be effectively managed for conservation purposes, and recreational and subsistence fisheries. The aim of this study was to investigate assemblages of juvenile fishes associated with a range of habitats in Mhlathuze Estuary and the adjacent Richards Bay Harbour. Habitats in these systems are typical of those in permanently open KwaZulu-Natal estuaries, and by virtue of their size they bestow considerable conservation status on the estuary and the harbour.
Fish ranging from postlarval to adult were sampled using a fine-mesh seine net. Endemic and conservation worthy species, as well as fishes valued in recreational and subsistence fisheries were recorded in both systems. Distinct assemblages of fishes were associated with different habitats. Differences in species compositions were largely due to habitat structure rather than the influences of physico-chemical variables. Eelgrass, Zostera capensis, is an important nursery for many fishes not found in other habitats. Presently in KwaZulu-Natal Zostera only occurs in Mhlathuze Estuary and St Lucia. Fish diversity in several other systems where it historically occurred may now be limited by its absence. Deep access channels, clear waters and an abundance of prey items render the harbour an attractive foraging ground for piscivorous fishes. Mangroves in this system play an important role as predation refugia for high densities of small fishes. Other habitats in both systems, including mudflats and sandbanks were also important. Many species showed a high preference for only one habitat type and the loss of any habitat in the estuary or the harbour would reduce biodiversity and the value of these systems as nursery areas.
Comparison of fishes associated with structurally similar habitats in different systems indicated that physico-chemical variables do influence the role of habitat in estuarine ecosystems. Sandbanks in the clear waters of the harbour supported different assemblages of fishes from sandbanks in the more turbid estuary. This could be related to turbidity preferences of selected species as well as higher predation threat in the harbour. Mangroves in both systems also supported different fish communities, probably also as a result of different predation risk and therefore indirectly attributable to turbidity differences in the two systems.
Results from this study highlight the conservation importance of the systems studied. Whilst it is probable that a multitude of species-specific responses to physical and biological factors influence fish assemblages associated with different habitats, results are broadly applicable to other KwaZulu-Natal estuaries. The distinct roles of structurally different habitats, and the influence of physico-chemical conditions, in the nursery function to different fish assemblages, illustrate the need for clearly stated management objectives at a provincial level.
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Submitted to the Faculty of Science in fulfilment of the requirements for the Degree of Master of Science in the Department of Zoology, University of Zululand, 2002
Keywords
Estuaries, Environmental sciences and ecology