Effects of simulated high-magnitude floods on first-feeding salmonids at high density in a seminatural environment

Abstract

• •
  We simulated spring floods to assess how they affected predator–prey interactions.
• •
  Trophic availability was low in early spring and increased later in the season.
• •
  Flooding reduced salmonid survival in early spring and growth in late spring.
• •
  In low-productivity systems, alevins were highly active and aggressive.
• •
  In high-productivity systems, alevins were highly active after flooding to feed.

Climate change alters the 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, 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 predator–prey dynamics by desynchronizing alevin emergence and macroinvertebrate availability. In a seminatural channel, we quantified macroinvertebrate abundance, monitored survival, growth, behaviour and standard metabolic rates (SMR) of just-emerged alevins at high density (32 alevins/m²) in control reaches and in reaches previously impacted by a simulated high-magnitude flood. The experiment was repeated in early and late spring. In early spring, system productivity was low and macroinvertebrate abundance was consistently low. Alevin survival was reduced by the flood. Alevins were highly active and aggressive regardless of treatment, probably driven by low trophic availability. In late spring, productivity increased. The flood significantly reduced macroinvertebrate abundance but high system productivity quickly compensated for this reduction. The simulated flood reduced alevin growth but did not affect survival. After the flood, alevins were active, probably to maintain a constant food supply. There were no differences in SMR between alevins in the control reach, whereas dominants displayed lower SMR than subordinates in the flooded reach. These subordinates were probably exposed to intense stress due to a lack of resources. Our experimental study helps to improve understanding of the behavioural and physiological processes driving survival and growth in salmonid early life stages.

Climate change alters the 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, 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 predator–prey dynamics by desynchronizing alevin emergence and macroinvertebrate availability. In a seminatural channel, we quantified macroinvertebrate abundance, monitored survival, growth, behaviour and standard metabolic rates (SMR) of just-emerged alevins at high density (32 alevins/m²) in control reaches and in reaches previously impacted by a simulated high-magnitude flood. The experiment was repeated in early and late spring. In early spring, system productivity was low and macroinvertebrate abundance was consistently low. Alevin survival was reduced by the flood. Alevins were highly active and aggressive regardless of treatment, probably driven by low trophic availability. In late spring, productivity increased. The flood significantly reduced macroinvertebrate abundance but high system productivity quickly compensated for this reduction. The simulated flood reduced alevin growth but did not affect survival. After the flood, alevins were active, probably to maintain a constant food supply. There were no differences in SMR between alevins in the control reach, whereas dominants displayed lower SMR than subordinates in the flooded reach. These subordinates were probably exposed to intense stress due to a lack of resources. Our experimental study helps to improve understanding of the behavioural and physiological processes driving survival and growth in salmonid early life stages.