Effects of body size and environment on diet-tissue δ15N fractionation in fishes

Nitrogen stable isotope natural abundance data are often used in trophodynamic research. The assumed nitrogen diet-tissue fractionation (Delta delta N-15) determines conclusions about trophic level, potential food sources and ontogenetic diet shifts. Delta delta N-15 is usually assumed to be 3.0-3.4 parts per thousand per trophic level and unaffected by the size or age of animals or their environment. To assess the effects of body size, experimental duration and environmental conditions on fish tissue Delta delta N-15, two populations of European sea bass (Dicentrarchus labrax) were reared on constant diets of dab (Limanda limanda) muscle or sandeel (Ammodytes marinus) for 2 years under natural light and temperature regimes. Bass were sampled at approximately monthly intervals to determine Delta delta N-15 for muscle, heart and liver tissue. Mean values of Delta delta N-15 were 3.83 parts per thousand, 3.54 parts per thousand, 2.05 parts per thousand (sandeel diet) and 3.98 parts per thousand, 3.32 parts per thousand, 1.95 parts per thousand (dab diet) for muscle, heart and liver tissue respectively. The assumption that fractionation was independent of body mass was upheld for muscle and heart tissue, but not for liver. Time effects on muscle Delta delta N-15 were explainable by a sinusoidal function with a period of I year and wave height similar to 0.3 parts per thousand. Time resulted in increases in heart delta N-15 and decreases in liver delta N-15 which were small compared to background variation, equating to 1/6 of a trophic level over 2 years, and unlikely to have great significance in ecological studies. Heart and liver delta N-15 were also affected by temperature probably reflecting the metabolic functions of these tissues and their associated rates of turnover. However in heart the explanatory power of temperature appeared tied to that of time. Although the Delta delta N-15 for bass muscle on both diets approached 4 parts per thousand, the Delta delta N-15 values from this study, when combined with those from the literature, suggest that where fish species specific data are not available, a mean Delta delta N-15 for fish muscle of 3.2 parts per thousand should be applied (mean white muscle Delta delta N-15=3.15). The literature based mean Delta delta N-15 for whole fish was lower than that of white muscle suggesting that a separate Delta delta N-15 (2.9 parts per thousand) should be applied when sampling whole fish. (c) 2006 Elsevier B.V. All rights reserved.