GROWTH-HORMONE INCREASES GROWTH-RATE, APPETITE AND DOMINANCE IN JUVENILE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

The hypothesis that exogenous growth hormone elevates feeding motivation, and thereby increases dominance status in juvenile rainbow trout was tested by comparing the feeding behaviour of growth hormone-treated and control (vehicle-injected) trout in two experimental series. Growth rate, food conversion and hypo-osmoregulatory ability of the two groups were also compared. During a 4-day treatment period, specific growth rate was significantly higher in growth hormone-injected trout than in control fish. Further, growth hormone-treated fish had significantly greater hypo-osmoregulatory ability than control fish. Satiation feeding experiments on individual trout revealed a considerably higher appetite in growth hormone-injected trout, which ate twice as much as control fish over the 2 days of feeding. Also, food conversion efficiency was higher in growth hormone-treated fish than in control fish. Feeding competition trials involving similar-sized matched pairs of growth hormone-treated and control trout revealed that growth hormone-treated juveniles were significantly dominant over control fish when the pairs were selected before the 4-day hormonal treatment but in a second dominance experiment, where growth hormone-treated and control fish were paired after the growth hormone treatment, no significant difference was found between the treatment groups. Overall, the results support the hypothesis that exogenous growth hormone can increase appetite and dominance in juvenile rainbow trout. It is suggested that the endogenous production of growth hormone in rainbow trout is limited by mortality costs associated with maximal growth rates. © 1993 The Association for the Study of Animal Behaviour.