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 stimulat ion (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). Activ ity in MIS was correlated with swimming activity in the stream-water aquari um. The MIS results could not be used for predicting any social behaviour p atterns or the growth performance of a fish. No behavioural differences bet ween the two strains were noted. However, the sea-ranched strain grew faste r than the wild one, both in regard to the RNA/DNA ratio and the weight-spe cific 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 fas ter juvenile growth in brown trout. The ultimate mechanisms underlying the faster growth by the domesticated strain probably involves both contest and scramble competition.