We investigated habitat availability and fish assemblage structure in three local river widenings, completed 3–14 years ago, and five adjacent canalized reaches on the river Thur, a seventh-order river in Switzerland. To account for seasonal variability, surveys were repeated in winter and summer 2005. Results were compared with historical pre-disturbance data to evaluate whether the current abiotic and biotic conditions in the study reaches have attained historic near-natural levels.
Hydro-physical habitat diversity (depth, flow velocity, cover availability) was considerably greater in the two longer widenings (>900 m length) than in the canalized reaches and in the shortest widening (300 m length), with higher proportions of shallow or deep areas of different flow velocities. However, the comparison of current and historical near-natural shoreline lengths indicated that the current geomorphological complexity is still considerably impaired in all reaches.
No overall significant relationship was found between the reach type (canalized or rehabilitated) and the number of species or the total fish abundance which were strongly correlated with the availability of suitable cover and moderate flow velocity. However, highest winter abundances were observed in deep, well-structured backwaters of the rehabilitated reaches, documenting their significance as wintering habitats. Assemblage structure and composition were similar in canalized and rehabilitated reaches. Compared to the historical data, however, fewer and different dominant species were found, and guild composition changed towards a higher representation of generalists and tolerant species. Copyright © 2008 John Wiley & Sons, Ltd.