Use of freshwater fish to evaluate the wellbeing of selected rivers in KwaZulu-Natal, South Africa.
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Freshwater makes up just 0.01 % of the total water on earth but the fish species inhabiting it make up around 20 % of all vertebrate species. For centuries, human settlements have relied upon freshwater fish for food, recreation and the ecosystem services they provide. As a watershort country, South Africa has been unable to keep up with the increasing water demands from the ever-growing population. This has resulted in increased abstraction from rivers and the construction of instream barriers such as dams. The impacts of these are relatively poorly known for most South African fish species. Fish species are good indicators of river wellbeing as they are relatively easy to collect and identify, they are responsive to changes in environmental quality and they are mobile and long living. Fish have globally been established as ecological indicators and scientists and managers use attributes of fish related to various levels of biological organisation from cellular to community level. The ecologically relevant evaluations of community structures of fish can be used to evaluate the condition of many determinant factors of riverine ecosystems. Numerous multi-variate statistical analysis techniques have been established to allow for a robust statistical evaluation of sifts in community structures of fishes and relate changes in associated ecosystem variables to characterise causality. Additional community metric measure tools such as the Fish Response Assessment Index (FRAI) is being used throughout South Africa successfully to evaluate the wellbeing of fish communities and identify causes of probable shifts in communities. The FRAI is based on fish species intolerances and preferences, and their response to drivers of change in riverine ecosystems. By comparing the community structure changes of fish communities to outcomes of community metric measures the lines of evidences can be used to validate outcomes and reduce uncertainty in the application of the approach for the region. Labeobarbus natalensis, locally known as the KwaZulu-Natal (KZN) yellowfish or scaly, is ubiquitous in KZN, South Africa. Populations that have historically been found in most rivers within the province but have apparently declined in response to increased use of water resources. Major determinants of their decline include habitat loss, altered water quality and quantity, and the establishment of barriers such as dams and weirs that hinders migrations. The probable occurrence of Labeobarbus natalensis in KZN rivers and environmental factors and anthropogenic determinants affecting their presence/absence were evaluated.Assessments were carried out at 40 sites on 23 river systems in KZN, South Africa. Four seasonal surveys were undertaken between January 2015 and April 2016. The rapid assessments included monitoring fish communities, water quality and habitat availability/conditions. Fish were collected using a range of sampling techniques appropriate to the habitats observed in the rivers. Sampling methods included the use of electrofishing and passive and active netting techniques. Fish collected were identified, measured to total length (TL) and then released alive at their capture location. The diversity, abundance and population structures of fishes were evaluated and compared with known historical distributions. Community structure analyses were carried out using multivariate statistical procedures for the fish community structures in relation to the drivers of community structure changes. In addition to the two community-based structure analyses which were done, a population-level analysis was carried where TL was used to assess the shape and wellbeing of populations. Outcomes includes significant shifts in fish communities between catchments and within catchments and between seasons. Many communities were observed to have been significantly correlated with water quality, quantity and habitat variability associated with different land use scenarios. The FRAI results varied representing communities in a largely natural to critically modified condition that were closely correlated to changes in community structures analysed statistically. The general reduction in the wellbeing of many communities throughout KZN were partially attributed to the drought that took place during the study period. Additional stressors identified included the increase in range of alien predators and competing fish species and a range of land use activities. The FRAI scores indicated that study sites where agricultural activities were the dominant land use type were of most concern, as the fish communities occurred in severely and critically modified conditions. Sedimentation impacts affecting instream habitats and water quality from poor agricultural practices likely resulted in shifts in the dominant substrate type from cobbles/gravel to sand/silt. It was found that the semi-rheophilic KZN yellowfish had a preference for fast flowing water and cobbles/boulders as a substrate type and as a cover feature. Furthermore, invasive fish, abstraction and industrial use all had negative impacts on the state of the rivers and the KZN yellowfish population wellbeing. The drought, which was impacting the northern parts of KZN the most, likely accelerated the decline in fish populations. The vulnerability of substrates such as gravel, cobbles, and boulders to sedimentation has the potential to result in a population shift, away from L. natalensis and towards species like Oreochromis mossambicus and Micropterus spp. The KZN yellowfish populations in the Mkuze and Mlazi River systems in particular were observed to be in a poor state with low abundances and poor population structures of the species in the Mkuze River and no yellowfish were observed in Mlazi River system. Contrastingly, the KZN yellowfish populations in the Thukela and uMngeni River systems in particular, were in good health. They showed good population structure and good recruitment. The overall wellbeing of KZN yellowfish populations in many river systems has declined in response to consistent increases in stressors observed in the province due to increasing use of water resources and expansion of alien fish distributions. Increased conservation efforts are required to curb the overall decline of the wellbeing of freshwater fishes in KZN observed during the study. The spread of alien species of fish is of particular concern, along with habitat loss which is of high ecological importance to native fish species. The river catchments which are situated in areas of high anthropogenic use were found to be the most impacted, followed by areas with intense agricultural utilisation. The drought during the study period likely exacerbated the aforementioned drivers of change, an area which requires further study. This study investigated specific threats and environmental driving factors that impact freshwater fish populations in KZN. The outcomes of the study include information on fish communities that can facilitate the identification of key conservation areas for local riverine conservationists and demonstrate the successful use of multiple lines of evidence to monitor and evaluate fish community wellbeing in KZN.
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