Physiological effects of dominance hierarchies within groups of brown trout, Salmo trutta, held under simulated natural conditions

While the existence of dominance hierarchies within natural populations of salmonids is well known, little is known about the physiological consequenc es of these social interactions. To investigate such physiological effects, replicate groups of four brown trout (Salmo trutta) were held under simula ted natural conditions in an artificial stream tank. Behavioural observatio ns allowed the fish to be ranked for dominance. After two weeks, physiologi cal status was assessed through measurements of specific growth rate, condi tion factor, plasma cortisol and ion concentrations, haematocrit, leucocrit , hepatosomatic index, hepatic glycogen concentration, interrenal cell nucl ear area and gill epithelium chloride cell density. Weight gain in the firs t-ranking (dominant) fish was significantly higher than in the second-ranki ng fish. In addition, the condition factor of the second-ranking fish decre ased over the experimental period while those of the first- and third- rank ing fish increased, resulting in significant differences among the three gr oups. The only other physiological parameter which varied significantly amo ng the ranked fish was chloride cell density, which was significantly highe r in the second-ranking fish than in the dominant fish. Cortisol concentrat ions were low in all fish and did not vary significantly with dominance sta tus. Overall, the least beneficial position, in physiological terms, appear s to be the second rank in the dominance hierarchy.