Comparison of the isotopic composition of fish otolith-bound organic N with host tissue

Abstract

The 15N/14N ratio of the fish-native organic matter preserved in fish otoliths (or δ15Noto) may allow for reconstruction of fish trophic history and changes in food webs. To support this application, ground-truthing data are needed on the relationships among the δ15N of diet, of fish tissue (e.g., white muscle tissue, δ15Nwmt), and δ15Noto. Using a highly sensitive method for N isotope analysis, δ15Noto was compared with δ15Nwmt in 24 teleost species. Within a species, the difference between δ15Noto and δ15Nwmt (Δδ15No-w) varied little across individuals, confirming the utility of δ15Noto to reconstruct δ15Nwmt changes for a given species. Across species, δ15Noto and δ15Nwmt were highly correlated. However, Δδ15No-w varied systematically across species. Phylogeny, the concentrations of total N and amino acids, and life history were ruled out as the main cause for the observed variation in Δδ15No-w. δ15Noto was lowest relative to δ15Nwmt in species producing larger otoliths. We propose that δ15Noto is elevated by isotopically fractionating metabolism of the organic matrix, which is less important when otolith growth is fast and thus when the otolith is large.