The influence of social interaction on the acclimation of rainbow trout, Oncorhynchus mykiss (Walbaum) to chronic stress

To establish the time required for fish to acclimate to experimental conditions, comprising holding within a spatially restrictive environment, rainbow trout were transferred from rearing tanks (1500 1) to smaller volume holding tanks (50 1). The fish were maintained singly, in pairs, in groups of five fish, and in groups of 10 fish, and selected indices of physiological stress were monitored at intervals following the onset of confinement. In all cases transfer and confinement provoked a stress response in the fish, indicated by significantly elevated plasma cortisol levels, and significantly reduced numbers of circulating lymphocytes. Recovery of these parameters to levels similar to unstressed control fish, was most rapid among the fish confined singly, requiring 2 to 4 weeks, although weight gain, indicative of feeding, was apparent within 1 to 2 weeks. Acclimation of the fish confined as pairs was severely confounded by the development of hierarchical relationships between the fish. This resulted in some individuals displaying rapid acclimation and growth, in contrast to other fish within the groups which failed to acclimate, exhibited signs of chronic stress, and ultimately succumbed to bacterial infections. Similar, though less severe, evidence of social stress was observed among the fish confined as groups of five. A decline in the coefficient of variance for weight within the fish confined as groups of 10 suggested an absence of growth inhibition due to social interaction among these fish. The implications of these results for experimental design are discussed.