Variation in aggressive behaviour and growth rate between populations and migratory forms in the brown trout, Salmo trutta

Aggressiveness of juvenile salmonid populations has been suggested to correlate positively with the time the fish spend in the stream. Consequently, resident populations are expected to be more aggressive than migratory populations. Aggressiveness and growth rate have been found to correlate positively at the individual level, but no studies have compared populations. We studied variation in aggressiveness and growth in 10 Finnish brown trout populations differing in their migratory behaviour (sea-run, lake-run and resident). Contrary to expectations, we found the sea-run populations to be more aggressive than the lake-run and resident populations. As all the study fish were reared under similar conditions, it is likely that the differences in aggression have a genetic basis. We also found a positive correlation between aggression and growth rate among the populations. This result supports earlier findings of a positive connection between aggressiveness and growth rate, but is, to our knowledge, the first time this phenomenon has been observed at the population level. (C) 2001 The Association for the Study of Animal Behaviour.