Cooperative Fish and Wildlife Research Units Program: Arizona
Education, Research and Technical Assistance for Managing Our Natural Resources

Increased stream temperatures through global climate change and urbanization will have important implications for fishes worldwide. While there is some information on the effects of elevated water temperatures on Apache trout Oncorhynchus Apache, less is known about effects of increased and fluctuating water temperatures over extended time periods, typical of Southwestern desert streams. Therefore, we applied static and fluctuating temperature regimes to determine how each affected the growth and survival of Apache trout. Using a recirculating water system, we tested static temperatures of 16, 19, 22, 25, and 28°C, temperatures that fluctuated +3°C from 16, 19, 22 and 25°C, as well as temperatures that fluctuated +6°C from 19 and 22°C. After a 14 day acclimation at 16°C, 12 days of temperature ramping and 30 day test trials, we determined the LT50 as well as mean growth for each temperature treatment. Results showed the LT50 for Apache trout under static conditions to be 22.9°C, 23.4°C under +3°C fluctuations and 22.9°C under + 6°C fluctuations. Growth decreased as temperatures approached the LT50. When comparing static conditions with fluctuating conditions with the same midpoint, growth was less under fluctuating conditions than under static conditions if the temperature fluctuation approached the thermal limit (CTMax). Reduced survival of individual fish, inhibited growth and changes in fish behavior caused by prolonged increased stream temperature will further affect the plight of the species. Therefore temperature tolerance information is critical to those restoring streams for Apache trout and in identifying new stocking locations.