Influence of diet on stable carbon isotope composition in otoliths of juvenile red drum Sciaenops ocellatus

Stable carbon isotope values of otoliths can provide long-term records of the environmental conditions experienced by fish. However, because otoliths contain carbon from metabolic sources, it is necessary to determine the proportion of metabolically derived carbon to properly interpret otolith delta C-13 values. To evaluate the relative influence of dietary carbon and the dissolved inorganic carbon (DIC) of ambient seawater on the delta C-13 isotopic composition of fish otoliths, juvenile red drum Sciaenops ocellatus were raised for 6 and 9 mo in tanks with flow-through ambient seawater and fed diets differing by 2.1 parts per thousand (p < 0.001) in carbon isotope composition (delta C-13). At the end of the experiment, muscle tissue from the 2 treatments reflected the isotopic composition of the diets and differed by 2.1 parts per thousand (p < 0.001). Muscle tissue from both groups was enriched by 1.5 parts per thousand in C-13 relative to the respective diets. The carbon isotope values of otoliths were enriched by similar to 16 to 17 parts per thousand relative to the respective diets, indicating that DIC was the dominant source (85 to 92%) for otolith carbon relative to carbon derived from diet. However, otoliths from the 2 treatments differed by 1.26 parts per thousand and expressed 60% of the difference in the isotope values of the diets. This result indicates that the nutritional characteristics of food may be a factor influencing the incorporation of metabolic carbon in the otolith. This is one of the first studies to provide evidence that the nutritional value of food influences otolith delta C-13 value.