Growth enhanced brown trout show increased movement activity in the wild

Rapid growth is often associated with several fitness-related benefits. However, in most organisms growth rates are rarely maximized, suggesting that trade-offs limit the benefits of rapid growth. To enable sustained high growth, behavioural modifications incurring costs may be required, and these may be responsible for evolution of sub-maximal growth rates. In a field experiment it was tested whether rapid growth in brown trout is achieved by increasing potentially costly movement activity. Causal relations were obtained by manipulating the growth rate using bovine growth hormone (GH) implants and monitoring movement activity of stream-dwelling brown trout for two months using radiotelemetry. GH-treated trout grew significantly faster than sham-treated trout both in terms of body mass and length. The GH-treated trout also had a significantly larger absolute movement during the experiment, moving about 68% more than the sham-treated trout. There were no GH effects on diel movement or home range between the two treatments. Plasma GH levels were lower in the GH-treated fish, indicating negative feedback inhibition of native GH secretion, and that the bovine GH implant was still functioning at the end of the study. This is the first study to show increased movement activity in GH-enhanced fish in the wild. Because increased movement is expected to lead to increased metabolic costs, the higher movement activity in the GH-treated trout was likely associated with increased foraging activity. The results demonstrate that increased movement activity is associated with rapid growth in brown trout. Such movement activity may be unfavourable under certain ecological conditions or life stages (i.e. high risk of predation or low resource levels), and may thus represent one component of the cost limiting evolution of maximum growth rates.