Hydrogen stable isotopes of animal tissues are well established tracers of migration ecology in terrestrial ecosystems. Recent research has highlighted delta H-2 as a potential tool in studies of aquatic ecosystems, particularly as a robust tracer for quantifying the importance of allochthonous subsidies. Although the use of delta H-2 has clear potential, some uncertainties remain, in particular with regard to the contribution of dietary water to consumer delta H-2. Here, we quantify the contribution of dietary water to delta H-2 in two salmonid fishes, Atlantic salmon (Salmo salar L.) and Arctic charr (Salvelinus alpinus L.), reared on diets of known isotopic composition. Furthermore, we examined the capacity of fins (adipose and caudal) to provide a non-lethal means of estimating consumer delta H-2. The proportion of deuterium derived from environmental water of all tissue was substantial in both Atlantic salmon (mean = 0.43 +/- A 0.1 SD) and Arctic charr (mean = 0.48 +/- A 0.15 SD) but varied considerably between both individuals and tissue type. White muscle proved to be the least variable of the tissues analysed. Although fins proved to be a possible non-destructive substitute, a degree of caution is recommended with their use, as the proportion of dietary water contributing to the deuterium of fins was considerable more variable.