Tissue-specific response of δ15N in adult Pacific herring (Clupea pallasi) following an isotopic shift in diet

The objective of this study was to measure the tissue-specific response of isotope delta N-15 to changes in isotopic signature of diet in an adult Pacific herring, Clupea pallasi, and to examine the importance of growth and metabolism in this shift. This was accomplished by placing wild adult Pacific herring in captivity and monitoring isotopic shift in tissues with a corresponding isotopic shift in diet, and the application of a metabolism/growth mixing model. Tissues examined were blood, eye, heart, liver, and white muscle. One group of herring was given a delta N-15 diet depleted by approximately 5.4 parts per thousand, and another given a N-15-enriched diet labeled with 98 atom% L-phenylalanine. This study showed that (i) isotopic response of individual tissues following an isotopic shift in diet varied in both rate of change and fractionation level, (ii) most of this isotopic shift is due to growth, and (iii) white muscle and liver tissue appeared the most responsive to isotopic shift in diet, reaching isotopic equilibrium with diet in a matter of months (not years). For trophic studies using delta N-15, these results indicate that field measurement of Pacific herring should be done after much of summer growth has occurred.