Fish biodiversity in different types of tributary mouths located within impounded sections of Swedish boreal rivers

Large boreal rivers in Sweden are generally impounded by hydropower dams and a large proportion of main stem shallow flowing habitats have been lost. Tributaries often con-tain the last undisturbed habitats and could be important for the conservation of species diversity. In particular, tributary mouth areas could be biodiversity hot-spots, due to their vicinity to the main stem and favorable environmental conditions. In this study, we inves-tigate whether tributary mouth areas in two impounded boreal rivers (Ume River and Lule River) could be regarded as biodiversity hot-spots for fish. Based on standardized elec-trofishing in 20 tributary mouths, we find that overall fish diversity is generally low. The highest species richness and diversity was found in mouth areas dominated by intermedi-ate substrate sizes (gravel - cobble). Few, if any, species were found in areas where fine sediments (smaller than sand) dominated. The tributary mouth areas had similar species richness and diversity as areas in the tributaries located 1-km upstream of the mouth, but the fish community composition often differed between these two types of sites. Manage-ment action favoring fish diversity in the tributary mouth areas could include protection or rehabilitation of areas dominated by medium sized substrate and reduction of erosion and transport of fine sediments in the tributaries. Overall, we find no support for trib-utary mouths being hot-spots for fish biodiversity and while some patterns in diversity gives hints on suitable management action, it is important to further understand impacts in tributaries and their mouths and the temporal dynamics of the fish community.(c) 2022 The Author(s). Published by Elsevier B.V. on behalf of European Regional Centre for Ecohydrology of the Polish Academy of Sciences. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Automated summary

Large boreal rivers in Sweden are significantly impacted by hydropower dams, resulting in the loss of main stem habitats. This study examines the potential of tributary mouth areas as biodiversity hotspots for fish in two impounded boreal rivers. The results show that overall fish diversity is generally low in these areas, with higher species richness and diversity found in mouth areas dominated by intermediate substrate sizes. Fine sediments dominate areas with few or no species. The fish community composition differs between tributary mouths and areas further upstream. Management actions to enhance fish diversity in tributary mouths could include protecting or restoring areas dominated by medium-sized substrate and reducing erosion and the transport of fine sediments. While tributary mouths do not appear to be hotspots for fish biodiversity, understanding the impacts and temporal dynamics of the fish community in tributaries and their mouths is crucial for appropriate management strategies.