Small-scale hydropower is developing rapidly in many countries in response to policies of encouraging renewable energy and reducing reliance on fossil fuels. This rapid increase in the construction of hydroelectric turbines provides a substantial risk to migrating biota, especially fish. Some turbines, such as the Archimedes screw design, are regarded as relatively friendly to fish but have not yet been assessed for their potential impacts on threatened lamprey species. To assess the risk of impingement and the patterns of movement by emigrating river lamprey Lampetra fluviatilis transformers and drifting larval ammocoetes at the site of an Archimedes screw turbine in north-east England, drift nets were set over the periods of January to June 2009 and November 2009 to May 2010. Drifting Lampetra sp. larvae were recorded in all sampling months, November to June, while emigrating lampreys were recorded in all months but June (93% captured between December and April), reflecting a higher period of impingement risk than expected. Night-time catches were 24- and 8-fold higher for transformers and larvae, respectively, than daytime catches. Catch per unit water volume data in different channel areas suggest that lamprey larvae behaved as passive particles within the river flow but that transformers selected areas of higher flow. Damage rates of lampreys passed through the screw were low (1.5%), suggesting minor impacts on downstream-moving larval and juvenile lampreys. However, the cumulative potential impacts of multiple hydropower sites on downstream fish passage, including lampreys, should be considered by regulatory agencies when planning hydropower development within catchments.
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