Recruitment of juvenile marine fishes into permanently open and seasonally open estuarine systems on the southern coast of South Africa
- Authors: Whitfield, Alan K , Kok, H M
- Date: 1992
- Subjects: Marine fishes -- Migration -- South Africa , Estuarine fisheries
- Language: English
- Type: Text
- Identifier: vital:15020 , http://hdl.handle.net/10962/d1019801 , ISBN 0-86810-232-6 , Ichthyological Bulletin of the J.L.B. Smith Institute of Ichthyology; No. 57
- Description: The recruitment of juvenile marine fishes into the Knysna and Swartvlei estuaries was monitored over 30 months using seine, scoop and cast nets. The relative abundance, seasonality and growth of the dominant species are presented, and includes the Cape stumpnose Rhabdosargus holubi, white steenbras Lithognathus lithognathus, blacktail Diplodus sargus, strepie Sarpa salpa, Cape moony Monodactylus falciformis, southern mullet Liza richardsonii, groovy mullet Liza dumerilii, striped mullet Liza tricuspidens, flathead mullet Mugil cephalus, freshwater mullet Myxus capensis and leervis Lichia amia. Information on a further 12 species is also provided. Recruitment of most fish species into the Swartvlei and Knysna estuaries reaches a peak during summer, which coincides with maximum food resource availability and corresponds to the time when systems along this section of the coast are often open to the sea. Artificial winter breaching of the Swartvlei mouth has occurred in the past, and has generally led to the premature closure of the system and loss of the ‘head’ of water needed for the summer opening. This type of mouth manipulation leads to reduced availability of estuarine nursery areas for marine fishes along the southern Cape coast. Comparisons between the recruitment of juvenile fishes into the Knysna and Swartvlei estuaries indicate that higher densities of most species were recorded in the former system. This is attributed mainly to the fact that the Knysna mouth is deep, permanently open, and has a strong marine influence when compared to the shallow, narrow and seasonally closed Swartvlei mouth. However, in a regional context both Knysna and Swartvlei are large, unpolluted systems, which serve as important nursery areas for many species of fish. On this basis alone, these contrasting estuarine systems should be allocated the highest possible conservation status. , Rhodes University Libraries (Digitisation)
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- Date Issued: 1992
Fish community structure in three temporarily open/closed estuaries on the Natal coast
- Authors: Harrison, Trevor D , Whitfield, Alan K
- Date: 1995
- Language: English
- Type: Text
- Identifier: vital:15026 , http://hdl.handle.net/10962/d1019890 , ISSN 0073-4381 , Ichthyological Bulletin J.L.B. Smith Institute of Ichthyology; No. 64
- Description: The fishes of three small Natal estuaries, the Mhlanga, Damba and Zotsha were sampled over a period of two years. A total of 68 fish taxa representing 24 families, 39 genera and 55 species were captured during this study. Forty seven fish taxa were recorded in the Mhlanga estuary of which Gilchristella aestuaria, Oreochromis mossambicus, Valamugil cunnesius, Valamugil sp. and juvenile mugilids numerically dominated. In terms of biomass, O. mossambicus, V. cunnesius, Liza alata, Myxus capensis and Mugil cephalus dominated the ichthyofauna of the Mhlanga system. In the Damba estuary, 24 fish taxa were recorded, the most abundant being Glossogobius callidus,M. capensis and 0. mossambicus. M. capensis, M. cephalus, O. mossambicus and G. callidus dominated the fish biomass captured in the Damba system. A total of 56 fish taxa were recorded in the Zotsha estuary during this study, with the ichthyofauna numerically dominated by juvenile mugilids, G. aestuaria, O. mossambicus, Rhabdosargus holubi, Terapon jarbua, Ambassis productus and G. callidus. The species which dominated the fish biomass in the Zotsha system were 0. mossambicus L. alata, Valamugil robustus, V. buchanani, M. capensis, M. cephalus and V. cunnesius. Classifying the species according to whether they were resident estuarine, freshwater, estuarine-dependent marine or marine species revealed that the first three groups were all well represented in the systems. Oreochromis mossambicus was the dominant freshwater species in all three estuaries. Gilchristella aestuaria and Glossogobius callidus were the principal estuarine species in the Mhlanga and the Damba respectively, with G. aestuaria, A. productus and G. callidus being the dominant estuarine species captured in the Zotsha. The principal estuarine-dependent marine fishes captured in the Mhlanga were V. cunnesius, Valamugil sp., juvenile mugilids, M. capensis, M. cephalus and L. alata. In the Damba, M. capensis and M. cephalus