A lack of knowledge on the transition from the resident to the migratory phase has led to a series of studies on the silvering process. Silvering marks the end of the sedentary growth phase and the beginning of the migratory phase. A six-stage classification was developed to describe the physiological and morphological events that occur during this metamorphosis and the subsequent migration. Stages corresponded to a growth phase (I to FII), a premigrant stage for females (FIII), and migrating stages for both sexes (FIV, FV, and MII). Here, the objective was to develop a silver index using only external measurements to assess the degree of metamorphosis of eels, based on the same data set that was used in the former study. It consisted of a large number of both resident and migratory eels that were sampled at different times of the year with different types of fishing gear and at several locations representing various types of habitats. Discriminant Analysis was applied on external measurements only: (body length, body weight, pectoral fin length, and eye diameters). Total percentage of correct reclassification into the six silvering stages was 82%. The silver index (classification functions) was able to identify 91% of the migratory eels. This method, associated with proper sampling, could be utilized for the quantification of potential spawners given that they all reach their spawning grounds in the Sargasso Sea.