Ontogenetic patterns in thermal adaptation of fish vs. long-term temperature trends in large rivers

Abstract

Rheophilic fish species characteristic of large river systems are finely tuned during early ontogenetic development to the seasonal temperature regime of their main nursery habitats in the inshore zones of rivers. River regulation and the construction of hydropower dams have disrupted the balance between requirements and the field conditions. Frequently, temperatures fall below the optimal range in regulated rivers due to faster runoff and reduced inshore retention capacity. Under such conditions, growth through the “critical period” during the life history is retarded resulting in high mortality rates. This is a main reason for the decline of stocks and a critical state of recruitment of many riverine species. The trends caused by long-term climatic changes have to be similarly addressed from the point of view of match or mismatch between requirements and the predicted changes in environmental conditions. Long-term trends in the temperature development have been analyzed for the Austrian Danube. The potential effect of such trends is tested against the temperature dependence of embryogenesis and a growth model developed for one of our target species, the cyprinid Chondrostoma nasus. Field data on individual daily growth rates of larval fish, based on otolith analysis, show that the present conditions are critical. The consequences of global change for rheophilic fish species in large river systems are discussed.