Juvenile fishes as functional describers of fluvial ecosystem dynamics: Applications on the river rhǒne, France

Biological indicators may signal changes in freshwater quality but offer little information on long‐term trends in ecosystem succession. As an improvement over biological indicators in river management we propose the use of ‘functional describers’, abiotic and biotic elements that reflect alterations in ecosystem function and succession through changes in their concentration or abundance, including the appearance and/or extinction of organisms or groups of organisms. Juvenile (0 +) fishes have been found to be useful functional describers in many fluvial situations encountered in Europe, not least because the ontogeny of European fishes is strongly reflected in the type of habitat chosen for reproduction. Using the regulated River Rhǒne, France, as an example, the absence or presence of 0 + juveniles is shown to provide a direct reflection of a biotope's potential as a spawning area, particularly if recent hydrological conditions have not facilitated 0 + fish displacement and the lone presence of a predator species has not suggested the elimination of other fish species. Identification of the ecological function and successional state of a fluvial ecosystem is aided by consideration of species reproductive styles and tolerances. When time, labour, and/or funding is limited, the function and succession of numerous biotopes within a catchment or large fluvial system can be monitored cost effectively by analysing the 0 + fishes assemblages via the functional describer approach.