Early impacts of the largest Amazonian hydropower project on fish communities

Impacts of Belo Monte dam on fish co-occurrence and artisanal fishing

The operation of large hydroelectric dams, such as Belo Monte in the Brazilian Amazon, significantly disrupts local freshwater ecosystems, impacting the socio-economic fabric and food security of dependent communities. To investigate and simulate future dam-induced impacts on fish species crucial for subsistence and artisanal fishing, this study leveraged subsistence fishing data from four areas of the Xingu River: the reservoir, upstream, the de-watered reach, and downstream. Using network analysis and graph theory, we examined the temporal and spatial variations in fish species composition, focusing on species that co-occur in fishing catches and their ecological roles within the network. Results showed a 34 % reduction in total biomass, from 8442.51 kg (2012-2015) to 5537.42 kg (2016-2020). The analysis revealed marked changes in species diversity and catch composition, with substantial shifts in heavily altered regions. For instance, in the de-watered reach, traditional species such as curimatã (Prochilodontidae) were replaced by others like pacu (Serrasalmidae), reflecting ecological shifts and the replacement of a detritivorous species by an omnivorous one. Directed simulations projected the dynamics of species loss, revealing significant alterations in co-occurrence patterns and network centrality, particularly in the de-watered reach. These simulations demonstrated that species such as pacu, pescada (Sciaenidae), and tucunaré (Cichlidae) remained central to the network despite ongoing ecological transformations. While this study provides valuable insights into environmental shifts, the absence of direct socio-economic data highlights the need for future research to understand the broader impacts on artisanal fisheries better. These results underscore the profound transformations in fish diversity and emphasize the critical need for adaptive management strategies to mitigate these impacts, ensuring the resilience of local fishing communities. While our findings offer valuable ecological insights, the absence of direct socio-economic data highlights the need for future research to better understand the broader impacts of hydroelectric dams on artisanal fisheries.

Ruin-of-the-rivers? A global review of run-of-the-river dams

The classification of a hydropower scheme as run-of-the-river (or run-of-river; ROR) evokes an image of a low-impact installation; however, examination of eight case studies worldwide shows that substantial negative societal and ecological impacts are tied to them, albeit in somewhat different ways. We conclude that ROR dams not only potentially displace communities, disrupt livelihoods, and degrade environments in surrounding areas, but they also divert water from areas of need, impact aquatic ecology through habitat destruction and disruption of fish migrations, emit non-trivial amounts of greenhouse gases over the lifespan of the project, and disrupt streamflow in downstream river sections. While these negative impacts vary on a case-by-case basis, medium and large ROR dams consistently have multiple and cumulative impacts, even when not having appreciable reservoirs. We contend that many impactful dams do not qualify as low-impact ROR projects, despite being defined as such. Such mislabeling is facilitated in part by the ambiguous definition of the term, which risks the ROR concept being used by proponents of impactful structures to downplay their negative effects and thus mislead the public or gain status, including within the Clean Development Mechanism in relation to mitigating climate change.

An in-situ study in the Xijiang River basin revealed adverse effects of total dissolved gas supersaturation on fish

High dam discharge can lead to total dissolved gas (TDG) supersaturation in downstream rivers, causing fish to suffer from bubble trauma and even mortality. Focusing on the Datengxia hydropower station in the Xijiang River basin, we conducted in-situ experiments to explore the tolerance patterns of economic fish species, including Ctenopharyngodon idella, Hypophthalmichthys molitrix, and Cirrhinus molitorella, under the influence of TDG supersaturation at different compensation depths. Moreover, the development and recovery patterns of bubble trauma and the swimming ability of fish exposed to TDG supersaturated water were investigated. In-situ experiments showed that TDG supersaturation ranged from 112.2 % to 125.2 %, averaging 118.3 % at the experiment site. The results revealed that compensation depth is favorable in fish avoidance of TDG supersaturation. The survival rate of the experimental fish at the surface was lower than for those at the 0-3 m water depth. The survival rates of Ctenopharyngodon idella, Hypophthalmichthys molitrix, and Cirrhinus molitorella at the surface were only 30 %, 47.5 %, and 70 %, respectively, while at the 0-3 m water depth, the survival rates were 97.5 %, 87.5 %, and 87.5 %, respectively. Additionally, the survival rate of fish was related to their preferred water depth. The bubble trauma scores of the experimental fish in TDG supersaturated water significantly increased with exposure time and significantly decreased after recovery in freshwater. The relative and absolute critical swimming speed (Ucrit) of Ctenopharyngodon idella ranged from 10.91 to 12.98 BL/s and 83.3-102.9 cm/s respectively, and there were no significant changes in the Ucrit with increasing TDG supersaturation exposure.

