The corticosteroidogenic response of brown and rainbow trout alevins and fry to environmental stress during a "critical period".

Postemergent salmonid fry experience a "critical period" in their development, during which there may be a high mortality rate, so determining the long-term success of the year class. The mechanism(s) underlying this phenomenon are not fully understood but physiological stress may be a contributory factor. This study sought to establish at what point in their development, posthatch, salmonids become stress responsive in terms of activation of the hypothalamic-pituitary-interrenal (HPI) axis. Immunoreactive corticosteroids were measured in tissue extracts of rainbow trout at intervals from 4 weeks prehatch to 27 weeks posthatch and in brown trout from 5 to 22 weeks posthatch. Corticosteroids were detectable throughout this period in both species. Simultaneous determination of whole-body immunoreactive corticosteroid levels (IRC) and plasma cortisol levels in fingerling trout validated the use of whole-body levels as an indicator of stress-induced elevation of corticosteroids. However, the dynamics of the circulating cortisol response to stress were not fully conveyed by whole-body measurements. From 5 weeks posthatch IRC increased significantly within 30-60 min following mechanical disturbance and confinement, suggesting a functional HPI axis to be present. An ectoparasite (Ichthyobodo sp.) infestation during this period caused a substantial increase in IRC levels in both species indicating that the HPI axis in these fish was sensitive to naturally occurring stressors. The response of fry to acute and chronic stress resembled that of adult fish in terms of duration. These data indicate that the early pre- and postemergent stages of salmonid fish are likely to be able to benefit from the adaptive advantage of HPI activation during episodes of acute stress but may also be susceptible to the deleterious consequences of prolonged activation of the HPI axis under conditions of chronic stress.