Abstract: Identification of migratory animals, such as adult fish and the like, by means of implanted coded tags, with tag coding involving one or more higher atomic numbered chemical elements in stable, solid form (elements with atomic numbers 40-42, 44-53, 55-60 and 62-83), the elements being identified in the live animal by selective X-ray irradiation of the implanted tag and spectral analysis of the fluorescent X-ray radiation emitted by the tag. Rapid analysis of the fluorescent X-ray radiation to identify the coding element(s) with a high level of confidence is obtained by use of high intensity irradiation and controlled masking to essentially confine the irradiation to only the tag and the animal tissue immediately surrounding the tag and thereby improve the signal-to-noise ratio of the fluorescent X-ray radiation emitted by the coding element(s).