Siblingship tests connect two seemingly independent farmed Atlantic salmon escape events

Aquaculture escapees represent a threat to the genetic integrity of native populations, may spread infectious agents and display ecological interactions with wild fish. DNA-based identification methods are well established for tracing Atlantic salmon escapees back to their farms of origin. However, traditional genetic assignment approaches are not always able to single out the farm of origin in cases where several potential farm sources rear fish from the same genetic line, and display strongly overlapping allele frequencies. We investigated whether an alternative statistical approach, which involves ad hoc identification of sibling relationships, circumvents the challenge of overlapping allele frequencies. We analysed the following samples collected in 2013: (1) 221 farmed escapees captured in several rivers in the Ryfylke region of Norway, (2) 139 farmed escapees captured some 150 km away in an upstream fish migration trap in the River Etne, and (3) 779 farmed salmon sampled from 17 cages on 10 farms in Ryfylke. Siblingship tests increased the precision of identification of escapees back to their farm of origin over genetic assignment and population statistic approaches. Together with other non-genetic data, siblingship tests were also able to connect 2 seemingly independent escape events, demonstrating that some of the salmon escaping from 1 or 2 farms in Ryfylke took approximately 1 mo to migrate 150 km northwards before entering the River Etne. Finally, we demonstrated that the genetic background of the escapees captured in the River Etne during the course of an entire season was represented by the 3 major breeding programs in Norway.