Stress responsiveness affects dominant-subordinate relationships in rainbow trout

The magnitude by which plasma cortisol levels increase following exposure t o a stressor is a heritable trait in rainbow trout. The relative growth in coculture of F1 lines selected for high responsiveness (HR) and low respons iveness (LR) to a confinement stressor suggested that behavioral characteri stics related to food acquisition, aggression, or competitive ability might differ between the two lines. This hypothesis was tested using the F2 gene ration of the selected lines. The F2 lines clearly exhibited the characteri stics of the F1 parents, displaying significantly divergent plasma cortisol responses to a 1-h confinement stressor and a high heritability for the tr ait. Behavioral differences between the lines were assessed by observing th e outcome of staged fights for dominance in size-matched pairs of HR and LR fish. The identification of dominant and subordinate fish within each pair on the basis of their behavior was supported by the levels of blood cortis ol in the fish attributed to each group (dominant << subordinate). Fish fro m the LR line were identified as dominant in significantly more trials than were HR individuals. The results suggest that behavioral attributes that a ffect the outcome of rank-order fights are closely linked to the magnitude of the plasma cortisol response to stress in rainbow trout. Whether the lin k is causal or circumstantial is not yet evident. (C) 2001 Academic Press.