An examination of the effects of chronic static and fluctuating temperature on the growth and survival of spikedace, Meda fulgida, with implications for management

Stream temperatures are on the rise in small streams in southern Arizona. Although, the biology of the spikedace, Meda fulgida, has been relatively well studied, little information is known about the thermal tolerance of this species and how rising temperatures impact its survival. We used the chronic lethal method and the acclimated chronic exposure (ACE) method, for both static and fluctuating temperatures, to estimate the thermal tolerance of spikedace. All three methods utilize a slow acclimation period, which mimics the natural environment more successfully than traditional thermal methods. These methods also allow for the evaluation of sub-lethal effects of temperature such as slowed growth and susceptibility to disease. The ACE method using static temperatures provided the most conservative estimate of thermal tolerance. The estimated 30 d LT50 was 32.1 °C. Fish exposed to the 32 °C treatment experienced only 37% of the growth experienced by control fish. Multiple behavioral and physiological changes, indicative of stress, were noted at 30, 32 and 33 °C treatments. A modified ACE method allowed us to incorporate temperature fluctuations ranging from 4 to 10 °C. We observed 96.5% survival for fish exposed to the control (25 °C) treatment. Similar survival rates were observed for after 30 d exposure to a 24-34 °C fluctuation. We saw a significant decrease in survival for fish exposed to the 28-34 °C (mean survival=79.2%) and 30-34 °C fluctuation (mean survival=73.3%). Fish held at both the 24-34 °C and 30-34 °C fluctuations showed significantly lesser growth than the control treatment. Under natural conditions, tolerance may be affected by synergistic interactions with other environmental stressors such as predation and poor water quality. Determination of thermal tolerance for native fishes is critical to estimate available habitat and may provide insight into reasons for their decline.