Investigating the age at sea departure of returning Atlantic salmon is critical to better understand the role of emigration timing in survival. Among the methods available, the use of otolith chemistry can be challenging in anadromous fish, as the elements frequently used (i.e. magnesium, manganese, and zinc) can jointly reflect individual metabolism and environmental conditions. Here, we present a new unsupervised and robust method for successfully estimating the outmigration age of Atlantic salmon in the Bay of Biscay, based on otolith zinc concentrations. Data published in doi:10.1111/jfb.15829