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

The N-15/N-14 ratio of the fish-native organic matter preserved in fish otoliths (or delta N-15(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 delta N-15 of diet, of fish tissue (e.g., white muscle tissue, delta N-15(wmt)), and delta N-15(oto). Using a highly sensitive method for N isotope analysis, delta N-15(oto) was compared with delta N-15(wmt) in 24 teleost species. Within a species, the difference between delta N-15(oto) and delta N-15(wmt) (delta No-15-w) varied little across individuals, confirming the utility of delta N-15(oto) to reconstruct delta N-15(wmt) changes for a given species. Across species, delta N-15(oto) and delta N-15(wmt) were highly correlated. However, delta N-15(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 delta N-15(o-w). delta N-15(oto) was lowest relative to delta N-15(wmt) in species producing larger otoliths. We propose that delta N-15(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.