Dominance hierarchies in Arctic charr,Salvelinus alpinusL.: differential cortisol profiles of dominant and subordinate individuals after handling stress

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Abstract
We examined the effect of social rank on plasma cortisol dynamics after handling stress in juvenile Arctic charr, Salvelinus alpinus L., reared in groups of 200 individuals in 1.5 m3 circular tanks (rearing density 0.022 kg L–1). Fish obtained by dip-netting were categorized as either socially subordinate, dominant or as holding an intermediate position on the basis of size, skin coloration and occurrence of bite marks. In undisturbed fish, the highest cortisol levels were found in fish ranked as intermediate and the lowest in dominant fish. In contrast to what has been found under conditions of artificial rearing in small groups (two to ten individuals), cortisol was not significantly elevated in subordinates compared with dominant individuals. In small groups of fish, aggressive interactions and restricted access to food may be significant factors leading to increased stress in socially subordinate individuals. Following stress by dip-netting and transfer, the quickest and largest cortisol response was seen in dominant individuals. These results suggest that hypothalamus–pituitary–inter-renal/adrenal axis responsiveness as well as baseline cortisol production is influenced by social rank in fish, as has been shown in other vertebrates.