Primary and secondary indices of stress in the progeny of rainbow trout ( Oncorhynchus mykiss ) selected for high and low responsiveness to stress

A series of pooled gamete matings was carried out employing eggs and milt from mature male and female rainbow trout selected for a consistently high‐ or low‐responsiveness to stress, as indicated by post‐stress plasma cortisol elevation. Development of the progeny was closely monitored and the responsiveness to stress of the progeny of high‐responding parents and the progeny of low‐responding parents was assessed by two methods. For a period of 14 months, at approximately monthly intervals, the plasma cortisol elevation evoked by a standardized confinement stress was determined in fish from each group, and secondly, on one occasion, the time‐course of the plasma cortisol response to a 24‐h period of confinement was monitored. Progeny of high‐responding parents snowed a significantly greater cortisol response to stress than the progeny of low‐responding parents during both testing procedures. However, when the effect of a 14‐day confinement stress was examined, high‐responding progeny showed a more rapid recovery of plasma cortisol levels, while levels in the low‐responding progeny, although initially lower, showed a more sustained elevation. To assess the possible functional implications of these observations, circulating lymphocyte numbers, an immunologically important cortisol‐sensitive component of the blood cell complement, were determined. The duration of the lymphocytopenia observed following the onset of confinement was found to be related to the initial, not the sustained, cortisol response. These data suggest that manipulation of the sensitivity to stress of fish is feasible by selective breeding, but that careful.choice of the indices employed to identify traits considered desirable is necessary.