Costs of rapid growth: the risk of aggression is higher for fast-growing salmon

1. Agonistic behaviour can be an important trait in determining individual success under interference competition, but it also generates energetic and physiological costs. The ensuing trade-off is expected to be dependent on the individual state. This study tests whether aggression patterns of juven ile Atlantic Salmon Salmo salar are linked to the recent growth history and the developmental pathway adopted by fish (early vs late migrants). 2. By manipulating growth rates over 1 month in the autumn, three groups of fish were produced differing in body size and thereby in the length increm ent necessary to achieve the threshold size for migration in the following spring. Fish that had experienced reduced rations or abnormally low tempera tures during the manipulation period experienced 'catch-up' growth after th e resumption of standard growth conditions. 3. Aggression was strongly connected to growth bimodality and the underlyin g developmental polyphenism. Fast-growing (upper modal group, UMG) fish (wh ich would migrate in the spring) were more aggressive than slow-growing (lo wer modal group, LMG) fish (which would delay migration for another year), but they were also more vulnerable to being attacked by conspecifics. 4. Aggressive acts initiated by UMG fish were strongly biased towards fish of the same category (57-77% of aggressive acts being against other UMG fis h). Under illumination conditions simulating overcast nights, UMG individua ls sharply reduced their aggressiveness and were not selective in their att acks. The aggression by LMG fish was more evenly distributed between modal groups (44-74% of attacks were against UMG) regardless of light levels. 5. Recent growth history had no consistent effect on aggression rates, alth ough under overcast night conditions the rate of attacks received by non-ma nipulated UMG fish was twice as high as that for fish undergoing compensato ry growth. 6. These results demonstrate a link between developmental pathways and aggr ession patterns. They also show that individuals maintaining a fast growth strategy over the winter experience a more aggressive social environment th an individuals adjusting feeding rates to ensure safe maintenance levels. T he reduced involvement of LMG fish in agonistic encounters conforms to a st rategy of avoidance of those energetic and physiological costs that ultimat ely could compromise overwintering survival.