Assessing the impact of damming on river fishes: going beyond taxonomy

Freshwater ecosystems have been severely affected by the expansion of human development in the last century. Freshwater biodiversity is consequently in sharp decline due to increased demand over limited water resources. Degradation and loss of ecosystem services, as well as financial consequences of ecological freshwater problems are creating pressure for a change in attitude from governments and stakeholders concerning strategies for conservation management. Damming is one of the major causes of ecological change in rivers associated with human development. Knowledge of their impact on freshwater ecosystems has been mostly described from the perspective of taxonomic changes. However, this approach provides little information about biological function. Alternative and simpler ways to assess the impact of damming are crucial since the constructions of dams are in expansion, especially in tropical regions where the lack of taxonomical and ecological knowledge about fish species impose an obstacle to effective mitigation measures. A trait-based approach holds the promise to increase our mechanistic understanding of biological responses to disturbed habitats; it transcends taxonomy and it can be applicable across large geographic units. Fishes have a history as biological indicators of environmental quality. They are useful for testing new approaches as their responses to varied anthropogenic disturbances are relatively well documented and their sensitivities to the health of surrounding aquatic environments form one of the basis to monitor environmental degradation. The usage of fish traits as an alternative approach to assess impacted rivers is presented in a critical review where their potential advantages over traditionally used methods are discussed as well as their challenges. The aim of this study was to assess the impact of damming on fish fauna after dam closure using taxonomic and trait-based approaches. The experimental design considered both spatial – the longitudinal gradient from Upstream to Downstream caused by reservoir formation – and temporal before, after a short time period and after at least five years of dam closure effects of damming in rivers. The first case study was conducted on the Tocantins River in Brazil, where the hydroelectric dam Peixe Angical was constructed. It included three parts: (i) a first approach using the traditional taxonomic metrics which confirmed the evidence from other Neotropical rivers of an increase in species richness and abundance immediately after the filling phase and a reduction after only five years, when communities contracted to a level of diversity below that observed prior to dam closure; (ii) an alternative analysis using simple descriptors of community organization (Abundance Biomass Curves and Species Abundance Distributions) which showed a clear impact on the structure of fish assemblages and were more informative than traditional diversity indices; (iii) the application of a species-trait approach which showed that after five years of dam closure opportunistic strategies (e.g., nonmigratory; no parental care) appeared to be more successful under the changed conditions. Long-distance migratory, benthic, detritivore and species with parental care were the most negatively affected. Despite the limitations inherent to the study of this particular fish community (e.g., the lack of knowledge on specific traits categories that might also mediate the response of fish communities to the impact of damming) these results demonstrated that beyond a loss of species richness there is a change in the functional structure of fish communities associated with anthropogenic disturbance. The second case study was conducted in Oldman River in Alberta, Canada where the Oldman River Dam was constructed for irrigation purposes. Integrating taxonomic and trait-based approaches was found to be more informative in a temporal-spatial context of change. Biomonitoring plans should explicitly consider longitudinal spatial gradients created by reservoir formation in their implementation and related management actions. Overall, information is given to identify specific features that should be considered in the assessment and biomonitoring of impacted freshwater systems using fish communities for the application of true ecologically oriented river management.