Biology and ecology of fishes of the Senqu River, Lesotho
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In this study, fish were used as bio-indicators to evaluate the present ecological state (PES) in different reaches of the Senqu River downstream of the proposed Polihali Dam in Lesotho. However, information regarding fish of the Senqu River is limited, although the various Lesotho fish species received attention with the introduction of trout populations in the 1930s to 1960s. In the 1960s fishery studies and research shifted towards the biology and ecology of local species, with particular attention paid to the endangered Maluti minnow (Pseudobarbus quathlambae) and the migration of cyprinids from the lower Senqu/Orange River into Lesotho in order to spawn. However, there was renewed interest in the fish species of Lesotho with the launch of the Lesotho Highlands Water Project (LHWP) during the late 1990s. After considering the approaches established in Phase I and the outcome of the use of fish to contribute to the establishment of the ecological water requirements (EWR) for the construction of the Katse and Mohale Dams, the research project team decided to use fish as ecological indicators to evaluate the PES prior to the launch of Phase II of the LHWP which entails the construction of the Polihali Dam. Seventeen fish species are known to occur in the upper Orange/Senqu River and its tributaries. These include the longfin eel (Anguilla mossambica) that was able to migrate from spawning sites in the Atlantic Ocean up the Orange River into Lesotho. Due to the establishment of many dams acting as barriers to their migration in South Africa, the access to different habitats has now been restricted in the Senqu River in Lesotho, and in the Orange River in particular. Other indigenous fishes include the now protected Maluti minnow and the rock catfish (Austroglanis sclateri) (both of which have established permanent populations within Lesotho), the Orange-Vaal River mudfish (Labeo capensis), Orange-Vaal River smallmouth yellowfish (Labeobarbus aeneus), Orange-Vaal River largemouth yellowfish (Labeobarbus kimberleyensis) and the moggel (Labeo umbratus). Until recently large numbers of all these species migrated from the middle reaches of the Orange River into the Senqu River and its tributaries. Although many individuals of these cyprinids still migrate into the Senqu River from South Africa, today as much as 90% of their migration potential is disrupted due to a series of partial barriers in the Orange/Senqu River. In addition, seven more indigenous fishes occurred in the lower reaches of the Senqu River in Lesotho and still occur in the upper Orange River catchment. These include the chubbyhead barb (Barbus anoplus), goldie barb (Barbus pallidus), straightfin barb (Barbus paludinosus), threespot barb (Barbus trimaculatus), sharptooth catfish (Clarias gariepinus), southern mouthbrooder (Pseudocrenilabrus philander) and banded tilapia (Tilapia sparrmanii). Three alien species, the rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta) and common carp (Cyprinus carpio) were successfully introduced into Lesotho where they have established populations. Prior to the launch of Phase II of the Lesotho Highlands Water Project, an evaluation of the combined impacts of this development on the Senqu River and its fish was required to minimize the effect of this development on the receiving environment. To achieve this, the aim of this study was to determine the effects of flow and temperature changes on the attributes of fish in the Senqu River, using multiple lines of evidence (LoE). The latter included the use of established best scientific practice measures, or tools across multiple levels of biological organisation. Therefore the objectives of this study were to: 1. Determine the influence of flow and temperature changes on the recruitment of cyprinids in the Senqu River (Chapter 2). 2. Determine direct and indirect effects of flow alterations on the feeding biology of selected fish of the Senqu River (Chapter 2). 3. Use population structures to evaluate the wellbeing of fish communities in the Senqu River (Chapter 2). 4. Use the habitat preferences and migration requirements of fish to evaluate the effects of flow alterations in the Senqu River (Chapter 3). 5. Use the Fish Response Assessment Index (FRAI) and shifts in community structures of fish to evaluate the effects of anthropogenic activities in the Senqu River (Chapter 4). In this study, a winter survey (August 2013) and a summer survey (January 2014) were carried out on four main sites on the Senqu River and on one site on the Linakeng River. Seven species of fish were observed during the 207 electro-fishing and netting efforts carried out on the more than 1 km long reach of each site. In total, 692 smallmouth yellowfish, 154 rock catfish, 145 mudfish, seven juvenile moggel, seven sharptooth catfish and two largemouth yellowfish were collected. Trout are still common in the study areas and 44 rainbow trout were collected. In addition to the fish diversity assessments, information on the population structures, recruitment, feeding ecology, and habitat requirements of the fish was gathered and evaluated during the surveys. The data gathered during these surveys were compared to available historical data to evaluate the state of the local fish communities at each site. These outcomes revealed that the state of the fish communities in the study area has deteriorated to a moderately modified state in the Senqu River downstream of the Polihali Dam site (sites IFR P1 and IFR P2), and to a moderately modified to largely modified state downstream of the confluence of the Senqu and Malibamatso Rivers below the Katse Dam, and at the confluence of the Senqu and Senqunyane Rivers below the Mohale Dam (sites IFR 5 and IFR 6). In particular, the state of the fish communities at site IFR P1 directly downstream of the proposed site of the Polihali Dam has deteriorated to a moderately modified state. Main determinants (or driver variables) contributing to the altered state of the fish communities at this site were identified as habitat state alterations – including sedimentation and a reduction in cover features – and the effect of barriers on the upstream migration of cyprinids, in particular in the Upper Orange River and Lower Senqu River. Likewise, the state of the fish communities at site IFR P2 was observed to occur in a moderately modified state (slightly worse than IFR P1). In similar fashion, the main drivers contributing to the altered state of the fish communities at this site included habitat template modifications, reduction in the abundance of cover features and the effect of barriers on the migration of cyprinids in summer. The state of the fish communities at site IFR 5 has deteriorated to a moderately modified to largely modified state. Main drivers responsible for this include modifications to the distribution and abundance of velocity-depth habitat types associated with altered flows from the Katse Dam, upstream land-use practices primarily impacting the site during winter and summer, and the effect of barriers on upstream migrations of cyprinids. Interestingly, in the vicinity of site IFR 5, it was found that some adult yellowfish and mudfish individuals maintain viable populations in the Senqu River and do not migrate into the lower reaches of the river or into the Orange River in South Africa. The state of the fish communities at site IFR 6 was also observed to be in a moderately modified to largely modified state (worse in winter). Here the extensive alterations to instream habitats – particularly substrate types – have reduced the average depth, and altered the flowdependent habitat profiles at the site. Downstream barriers were again identified as important driver variables affecting the wellbeing of fish. Based on this information, together with additional historical and regional evidence, recruitment data, feeding biology data and habitat preference data, the IFR (instream flow requirement) for the fish could be determined. In addition, the FRAI was used to assess the integrity of the fish communities. Recruitment data were obtained by determining the age of larval smallmouth yellowfish, mudfish, rock catfish and rainbow trout. Winter recruitment was evident for smallmouth yellowfish, mudfish and rock catfish between April and June, and from November to December during the summer. Rainbow trout recruitment was evident in September. Data on the feeding biology of rock catfish indicated that they rely more on macroinvertebrates as food source in winter, whereas smallmouth yellowfish rely more on algae and diatoms as food source in summer periods. Environmental variables such as flows, depths, substrata and temperatures were recorded for each site in the study. Throughout this process, these multiple variable flow requirement states were used to establish aspects of the volume, timing and duration of flows required to maintain the local fish communities. These aspects included base flows required to maintain suitable habitats for recruitment and resident fish, flows that provide key ecological cues for species, and flood flows to maintain key ecological processes.
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