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Scientists devised a method for extracting information from long-term fish-monitoring datasets collected by environmental agencies as part of routine national and international fish monitoring programmes – the resultant study is the largest of its kind to date for these small-scale hydroelectric power plants.

The findings have been published on 18 May in the open-access science journal . The research was funded by the Natural Environment Research Council (NERC) and the Environment Agency.

Dr Gary Bilotta, lead author of the paper, explained: “Globally, electricity generation from hydroelectric power is forecast to increase significantly over the next two decades, contributing 50 per cent of all renewable electricity generation by 2035. Currently the majority of global hydroelectric power comes from large-scale storage-type schemes whereby rivers are dammed to create reservoirs.

Dr Gary Bilotta

“In Europe, the documented environmental effects of such schemes  and recognition that most opportunities for economic and politically-acceptable large schemes have already been developed, has turned development attention to smaller designs, particularly run-of-river schemes.”

According to Dr Bilotta ‘run-of-river’ hydroelectric power schemes operate without water storage, using river flow. Channel structures, typically pre-existing weirs, are normally used to regulate water levels, allowing a proportion of flow to be diverted through a turbine before returning it to the river downstream.

“It has been shown that these types of hydroelectric power schemes consume less water per unit of electricity generated compared to other sources of energy, and that they have significantly lower greenhouse gas emissions than other sources of energy, but until our research published this week, their physical and ecological impacts had not been well studied.”

As part of this latest study, the researchers used a ‘Before-After-Control-Impact’ (BACI) design whereby the before-after responses of fish communities in proximity to 23 run-of-river hydroelectric power schemes were compared to the before-after responses of fish communities at 23 local paired ‘control’ sites that lacked the influence of run-of-river hydroelectric power schemes. The control sites helped the researchers to distinguish changes attributable to run-of-river hydroelectric power schemes from changes over time related to regional trends or events such as floods and droughts.

The researchers analysed six aspects of fish community composition (number of species, number of fish, number of Atlantic salmon, number of one-year-old Atlantic salmon, number of brown trout, and number of one-year-old brown trout) that might be expected to be affected by hydroelectric power and are widely monitored by the Environment Agency.

The results showed no statistically significant change in five of the metrics, but a statistically significant but very small decrease in the mean number of fish species (-0.06 species per 100 m2), in the period after hydroelectric power construction relative to the period before construction. In comparison at control sites, at the same time, there was a small increase in the mean number of species (+0.13 species per 100 m2).

The researchers highlight that these findings warrant further investigation to establish the likely mechanisms of community composition change and to better understand longer-term trends in community composition.