The early life history stages of riverine fish: ecophysiological and environmental bottlenecks.

Fish are good indicators of the environmental health of rivers and their catchments as well as important conservation targets. Bioindication has to be based on an understanding of the requirement of characteristic species with regard to: (a). The match/mismatch between reproductive strategies and environmental conditions. (b). The niche dimensions of critical stages vis-a-vis the key conditions. (c). The availability of microhabitats along the ontogenetic niche profiles, i.e. the connectivity from spawning substrates to early life history microhabitats. The main conditions for the embryonic period are temperature and oxygen supply which are responsible for embryonic mortality, the duration of the period, and size and condition of newly-hatched larvae. For the exogenously feeding larvae the functional of food acquisition, growth and bioenergetics to temperature, food availability and current velocity is decisive. Studies concentrated on Chondrostoma nasus, a target species for monitoring and conservation in large European river systems. Results obtained in experimental studies are compared with those from field studies in order to evaluate the match/mismatch between performances and microhabitat choice and population dynamics in the field. Discrepancies between requirements and field conditions in regulated rivers underline the significance of inshore zones as microhabitats (expressed in the 'Inshore Retention Concept') and the requirement for ecophysiological studies on target species for river restoration and conservation.