Movement patterns of seaward migrating European eel ( Anguilla anguilla ) 1 at a complex of riverine barriers : implications for conservation

Movement patterns of seaward migrating European eel (Anguilla anguilla) 1 at a complex of riverine barriers: implications for conservation 2 A. T. Piper, J. C. Svendsen, R. M. Wright & P. S. Kemp 3 4 International Centre for Ecohydraulics Research, School of Civil Engineering and the 5 Environment, University of Southampton, Highfield, Southampton SO17 1BJ, UK;, National 6 Institute of Aquatic Resources, Section for Freshwater Fisheries and Ecology Technical 7 University of Denmark, Vejlsøvej 39, DK-8600 Silkeborg, Denmark; 8 Interdisciplinary Centre of Marine and Environmental Research (CIIMAR), University of 9 Porto, Rua dos Bragas 289, 4050-123 Porto, Portugal; 10 Environment Agency, Rivers House, Threshelfords Business Park, Inworth Rd, Feering, 11 CO5 9SE, UK 12 13 Correspondence: Adam T Piper ([email protected]) 14 Running headline: Downstream migration patterns in European eel 15 16 ABSTRACT 17 River infrastructure such as weirs and hydropower stations commonly present migrating fish 18 with multiple potential passage routes. Knowledge of the cues fish use to navigate such 19 environments is required to protect migrants from hazardous areas and guide them towards 20 safe passage, however, this is currently lacking for many species. Employing high-resolution 21 positioning telemetry, this study examined movements of downstream migrating adult 22 European eel, Anguilla anguilla, as they encountered a complex of water control structures in 23 one location on the River Stour, southern England. The distribution of eels across five 24 potential routes of passage differed from that predicted based on proportion of discharge 25 alone. Certain routes were consistently avoided, even when the majority of flow passed 26 through them. Passage distribution was partially explained by avoidance in the vicinity of a 27 floating debris boom. Movement paths were non-randomly distributed across the forebay and 28 eels moved predominantly within a zone 2-4 m from the channel walls. Understanding of 29 avoidance and structure-orientation exhibited by eels will help advance effective guidance 30 and downstream passage solutions for adults. 31