RATIONALE: Otolith δ18O and δ13C values have been used extensively to reconstruct thermal and diet histories. Researchers have suggested that individual growth rate and size may have an effect on otolith isotope ratios and subsequently confound otolith based thermal and diet reconstructions. As few explicit tests of the effect of fish in freshwater environments exist, here we determine experimentally the potential for related growth rate and size effects on otolith δ18O and δ13C values. METHODS: Fifty Arctic charr were raised in identical conditions for two years after which their otoliths were removed and analyzed for their δ 18O and δ13C values. The potential effects of final length and the Thermal Growth Coefficient (TGC) on otolith isotope ratios were tested using correlation and regression analysis to determine if significant effects were present and to quantify effects when present. RESULTS: The analyses indicated that TGC and size had significant and similar positive non-linear relationships with δ 13C values and explained 35% and 42% of the variability, respectively. Conversely, both TGC and size were found to have no significant correlation with otolith δ18O values. There was no significant correlation between δ 18O and δ13C values. CONCLUSIONS: The investigation indicated the presence of linked growth rate and size effects on otolith δ13C values, the nature of which requires further study. Otolith δ18O values were unaffected by individual growth rate and size, confirming the applicability of applying these values to thermal reconstructions of fish habitat.