Long‐term monitoring on a new channelized stream section: changes in mesohabitat, composition, and size structure of fish assemblages

Abstract

The construction of riverine structures (weirs, bridges, or channelization) on riverbeds causes alterations in the flow regime and channel geometry. Once a new stretch is created, species must colonize it. The ecological succession processes that are entailed are decisive for adequate recovery after alterations, and understanding these processes would enable elaborate efficient restoration actions. We analyzed environmental variables and the colonization and succession patterns of fish assemblages in a new channelized stretch of a river from its construction to the present (from 1996 to 2020) in the northern Iberian Peninsula. During the studied period, habitat diversity increased, and mesohabitat became more complex. Depths, depth diversity, and the number of pools in the new channel increased with time. Water temperature decreased because of the new shade provided by riparian forests. The size‐related variables of the fish community (size diversity, mean, and maximum length) increased in the new section, achieving similar values to those in the control section. The slopes of the fish size spectra showed a slow evolution over 25 years from a fish assemblage dominated by small fish to a more size‐diversity fish community. Our results suggest that habitat complexity shape fish assemblage and structure. Moreover, size‐related variables can be effective ecological indicators of fish colonization and succession processes. Finally, small‐scale restoration measures in the riverbed and riparian forest are expected to increase the effectiveness of future restoration projects in rivers.