Effect of confinement on water-borne and whole body cortisol in wild and captive-reared rainbowfish (Melanoteania duboulayi)

Whole body and water-borne cortisol levels were measured in captive reared and wild Rainbowfish (Melanoteania duboulayi Castelnau 1878) subjected to social isolation by confining them in a beaker for 30 min to induce an acute stress response. Wild fish had higher levels of cortisol before and after exposure to a mild stressor and also showed the greatest stress response. The differences in stress responses are likely the result of artificial selection in the captive environment. Importantly, there was a strong linear relationship between whole body and water-borne cortisol in wild and captive reared populations (r² =0. 95 and 0.84, respectively) suggesting that non- invasive assays for cortisol provide a valuable alternative for whole body cortisol levels in small fishes.