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 (Δδ15N) determines conclusions about trophic level, potential food sources and ontogenetic diet shifts. Δδ15N is usually assumed to be 3.0–3.4‰ 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 Δδ15N, 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 Δδ15N for muscle, heart and liver tissue. Mean values of Δδ15N were 3.83‰, 3.54‰, 2.05‰ (sandeel diet) and 3.98‰, 3.32‰, 1.95‰ (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 Δδ15N were explainable by a sinusoidal function with a period of 1 year and wave height ∼ 0.3‰. Time resulted in increases in heart δ15N and decreases in liver δ15N 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 δ15N 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 Δδ15N for bass muscle on both diets approached 4‰, the Δδ15N values from this study, when combined with those from the literature, suggest that where fish species specific data are not available, a mean Δδ15N for fish muscle of 3.2‰ should be applied (mean white muscle Δδ15N = 3.15). The literature based mean Δδ15N for whole fish was lower than that of white muscle suggesting that a separate Δδ15N (2.9‰) should be applied when sampling whole fish.