Assessing the Thermal Tolerance of Apache Trout at Critical Life Stages

Apache trout Oncorhynchus gilae apache were placed in different static and fluctuating temperature regimes to determine how they affected survival.  Using a recirculating water system, we tested post-hatch survival of Apache trout eggs in static temperatures of 15, 18, 21, 24, and 27°C, and also temperatures that fluctuated ±3°C sinusoidally from 15, 18, 21, 24 and 27°C.  We determined the LT50 of Apache trout eggs to be 17.1°C for static temperatures treatments and 17.9°C for ±3°C fluctuating treatments.  There was no significant difference in survival between static temperatures and fluctuating temperatures that shared the same mean temperature.  Using the same recirculating water system, growth and survival of Apache trout fry was assessed at 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.  We determined the LT50 for Apache trout fry under static conditions to be 22.9°C, 23.1°C under ±3°C fluctuations and 22.9°C under ±6°C fluctuations.  There was no significant difference in thermal tolerance between static treatments and fluctuating treatments with the same midpoints.  Treatments that had an average daily temperature ≥22°C or that fluctuated above 22°C for parts of the day had significantly lower growth than lower temperature treatments.  Streams that contain Apache trout should have mean summer stream temperatures below 22^oC to support fish, and spring temperatures below 15^oC to ensure maximum hatch rates and post-hatch survival.