were the dominant estuarine-dependent marine species and in the Zotsha juvenile mugilids, R. holubi, T. jarbua, M. capensis, V. cunnesius, V. robustus, M. cephalus, L. alata and V. buchanani were the principal estuarine-dependent marine species. The results of this study indicate that the estuaries are dominated at different periods by different assemblages of fishes. This is linked to the spawning and migration patterns of the various species as well as the hydrological regime of each estuary. During the winter these systems are normally closed with relatively deep waters and high food resource and habitat availability. Freshwater and estuarine species mainly inhabit the upper reaches of the systems while estuarine-dependent marine species, which dominate the fish community, mainly occupy the middle and lower reaches. When these estuaries open with the onset of the spring/summer rains, adult and sub-adult estuarine-dependent marine species emigrate to the marine environment and juveniles begin recruiting into the systems. Spring is also the peak breeding period of resident estuarine and freshwater species, resulting in an increase in the contribution of these fishes to the overall ichthyofauna during this period. When closed estuaries open the water level falls and this results in the fishes concentrating in the lower reaches of the system where moderate water depths are present, thus further contributing to an increase in the proportion of freshwater and estuarine species in this region. The breaching of closed estuaries also results in a reduction in food resources and habitat availability. Competition and possible increased vulnerability to avian predation (due to the shallow nature of the systems), may contribute to a decrease in the proportion of estuarine and freshwater species in summer. The prolonged recruitment of Of-juveniles of estuarine-dependent marine species results in an increase in the proportion of these fishes present in the estuaries during summer. In autumn, the systems normally close, water levels rise and available food resources and habitat increase. This allows the redistribution of freshwater and estuarine species upstream, leaving estuarine-dependent marine species to dominate the middle and lower reaches. Although temporarily open/closed estuaries along the Natal coast may not be as diverse as permanently open estuaries in terms of their ichthyofauna, their importance must not be underestimated, since by providing a series of sheltered habitats along the coast they may contribute significantly to the viability of estuarine-dependent marine fish stocks. , Rhodes University Libraries (Digitisation)
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- Date Issued: 1995
A survey of selected Eastern Cape estuaries with particular reference to the ichthyofauna
- Authors: Vorwerk, Paul D , Whitfield, Alan K , Cowley, Paul D , Paterson, Angus W
- Date: 2001
- Language: English
- Type: Text
- Identifier: vital:15032 , http://hdl.handle.net/10962/d1019908 , ISSN 0073-4381 , Ichthyological Bulletin J.L.B. Smith Institute of Ichthyology; No. 72
- Description: The physical environment and ichthyofauna of 10 estuaries, the East Kleinemonde, Klein Palmiet, Great Fish, Mtati, Mpekweni, Mgwalana, Bira, Gqutywa, Ngculura and Keiskamma, was sampled over a four year period. The ichthyofauna of each estuary was sampled once during winter and once during summer in that period, with the physical environment being sampled on two occasions per season. This investigation provides baseline ichthyofaunal and physical information for these estuaries, the majority of which have never been studied before. The fish data presented for these systems includes species composition, relative abundance, richness and diversity, longitudinal distributions and length frequency data. Descriptions of the physical environment within each estuary are also presented. The permanently open estuaries had a greater proportion of marine and freshwater species relative to the temporarily open/closed systems which were dominated by marine species dependent on estuaries and estuarine resident species. The permanently open estuaries had a higher Margalef's species richness index relative to the temporarily open/closed systems but the Shannon-Wiener species diversity index did not follow any discernible trend. There were minor longitudinal distribution trends when analysing the community as a whole, with patterns for individual species being more pronounced. The length frequency histograms for estuarine resident species differed between estuary types, while those for the marine migrant species were similar in the different estuary types.\ , Rhodes University Libraries (Digitisation)
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- Date Issued: 2001