Fish zonations and guilds as the basis for assessment of ecological integrity of large rivers

Longitudinal zonation concepts describe the downstream changes in chemico-physical and biological properties of rivers. Including information on ecological fish guilds can enhance the usefulness of fish zonation concepts, in a way that they can be used as tools for assessment and management of the ecological integrity of large rivers. We present an ecological characterization of fish zones and fish communities in near-natural and in regulated large rivers in Europe (the River Doubs in France and the Rivers Rhine and Meuse in the Netherlands), using guild classifications of several life-history traits of fish and national Red Lists of threatened species. The Doubs data set was also analyzed using indices of the sensitivity of fish species to environmental degradation and indices for eurytopy. In these rivers, the number of ecological guilds per zone increases downstream, and there are clear shifts in the structure of the guilds. Flow preference and reproduction ecology of river fish are closely linked. The proportion of rheophilic species in the fish community decreases downstream, and the proportions of limnophilic and eurytopic species increase. Lithophilic and psammophilic spawners are dominant in the upper zones, whereas the lower zones are dominated by phytophilic and phytolithophilic spawners. The proportion of zoobenthivorous and periphytivorous species decreases downstream, and the proportion of zooplanktivorous and phytivorous species increases. However, because the European fish fauna mainly consists of feeding generalists, the discriminative abilities of simplistic feeding guild classifications are not very high. Guilds of sensitive, stenoecious species that share life history strategies that are highly adapted to specific riverine conditions (rheophils and limnophils) have declined far more than generalist species that can survive in a wide range of habitats that are not characteristic of natural river ecosystems. Because of the subsequent over-abundance of the eurytopic species the original longitudinal fish zonations are hardly recognizable anymore in heavily impacted large rivers such as the River Rhine. Hence these rivers do not meet the criteria for ecological integrity. Within a specific fish region, a suitable way of analyzing and monitoring the impact of human disturbance on the structure of the fish community is by comparing the guild structure of the present state of a fish zone with that of the reference situation.