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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 adult sea lamprey detected by the video fish-counting stations situated in the Dordogne River (black) and the Garonne River (grey) from 1993 to 2019. 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. On the contrary, the absence of some bars from 2013 to 2019 on the barplot is not due to technical problems and indicates no effective lamprey passages. A global same temporal pattern was observed in both rivers, with three time periods. From 1993 to 2000, the number of lampreys was quite low, averaging 4,995 and 1,076 individuals per year in the Dordogne and the Garonne, respectively. This number globally increased between 2000 and 2010, reaching records of 18,344 individuals in 2003 in the Garonne and 39,069 in 2009 in the Dordogne. Finally, the annual number experienced a severe drop since 2009: no lamprey has been counted in the Garonne River since 2013 and only 11, 4, 34 and 0 individuals have been counted during the last four years in the Dordogne River.