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

Frances Eley,Jim Lyons,Graeme Peirson,Jon Hateley,Karen Twine,Stuart Manwaring

doi:10.3390/w13202932

Abstract

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.

Highlights

This study represents the longest single continuous and standardised application of horizontal hydroacoustic methodology for monitoring a lowland river’s fish populations.These hydroacoustic data are a comprehensive long-term baseline of fish abundance and distribution, based entirely on non-destructive sampling
3 = 83.62 ± 5.96; Reach 4 = 50.49 ± 2.94 fish 1000 m−3 ). These values compare with the long-term average for mobile horizontal hydroacoustic surveys on large lowland UK Rivers of 53.63 ± 21.53 fish 1000 m−3 [46]
This may result from a lack of data on the natural environment, this study showing the value of consistent long term data collection and the potential errors in the extrapolation of limited temporal data

Summary

Introduction

This study represents the longest single continuous and standardised application of horizontal hydroacoustic methodology for monitoring a lowland river’s fish populations These hydroacoustic data are a comprehensive long-term baseline of fish abundance and distribution, based entirely on non-destructive sampling. This information is complemented with species composition derived data from a long-term time series of electric fishing surveys, collected with the same sampling frequency and collection dates. These longstanding data provides a unique opportunity to investigate temporal and spatial shifts in the fish population on a world-renowned river coarse fishery. The River Thames is a valuable socioeconomic angling venue with annual local licence sales of approximately 250,000 and

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Discussion

Conclusion