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This paper reports the results from mobile hydroacoustic surveys carried out between 1994 and 2018, to assess the fish stocks in four impounded reaches, covering 19.8 km of the River Thames, England. The data are complemented with electric fishing boom boat results, collected at the same study reaches and time periods. Hydroacoustic surveys used inter-calibrated dual and split-beam scientific echosounders, with the transducers beaming horizontally across the river to provide fish abundance and distribution estimates. Electric fishing surveys provided catch per unit effort estimates and information on size structure and species composition. Catch data from the margins of the study reaches were dominated by roach (Rutilus rutilus), with bleak (Alburnus alburnus) dominant in mid-river. Hydroacoustic data demonstrated patchy spatial distribution, often associated with natural and anthropogenic habitat features. Cyclical peaks and troughs in both hydroacoustic and electric fishing abundance were found. There were periods of correspondence with electric fishing abundance estimates, but also periods of significant divergence. The concept of ‘Shifting Baseline Syndrome’ is discussed with reference to these data, illustrating the importance of viewing long term quantitative information when using meaningful reference points. The potential impact of river temperature and flow on the fish population data are discussed.

An Assessment of Hydroacoustic and Electric Fishing Data to Evaluate Long Term Spatial and Temporal Fish Population Change in the River Thames, UK