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 juvenile 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 increment necessary to achieve the threshold size for migration in the following spring. Fish that had experienced reduced rations or abnormally low temperatures during the manipulation period experienced 'catch-up' growth after the resumption of standard growth conditions. 3. Aggression was strongly connected to growth bimodality and the underlying developmental polyphenism. Fast-growing (upper modal group, UMG) fish (which would migrate in the spring) were more aggressive than slow-growing (lower 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 fish). Under illumination conditions simulating overcast nights, UMG individuals sharply reduced their aggressiveness and were not selective in their attacks. 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, although under overcast night conditions the rate of attacks received by non-manipulated UMG fish was twice as high as that for fish undergoing compensatory growth. 6. These results demonstrate a link between developmental pathways and aggression patterns. They also show that individuals maintaining a fast growth strategy over the winter experience a more aggressive social environment than individuals adjusting feeding rates to ensure safe maintenance levels. The reduced involvement of LMG fish in agonistic encounters conforms to a strategy of avoidance of those energetic and physiological costs that ultimately could compromise overwintering survival.