Relatedness and body size influence territorial behaviour in Salmo salar juveniles in the wild

Wild Atlantic salmon, Salmo salar L., juveniles defend territories to enable exclusive access to food resources, and kin selection benefits may be accrued where territorial boundaries of relatives overlap. This study explored space sharing events between pairs of sibling and non-sibling fish as a measure of territoriality and resource competition in a small chalk stream using passive integrated transponder (PIT) technology. The time period between fish detections in a shared space was closer between pairs of siblings (sibling pairs mean = 60.48 ± 51.84 min; non-sibling pairs mean = 348.8 ±65.94 min). These results suggest that the territorial boundaries of related fish often overlap, thus increasing the likelihood of siblings accruing kin selection benefits. The findings from this study also suggest that outcomes of competitive interactions among dominant and subordinate fish are less pronounced when fish are related.