Temporal Variation of Meristic Traits within an Atlantic Salmon (Salmo salar) Stock, and Implications for Stock Identification

The assumption is generally made in stock identification studies that variation within stocks is not confounded with variation among them. To evaluate whether this assumption is tenable we test the null hypothesis of temporal stability in five key meristic traits of a single stock of Atlantic salmon (Salmo salar). We used smolts collected over a 10-yr period in the downstream migrations from one small river system. Individuals were tagged when captured and preserved, so we can estimate variability within, as well as among, runs. The meristic traits are remarkably homogenous within runs and cohorts. However, the null hypothesis of stability over time among runs and cohorts is strongly rejected for each trait. We conclude that temporal variation is both extensive and persistent. There is limited covariation among the traits, but they are largely independent of scores of environmental variables and of variation in the biological attributes of the stock. Because among-group variability is meaningful only in context of within-group variation, and because the within-group variation is large, our results have serious implications for sampling design in stock identification studies.