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

Abstract

The 15 N/ 14 N ratio of the fish-native organic matter preserved in fish otoliths (or δ 15 N oto ) 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 δ 15 N of diet, of fish tissue (e.g., white muscle tissue, δ 15 N wmt ), and δ 15 N oto . Using a highly sensitive method for N isotope analysis, δ 15 N oto was compared with δ 15 N wmt in 24 teleost species. Within a species, the difference between δ 15 N oto and δ 15 N wmt (Δδ 15 N o-w ) varied little across individuals, confirming the utility of δ 15 N oto to reconstruct δ 15 N wmt changes for a given species. Across species, δ 15 N oto and δ 15 N wmt were highly correlated. However, Δδ 15 N o-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 Δδ 15 N o-w . δ 15 N oto was lowest relative to δ 15 N wmt in species producing larger otoliths. We propose that δ 15 N oto 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.