Contrasting effects of a catastrophic flood on survival, growth and behaviour of first-feeding salmonids at high density throughout spring in a semi-natural environment

Climate change alters thermal and hydrological regimes of rivers, increasing both water temperature and extreme event frequency. In spring, temperature regulates the timing of alevin emergence in brown trout (Salmo trutta L.) as well as macroinvertebrate abundance, which are the main food items for just-emerged alevins. In parallel, severe floods drastically reduce macroinvertebrate abundance and diversity. Consequently, climate change may disrupt prey-predator dynamics by desynchronising alevin emergence and macroinvertebrate availability. In a semi-natural channel, we quantified macroinvertebrate abundances, monitored survival, growth, behaviours and standard metabolic rates (SMR) of just-emerged alevins at high density (32 alevins m-2) in control reaches and in reaches previously impacted by a catastrophic flood. The experiment was repeated in early and late spring. In early spring, macroinvertebrate abundance was globally low. Survival of alevins was reduced by the occurrence of the flood (61% vs. 37% in control and flooded reaches, respectively). Alevins were very active and aggressive whatever the treatment, probably driven by a globally low trophic availability. In late spring, productivity was boosted. The flood significantly reduced macroinvertebrate abundance and alevin growth (1.22 vs. 0.24) but did not affect the alevin survival. After the flood, alevins were active, probably to maintain a constant food supply. There were no differences in SMR between alevins in control reach, while the dominants displayed lower SMR than the subordinates in the flooded reach. Our experimental study helps to understand the behavioural and physiological processes underlying the mechanisms driving both survival and growth of salmonid early life stages.