Both contest and scramble competition affect the growth performance of brown trout, Salmo trutta, parr of wild and of sea-ranched origins

The effect of contest and scramble competition on the growth performance of wild and sea-ranched juvenile (0+) brown trout, Salmo trutta, originating from the River Dalalven, Sweden was scrutinised. In a mirror image stimulation (MIS) experiment, and in a 35 000 1 stream-water aquarium the trout was studied for three weeks (20 individuals in each of four replicates). Activity in MIS was correlated with swimming activity in the stream-water aquarium. The MIS results could not be used for predicting any social behaviour patterns or the growth performance of a fish. No behavioural differences between the two strains were noted. However, the sea-ranched strain grew faster than the wild one, both in regard to the RNA/DNA ratio and the weight-specific growth rate. Because the strains had the same genetic background and prior to the experiments were raised under similar hatchery condition, the results of this study suggest that the sea-ranching process selects for faster juvenile growth in brown trout. The ultimate mechanisms underlying the faster growth by the domesticated strain probably involves both contest and scramble competition.