<p>Concern over the use of lethal sampling for routine ecological research work has driven the development of equations for equating non-lethally sampled tissue to the archival database of previously lethally sampled tissue, usually white dorsal muscle tissue. Here we develop a set of equations for Arctic grayling and bull trout for predicting dorsal muscle stable isotope values from adipose fin clips that consider the need for additional explanatory biological variables (length, capture location, and the adipose C:N ratio). Based on the use of information theoretic criteria we found the simple adipose fin-muscle relationship was not the “best” for making conversions for Arctic grayling δ¹³C and δ¹⁵N or bull trout δ¹³C, but worked adequately for bull trout δ¹⁵N. Length and C:N ratio improved the accuracy of the conversions, with length generally being found to be the most important based on its estimated importance weight. Models with adipose C:N ratio only improved conversion accuracy for&#xa0;δ¹³C.&#xa0;Given the potential for error propagation in derivative ecological analyses based on the converted data and the consequences of such errors for ecological inferences based on those analyses, conversion equations should be made as accurate as possible. When developing conversion equations that include length as a covariate it is recommended that an adequate length range be used that includes the lengths over which any ontogenetic shifts in diet might occur. Use of these guidelines will help improve the ecological analyses and inferences based on using fin tissue as a non-lethal alternative for lethal sampling, thereby helping to minimize the sacrificing of fish.</p>