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 de velopment, during which there may be a high mortality rate, so determi ning the Long-term success of the year class. The mechanism(s) underly ing this phenomenon are not fully understood but physiological stress may be a contributory factor. This study sought to establish at what p oint in their development, posthatch, salmonids become stress responsi ve in terms of activation of the hypothalamic-pituitary-interrenal (HP I) axis. Immunoreactive corticosteroids were measured in tissue extrac ts of rainbow trout at intervals from 4 weeks prehatch to 27 weeks pos thatch and in brown trout from 5 to 22 weeks posthatch. Corticosteroid s 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 corti costeroids. 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 a xis to be present. An ectoparasite (Ichthyobodo sp.) infestation durin g this period caused a substantial increase in IRC levels in both spec ies indicating that the HPI axis in these fish was sensitive to natura lly occurring stressors. The response of fry to acute and chronic stre ss resembled that of adult fish in terms of duration. These data indic ate that the early pre- and postemergent stages of salmonid fish are l ikely to be able to benefit from the adaptive advantage of HPI activat ion during episodes of acute stress but may also be susceptible to the deleterious consequences of prolonged activation of the HPI axis unde r conditions of chronic stress. (C) 1994 Academic Press, Inc.