- Title
- Ichthyofaunal and isotope changes along the rivere-stuarine continuum in a fluvially dominated Southern African coastal system
- Creator
- Nashima, Festus Panduleni
- Subject
- Fishes -- Ecology
- Date Issued
- 2020
- Date
- 2020
- Type
- Thesis
- Type
- Doctoral
- Type
- PhD
- Identifier
- http://hdl.handle.net/10948/48555
- Identifier
- vital:40890
- Description
- Catchment to coast and the continuum concept recognize linkages and make it imperative to take an ecosystem approach to catchment management, water allocation, fisheries management and environmental management as a whole. Fishes that utilise estuarine environments are influenced by physical and chemical conditions as well as biological interactions. Community structure, drivers and trophic linkages between species of different origin, utilising the low salinity waters along the riverestuarine continuum remain poorly understood in the lower Orange River and estuary. This body of work investigated the spatio-temporal dynamics structuring fishes in the lower Orange River Estuary Continuum (OREC) within the context of River Continuum (Vannote et al., 1980). Eight years (2004 - 2018) of seasonal sampling at 18 sites comprised of multi-species catches were analysed, including three years (2016 - 2018) exploring trophic linkages between the dominant species of marine, estuarine and freshwater fishes in the system. In total, 30 species belonging to 16 families were collected in the lower OREC. The Family Mugilidae dominated all catches. Fish species composition was comprised mostly of freshwater (47%), followed by the marine (33%) and estuarine species (20%). Numerically, the marine euryhaline Chelon richardsonii and solely estuarine Gilchristella aestuaria were the dominant species in the estuary region (site 1 - 11) with 95% and 4% contribution, respectively. Chelon richardsonii also dominated all catches (67%) upstream into the river (site 12 - 18), followed by freshwater Labeobarbus aeneus (13%) and Pseudocrenilabrus philander (8%). This highlights the point that besides the marine Chelon richardsonii, other species albeit marine, estuarine and freshwater are utilising unique areas within the OREC. Temporal differentiation in fish abundance (catch-per-unit-effort, CPUE), diversity, richness and evenness were higher in the high-flow season than the low-flow season. Mean CPUE was higher at the mouth of the estuary and declined upstream into the river, whilst species diversity (including richness and evenness) increased upstream. High catches but low species diversity observed close to the mouth of the estuary were attributed to the domination of Chelon richardsonii. However, species diversity increased upstream in the river due to the dominance of freshwater fishes coupled with several marine and estuarine species. This diversity patterns were inconsistent with the general Remane and revised model for estuaries, hence a new RiverEstuarine Continuum concept suitable for OREC was developed. With the exception of Chelon richardsonii, spatial variability in the length-frequency distribution of most fishes was observed. Physico-chemical drivers provided valuable information on adaptation limits of species movement and distribution. Fishes were structured along a salinity gradient and interaction between temperature and salinity influenced community structure and diversity patterns in the OREC. Fish species utilising the lower OREC are predominantly zoobenthivores but a few detritivores/planktivores dominate numerically. Trophic inferences from stable isotope analysis showed increased carbon isotope values (δ 13C) with salinity toward the mouth of the estuary, whilst nitrogen isotope values (δ 15N) increased upstream into the river. Ontogenetic shifts in the SI ratios of all five fish species (marine Chelon richardsonii; estuarine Gilchristella aestuaria; freshwater Labeobarbus aeneus, Pseudocrenilabrus philander and Mesobola brevianalis) were observed in the OREC. Variability in seasons and salinity gradient influenced the SI ratios of several fish species. Seasonal changes influenced the δ 13C ratios of all species, except for Labeobarbus aeneus and Pseudocrenilabrus philander whereas the δ 15N ratios of all species (except for Mesobola brevianalis) differed significantly between the high-flow and low-flow season. This can be due to their feeding on specific life-history stages, such as eggs, larvae, gametophytes, sporophytes or zoospores of prey. Stable isotope ratios revealed that Mesobola brevianalis fed at a higher trophic level than Chelon richardsonii and Gilchristella aestuaria. Mesobola brevianalis feed on planktonic crustacean and insects. Chelon richardsonii feed on a wide range of prey, with prey switching between planktonic and detritivore food resources, whilst Gilchristella aestuaria feed selectively on zooplankton but switched to filter feeding modes in turbid water. Generally, δ 15N SI ratios increase with growth, however, for Labeobarbus aeneus, it was significantly higher in small size length than in large size individuals, probably because most of the juveniles assessed were caught upstream where δ 15N increases. Juveniles of Labeobarbus aeneus mainly feed on zoobenthos whilst in larger fish phytoplankton becomes increasingly important, suggesting the possibility that zoobenthos was more 15N enriched than plant materials. The trophic niches of Mesobola brevianalis were isotopically distinguishable from Chelon richardsonii and Gilchristella aestuaria during the low-flow season, revealing that these species are probably accessing carbon sources from different food chains. During high-flow season, the isotopic niche overlapped among several species, indicating that these species are probably deriving carbon sources from the same or similar food chains. Results from Layman isotope-based metrics revealed that Chelon richardsonii, Pseudocrenilabrus philander and Labeobarbus aeneus occupied the widest isotopic niche in the lower OREC, suggesting that these species are utilising food sources which may be derived from a wide variety of carbon sources. Mesobola brevianalis had the narrowest isotopic niche, however this does not imply "a narrow range of food sources’’ for the species. Overall, the potential carbon sources for selected fish species revealed that they are supported by a variety of food sources (mainly phyto-and zooplanktonic) or their food items constituted different basal sources which enable them to coexist. This study represents the first, integrated investigation of trophic relations among fishes of different origin using δ 13C and δ15N SI analyses. The current study provides a broader understanding of fish ecology within the River-Estuarine Continuum in the lower Orange River and estuary. By filling research gaps identified in the lower OREC, this synthesis of work also supports and informs management, conservation and responsible utilisation of estuarine fishery resources, in particular within the OREC.
- Format
- Xv, 201 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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