Fish age, instead of weight and size, as a determining factor for time course differences in cortisol response to stress

In many vertebrate species, including humans, the developmental stage directly influences an organism's reactivity to stress. For instance, fishes appear to exhibit stress insensitive periods early in development, which contributes to important alterations in stress responses. This phenomenon raises the important question of whether size or age influences fish stress responses, as there may be large discrepancies in size at any stage of ontogeny. We therefore posed the following question: what key role does the age and/or weight/length of fish play in contributing to different stress levels? To address this question, we conducted 2 simple experiments to compare the cortisol response to stress in the fish Rhamdia quelen. In the first experiment, we compared the cortisol response of fish of 2 different age groups (3 vs. 12 months) with the same body size, whereas in the second experiment we compared 2 groups of the same age (3 months) but with different body sizes. The results showed a similar stress response in fish of the same age but different size, but a large difference in the stress response in fish of the same size but different age. Both tests indicate that age is the determining factor for the functioning of the hypothalamo-pituitary-interrenal (HPI) axis, and is probably related to the stage of maturation. This study makes a significant contribution to our knowledge of the stress, behavior, and welfare of fish of different age classes, primarily with respect to the timing of measurements and the accurate determination of fish age, regardless of size. (C) 2012 Elsevier Inc. All rights reserved.