1. Introduction
Along the river course, characteristic fish associations are adapted to prevailing
temperature regimes. The longitudinal thermal profile of rivers resembles to some extent
the latitudinal geographical pattern. Local success of species is dependent on the match or
mismatch between requirements and field conditions during the early life history, which is
the critical phase, the bottleneck, deciding on success or failure of a species. The fine
tuning between requirements and environmental conditions has been considerably
disrupted by river engineering (dams, reduction of inshore retention [1]). Global warming
is a further factor which has to be taken into account.
The study analyzes (a) the thermal ranges and ontogenetic niche shifts of
characteristic species during the early life history, (b) its optima and constraints with
regard to temperature, (c) identifies long-term trends in the temperature development of
the Danube river and (d) analyzes the potential effects of such trends on characteristic
fish species.