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. 2020 Apr 9;10:6122. doi: 10.1038/s41598-020-62916-w

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Annual numbers of Northern pike (a) and European catfish (b) coming back in front of the video fish-counting stations in the Dordogne River (black) and the Garonne River (grey) from 1993 to 2017. The Tuillière fishway on the Dordogne River (at 220 km from the confluence between Dordogne and Garonne) and the Golfech fishway on the Garonne River (at around 270 km from the Garonne River mouth) (see Fig. 6) are equipped with permanent video fish-counting stations to monitor fish upstream movements. Numbers in the Dordogne River were not recorded from 2006 to 2008 due to technical problems on the video station. No other bars on the barplot indicate no passage. (a) Annual numbers of pike passages average 4 individuals (±5 SD) and 12 (±8 SD) in Garonne and Dordogne, respectively with maxima of 21 individuals in 2004 in Garonne and 31 individuals in 1999 in Dordogne. (b) Catfish passages at the video fish-counting stations occurred in 1995 with 3 and 15 individuals in Garonne and Dordogne, respectively. Those annual numbers progressively increased in both rivers to reach records of 1 134 individuals in 2007 in Garonne and 201 individuals in 2009 in Dordogne. Since those peaks, catfish numbers averaged 520 (±181 SD) individuals in Garonne and 94 (±34 SD) in Dordogne. Such temporal data showed that pike density is far less as catfish density.