Tissue-specific turnover rates and trophic enrichment of stable N and C isotopes in juvenile Atlantic cod Gadus morhua fed three different diets

A 150 d laboratory diet-shift study was conducted on juvenile Atlantic cod Gadus morhua L. to examine (1) stable nitrogen and carbon isotope turnover rates, (2) the tissue-specific relative contribution of metabolism and growth to isotopic change, and (3) diet-tissue enrichment of the isotopes in white muscle, heart, whole blood, and cranial bone collagen. Time-and growth-based models described the change in delta N-15 and delta C-13 of the tissues following a diet shift from pelleted food to either blue mussels Mytilus edulis L., sandeel Ammodytes marinus Raitt., or whiting Merlangius merlangus L. diets. Cod growth rates ranged from 1.5 to 2.0% d(-1), with the exception of whiting-fed cod that experienced a cessation in growth after Day 86. Isotope turnover rates, expressed as half-lives (T-1/2), differed little between tissues as expected in fast-growing fish. For delta N-15, heart tissue had the shortest T-1/2 while muscle had the longest (30.7 to 34.8 versus 35.7 to 77.9 d(-1); range among diets). Heart tissue also had the shortest delta C-13 T-1/2, whereas blood had the longest (25.5 to 38.5 versus 49.5 to 60.3 d(-1); range among diets). Diet-tissue enrichment varied between 0.8 and 5.1% (Delta delta N-15) and between 0.7 and 2.2% (Delta delta C-13). Growth accounted for most of the isotopic change, except in heart tissue of all cod and in all tissues of whiting-fed cod, where there was a significant effect of metabolism on isotopic change. Despite variability in enrichment among diets and tissues, fish feeding on the 3 diets could be distinguished based on their tissue-specific isotopic signatures.