Detecting pan-Atlantic migration in salmon (Salmo salar) using 137Cs

137Cs is a globally dispersed radioisotope that is transferred efficiently through the food chain. There is a strong east-west gradient of 137Cs in the waters of the North Atlantic due to anthropogenic inputs from Europe, with levels exceeding 10 Bq·m-3 in the Irish Sea and concentrations <1.5 Bq·m-3 in the West Atlantic. This range in values is subsequently reflected in fish, including Atlantic salmon (Salmo salar), caught in those waters. 137Cs concentrations in adult salmon, which had returned to the Ste. Marguerite River, Canada, reflected the entire range of values seen previously across the North Atlantic. In fact, 43% of fish had concentrations characteristic of the Faroe, Norwegian, North, and Irish seas. These results are in sharp contrast with what is generally believed about the migration of salmon and suggest that their marine life history is more panoceanic than previously thought.