Use of long-term underwater camera surveillance to assess the effects of the largest Amazonian hydroelectric dam on fish communities

The increase in the construction of mega dams in tropical basins is considered a threat to freshwater fish diversity. Although difficult to detect in conventional monitoring programs, rheophilic species and those reliant on shallow habitats comprise a large proportion of fish diversity in tropical basins and are among the most sensitive species to hydropower impacts. We used Baited Remote Underwater Video (BRUV), an innovative, non-invasive sampling technique, to record the impacts caused by Belo Monte, the third largest hydropower project in the world, on fishes inhabiting fast waters in the Xingu River. BRUV were set in a river stretch of ~ 240 km for 7 years, 2 before and 5 after the Belo Monte operation. We explored the spatial and temporal variation in fish diversity (α, β, and γ) and abundance (MaxN) using generalized additive models. We also investigated the variation of environmental variables and tested how much information we gained by including them in the diversity and abundance models. Belo Monte altered the flow regime, water characteristics, and fishery yield in the Xingu, resulting in changes in the fish community structure. Temporally, we observed sharp declines in α diversity and abundance, far exceeding those from a previous study conducted with more conventional sampling methods (i.e., catch-based) in the region. γ-diversity was also significantly reduced, but we observed a non-expected increase in β diversity over time. The latter may be associated with a reduction in river connectivity and an increase in environmental heterogeneity among river sectors. Unexpected signs of recovery in diversity metrics were observed in the last years of monitoring, which may be associated with the maintenance of flow levels higher than those previously planned. These results showed that BRUV can be a useful and sensitive tool to monitor the impacts of dams and other enterprises on fish fauna from clear-water rivers. Moreover, this study enhances our comprehension of the temporal variations in freshwater fish diversity metrics and discusses the prevalent assumption that a linear continuum in fish-structure damage associated with dam impoundments may exhibit temporal non-linearity.

Restoration of a fish-attracting flow field downstream of a dam based on the swimming ability of endemic fishes: A case study in the upper Yangtze River basin

The construction of fish passage facilities can mitigate the negative effects of dams and other water engineering construction on river connectivity and have a significant positive effect on the conservation of local fish diversity. To attract target fishes into fish passage facilities effectively, the optimal flow velocity range to attract fish must be determined. Three local endemic species of the Mishi Reservoir were considered as the protection targets. However, their swimming abilities remain unclear. Therefore, the induced swimming speed (Uind), critical swimming speed (Ucrit) and burst swimming speed (Uburst) of three fish species were tested. Based on these results, we identified the optimal flow velocity to attract fish, which falls within the range of 0.15-0.51 m/s. A validated three-dimensional hydrodynamic model was used to simulate different schemes. By comparing the flow field simulation results of different schemes, we obtained the optimal measure to restore the flow field, namely, a multiple engineering measure combining increased the fish attraction flow in the fish collection pond and the construction of a spur dike. This study offers a solution for the specific case and enhances the database of swimming characteristics of endemic fish in the upstream reaches of the Yangtze River. It also provides a valuable reference for designing fish-attracting flows and potential measures for restoring flow fields in similar future projects.

Freshwater species diversity loss embodied in interprovincial hydroelectricity transmission with ecological network analysis

A major strategy for addressing the imbalance in source-network-load distribution is interprovincial electricity transmission; however, this process also causes various environmental effects. Previous studies have mainly examined thermal power transmission, and few insights have been gained into the challenges hydroelectricity transmission poses for biodiversity conservation. Here, we innovatively incorporated the freshwater species diversity footprint into a hydropower environmental impact assessment, calculating the interprovincial transfer of freshwater species diversity embodied in hydroelectricity transmission. We proposed an evaluation model of an interprovincial hydroelectricity transmission network using freshwater species diversity as the ecological element and creatively identified significant nodes and paths of the network. Up to 28% of the transfer of freshwater species diversity was related to the demand for hydroelectricity consumption in Shanghai. 64% of the relationships in the hydroelectricity transmission network were implemented at the expense of ecological losses on one side. Shanghai and Sichuan provinces and some transmission lines related to them were significant nodes and paths for improving the overall status of the network. This research can help policymakers comprehend the challenges to freshwater species presented by interprovincial hydroelectricity transmission and serve as a reference for ecological compensation for hydropower development.