This project was funded by the Natural Environment Research Council (NERC), UK, through an Industrial CASE studentship (NER/S/C/2005/13466), with CASE partners, the Westcountry Rivers Trust. Additional funding was provided through the Atlantic Salmon Arc Project (ASAP), European Union INTERREG IIIB Project No. 040. We are grateful to Professor Paulo Prodohl (Queen’s University, Belfast) for helpful suggestions on the analysis. We thank J-L Baglinière (INRA, Rennes) for access to french scale samples and partners in the ASAP project for additional samples.