Trace elements incorporated into the growing surface of the fish otolith (ear stone) reflect the physical and chemical characteristics of the ambient water, although not necessarily in a simplistic manner. Since otoliths grow continuously without resorption throughout the life of the fish, fish population~ growing up in different water masses should produce otoliths of different elemental composition. The otolith elemental composition ('fingerprint') determined with isotope dilution -inductively coupled plasma mass spectroscopy (ID-ICPMS) proved to be an effective discriminator of adjacent population~ of Atlantic cod Gadus morhua off the coast of eastern Canada, with sufficient accuracy as a natural tag to determine population identity in a mixed population fishery. Classification of samples collected from the winter cod fishery on the eastern Scotian Shelf indicated that the annual winter migration out of the Gulf of St. Lawrence is more extensive than was previously believed.