Does increasing habitat complexity favour particular personality types of juvenile Atlantic salmon, Salmo salar?

The costs and benefits of a particular behavioural trait, such as boldness or aggression, may vary depending on the physical environment. We tested whether the common practice of adding physical structure (i.e. boulders) to streams to increase salmonid density has behavioural consequences, as open habitats are predicted to favour individuals that are more bold and aggressive. Wild young-of-the-year Atlantic salmon were captured from habitats of varying physical complexity and placed into seminatural stream enclosures for 11 days while their behaviour was observed and tested in both open and structurally complex environments. We found evidence for personality, or consistent individual behavioural differences across contexts, for avoidance and site attachment, with repeatabilities of 0.287 and 0.206, respectively, but not for activity or frequency of aggression. Fish were significantly more active and aggressive in the open habitats, and more site-attached in the complex habitats. Active and aggressive fish also grew more in the wild, while site-attached fish grew less in the wild, but more in the enclosures. However, contrary to our expectation, the complexity of the original habitat was not a significant predictor of personality. Our results suggest stream restorations involving increasing habitat complexity will alter the behaviour of young-of-the-year Atlantic salmon, but will not favour any particular personality types.