Growth hormone-induced effects on mortality, energy status and growth: a field study on Brown Trout (Salmo trutta)

1. Growth hormone (GH) treatment increases the growth rate and competitive ability of salmonids under laboratory conditions. Since fast growth should increase fitness, why is endogenous secretion of GH not higher in wild fish ? To address this question, three hypotheses were suggested. H-1: high GH l evels reduce antipredator responses and may therefore increase mortality fr om predation. H-2: high GH levels reduce longterm (e.g. over winter) surviv al by reducing allocation to critical energy reserves. H-3: GH is not benef icial for growth under natural conditions. 2, To test these hypotheses, the performance of GH-treated juvenile Brown T rout (Salmo trutta) and control (placebo) trout was compared in an enclosed stream section subjected to natural predation. Four experiments were condu cted during winter, spring, summer and autumn, respectively. 3, Mortality rates were not significantly different between GH-treated and control trout in any of the four experiments so H-1 was not supported. Ener gy reserves were generally lower in GH-treated fish, which is consistent wi th H-2, whereas growth rates in mass were higher in GH-treated fish than in controls so H-3 was not supported. This suggests that GH promotes growth a t the expense of investment in maintenance. 4. Judging from growth and mortality rates, the fitness of GH-treated and c ontrol trout appeared similar. Thus, escaped GH-manipulated fish may compet e successfully with wild fish. 5. Hatchery-raised trout with higher initial condition index suffered highe r mortality rates than more slender fish. This novel finding may be explain ed by reduced escape ability related to body morphology, reduced behavioura l responses towards predators by high-condition trout, or predator preferen ces for high-condition fish.