Condition dependent intra-individual repeatability of stress-induced cortisol in a freshwater fish.

The glucocorticoid (GC) stress response is thought to be an individual trait associated with behaviour and life history strategies. Studies exploring such relationships typically assume measured hormone values to be repeatable within an individual. However, repeatability of GCs has proven variable in wild animals and underlying reasons remain unknown. We assessed individual repeatability of circulating stress-induced cortisol, the primary GC in teleost fish, and glucose concentrations in a wild teleost fish held under consistent laboratory conditions. We also tested the hypothesis that the magnitude of intra-individual variability in stress-induced cortisol concentrations ("cortisol variability") is influenced by body condition. Wild-caught bluegill sunfish (Lepomis macrochirus) were subjected to repeated standardized stressors and blood sampled (3 times over 6 days) once cortisol concentrations peaked. Various indicators of fish condition, both whole body and physiological, were also measured. Overall, stress-induced circulating cortisol concentrations were repeatable but stress-induced glucose was not. Cortisol variability was related to Fulton's condition factor and size (eviscerated mass) where smaller fish in poor condition exhibited increased cortisol variability. The findings have implications for the interpretation of studies that examine correlates of GC concentrations as they suggest consistency in stress responsiveness is influenced by factors such as size and condition.