Critical Thermal Limits and Sensitivity in Developing Endangered California Fishes

Increases in water temperature due to climate change are of great concern for managers of the California Bay-Delta Estuary. Already, several native fishes in the Bay-Delta system have been in decline; however, it remains unclear how added temperature stress may contribute to these declines. The endangered Delta Smelt (Hypomesus transpacificus) and threatened Longfin Smelt (Spirinchus thaleichthys) are of specific concern in California. In efforts to predict how temperature may impact these fishes, we measured Critical Thermal Maxima (CTMax) and oxygen consumption rate (MO₂) at 14ºC and 20ºC in both species at 50-days post hatch, a vulnerable developmental stage. Delta Smelt had a greater CTMax at 27.6ºC compared to Longfin at 24.8ºC. MO₂ was greater in Delta Smelt exposed to 20ºC compared to the 14ºC handling control, reflecting an increased energetic cost at warmer temperatures. Longfin Smelt MO₂ was similar at 20ºC and 14ºC, suggesting these fish may have limited capacity to adjust their physiology, or there is a mismatch between oxygen demand and supply at this early developmental stage. Understanding the thermal limits and physiological responses to increases in temperature can help identify the vulnerability of our native, ecologically important fishes to predicted future increases in water temperature.