The potential for naturally occurring element concentrations and stable isotope ratios to provide biogeochemical tags of Sa/mo fruita, an andromous salmonid, was examined. Statistically significant differences were found in the concentrations of Li,Mg, Mn, Sr, Ba and Pb in the hydroxyapatite freshwater growth bands of wild Sa/mo fnltta scales collected from 12 catchments draining into the Irish Sea and Celtic Sea.However, only 50% of the fish studied could be correctly classified to their catchment of origin based on element concentrations in scales. The degree to which element concentrations in sea trout scales could be used as a stock discrimination tool offish in the Irish Sea and Celtic Sea is probably limited. Sr, Mn, Ba and Mg in recently deposited scale hydroxyapatite and otolith aragonite allowed the classification of Sa/mo tnltta to site of origin (n=6) in the Dee river catchment in Wales, u.K., to be determined with 86% and 89% classification accuracy respectively. There appeared to be regional patterning in scale and otolith chemistry in the upper, middle and lower regions of the catchment. Scales might offer a non-lethal biogeochemical tag, comparable in performance to otoliths, but further work needs to examine the degree of post-depositional change in scale hydroxyapatite. Concentrations of Sr and Mn in scales were correlated with concentrations in stream water, providing a unique opportunity to map high resolution catchment-wide variability in scale element concentrations using British Geological Survey (BGS)stream water chemistry data as a proxy for Sr and Mn in scales, at 792 sites in the Dee catchment (neighbouring sites were >41m apart). The regional patterning in Sr and Mn in scales among sites (n=12) in 3rd order tributaries in the upper, middle and lower regions of the catchment were not present in predicted scale chemistries at sites in 1 st and 2nd order tributaries (n=792). Some geographically isolated sites in the Dee catchment might be expected to show similar concentrations of Sr and Mn in scales suggesting that the technique is unlikely to be a useful fisheries research tool in the Dee catchment. The performance of olsN, 013C and oD in scales and concentrations of Sr, Mn, Ba and Mg in scales and otoliths of Sa/mo tnllta as biogeochemical tags of fish at sites (n=6) in the Dee catchment were compared. No significant differences in oD in scales were detected among the 6 study sites. However, significant differences were detected in Ol~ and OBC among sites. The olsN and OBC values allowed fish to be classified to their site of origin with 93% accuracy. This classification was superior to that achieved when using Sr, Mn, Ba and Mg in the hydroxyapatite of scales from the same fish (which achieved a classification accuracy of 88%). olsN and 013C could provide biogeochemical tags of Salmo fruita but further work needs to examine the degree of spatial variability in 01SN and OBC in Salmo fruita scales in the Dee catchment.