Stable isotope analysis shows limited assimilation of carbohydrates in muscle tissue of an omnivorous fish

Abstract Stable isotope analysis is key to understanding trophic relationships, but how different diet components are incorporated into tissues remains unclear. Understanding trophic relationships for omnivorous species is particularly challenging because they consume both plants and animal material that vary in nutritional quality, isotopic composition, and metabolic routing. Carbohydrates can fuel metabolism but may not be incorporated into tissues, leading to underrepresentation in isotope signatures of muscle tissue. This study examined how stable isotopes (δ13C, δ15N, δ2H) in channel catfish ( Ictalurus punctatus ) muscle reflect contributions from carbohydrate-rich Russian olive fruit ( Elaeagnus angustifolia ) and protein-rich black soldier fly larvae (Stratiomyidae). Over a 44-day trial, catfish were fed mixed diets ranging from 100% olive to 100% fly meal, plus a commercial feed control. Muscle tissue from each treatment was analyzed for isotopic ratios after the feeding period. While treatment feeds differed in isotopic value, muscle tissue only reflected the assimilation of fly larvae, not olive. Fish fed 100% olive showed no significant isotopic difference from pre-trial controls, indicating minimal assimilation. Hydrogen isotopes showed no dietary effect, suggesting limited incorporation into muscle tissue. These results suggest carbohydrate-rich diet items that are metabolized for energy may be underrepresented by stable isotope analysis of muscle tissue.