Effect of ingestion on the stable isotope signatures of marine herbivorous fish diets

Stable isotope analysis is widely used to investigate the diet–consumer relationships of free-ranging animals, including fishes. However, measuring the isotope signature of relevant diet items can be problematic when little is known about consumer diet choice, or when individual diet items are difficult to collect in the field. Here we tested the viability of using ingested material as a proxy for fresh diet items in a field study by examining the effect of ingestion on the δ15N and δ13C values of diet items commonly consumed by the marine herbivorous fish Odax pullus (Labridae). Ingestion had a significant effect on the δ15N or δ13C value of three of the five diet items considered. Two diet items showed a clear and consistent change in isotopic signature following ingestion, indicating that discrimination between light and heavy isotopes occurs during the early stages of processing in this stomachless fish. The potential for isotope signatures to change following ingestion suggests that caution should be taken when using ingested material as a proxy for diet items in studies of marine herbivory. Additionally, the predictable effect of ingestion demonstrates the potential for digestion to make a large contribution to the fractionation observed in trophic studies of marine herbivory.

► We compare the sampling of ingested vs. fresh material for stable isotope analysis.
► Sampling of ingested material may result in significantly altered δ13C and δ15N values.
► Stable isotopes (13C and 15N) are enriched at onshore locations in this study.