Relatedness on the Landscape: Do Juvenile Steelhead Trout Establish Territories Near Their Siblings?

Kin selection has been used to explain a wide array of social behaviors across many taxa. Laboratory experiments indicate that juvenile salmonids not only recognize kin, but also behave less aggressively toward them. However, field studies have been less conclusive regarding whether juvenile salmonids remain close enough to interact with siblings in the wild. Here, we analyzed the geographic distribution of juvenile steelhead trout (Oncorhynchus mykiss) in a small stream in Washington State to test the null hypothesis that siblings were distributed randomly across the landscape several (1-3) months after they emerge. Fish (N=1020) were genotyped at 96 single-nucleotide polymorphisms, and pedigree analysis was performed in COLONY2 to identify full-sibling groups. Spatial autocorrelation analysis revealed that individuals <1 km apart were more genetically similar than expected by chance. The correlation between genetic and geographic distance was highest for pairs of individuals within 0 and 100 m, due in part to the presence of full siblings. While the majority of sibling pairs were too far apart to interact, some sibling groups were co-located. We suggest future research focus on characteristics of siblings and environmental conditions associated with sibling groups that remain aggregated versus those that disperse over large or short distances.