Plasma cortisol response to stress in juvenile rainbow trout is influenced by their life history during early development and by egg cortisol content.

In this study, the consequences of the exposure of rainbow trout (Oncorhynchus mykiss) to a brief stress during early development were explored on the later response of fingerlings to stress. Firstly, we analyzed the ontogeny of cortisol production and that of the initial cortisol response to stress in developing fish. It is only at the eyed stage that the embryos started to produce some basal cortisol. The HPI (hypothalamic-pituitary-interrenal axis) was however not functional before hatching, as exposure of the embryos to a stress did not trigger any cortisol response. A cortisol response to an acute stress was detected 9 days after hatching. In a second set of experiments, we showed that a very brief stress applied at 3 different early stages (eyed, hatching, and yolk resorption) reduced the later cortisol response to stress of 5-month-old fingerlings. This reduction is not likely to be due to alterations in the fish interrenal sensitivity because the 5-month-old fingerlings responded to ACTH treatment (only one dose tested) within the same magnitude as the fish that were not stressed during early development. An experimentally induced increase in egg cortisol just after fertilization also induced a reduction in stress sensitivity of 5 month old fingerlings, which was dose-dependent This study shows for the first time that the responsiveness of the corticotrope axis in 5-6 months old rainbow trout was influenced both by early stress exposure and by initial egg cortisol levels. Whether the HPI was functional or not at the time the initial stress was applied only had a small influence on the later unambiguous effect of early stress exposure.