Kin-biased territory overlap and food sharing among Atlantic salmon juveniles

1. Based on kin selection theory, it has been predicted that close relatives should aggregate to accrue inclusive fitness advantages. However, studies of genetics have failed to find convincing evidence of kin association in wild populations of territorial fish such as juvenile salmonids, despite laboratory evidence of reduced aggression among siblings. In these genetic studies, calculations of the degree of kin association have been based on the assumption that the geographical distribution of individual fish is fixed over space and time. By comparison, recent work suggests that patterns of spatial distribution are fluid and that home ranges overlap considerably. Therefore, it is possible that benefits of kin selection depend not on the juxtaposition of fish per se, but on how they interact on the occasions when they associate. 2. To explore this issue,, an experiment was conducted to test whether dominant fish in pairs of Atlantic salmon Salmo salar (L.) juveniles were more likely to share food in high quality feeding patches with kin than non-kin and also whether kin-biased intimidation from dominant to subordinate fish was sustained in the absence of the dominant individual. 3. Siblings shared high-quality feeding territories for twice as long as pairs of non-kin. The food intake of dominant fish did not vary with relatedness of subordinate conspecific. However, once dominant fish were satiated, subordinate individuals consumed a greater proportion of food items when paired with kin than non-kin. 4. When dominant fish were removed from half the pairs of salmon, subordinate fish that had been paired previously with dominant kin increased their feeding activity to greater levels than subordinate fish that had been paired previously with dominant non-kin. 5. These findings indicate that subordinate fish in wild populations may gain twofold foraging advantages by close association with dominant kin: (i) by increased territory sharing, and (ii) by reduced time for food patch acquisition when dominant fish move from patches shared with subordinates. 6. We suggest that in territorial animals, the acquisition of kin selection benefits need not be dependent upon the fixed association of kin in adjacent territories, but can be accrued during occasional associations within a more fluid pattern of overlapping home ranges.