Stress and Social Hierarchy Rank in Coho Salmon

Dominance hierarchy was monitored over a 2-week period in triplicated groups of six parr of coho salmon (Oncorhynchus kisutch) by recording a variety of behavioral activities. Indices of stress were subsequently determined and compared with dominance statuses. Social position fell into three main categories: (1) the dominant fish; (2) two or three intermediate fish; and (3) two or three subordinate fish. Plasma cortisol concentration, interrenal nuclear diameter, and tail-beat frequency were lowest in the dominant fish and highest in the subordinate fish. Fish held alone had levels of plasma cortisol, interrenal nuclear size, and tail-beat frequency similar to those of the dominant fish. Hepatic glycogen varied directly with social position. It is plausible that dominance status is inversely related to a low-level, chronic state of stress in the fish, the dominant individuals being the least stressed.