Stable isotope discrimination factors of omnivorous fishes: influence of tissue type, temperature, diet composition and formulated feeds

Application of stable isotope data to trophic studies requires understanding of factors influencing the isotopic discrimination factor (Δ) between consumers and their prey resources. This is missing for many omnivorous species, despite their diet and environment potentially impacting Δ. The effects of temperature, diet (including formulated feeds) and tissue type on Δ 13 C and Δ 15 N were thus tested experimentally. A temperature experiment exposed three species to identical diets at 18 and 23°C, whereas a diet experiment exposed one species to four diets at 18°C. At 23°C, C:N ratios, Δ 13 C and Δ 15 N were generally elevated versus 18°C. After lipid correction, tissue/species-specific differences at 23°C in Δ 13 C and Δ 15 N were up to 0.73 and 0.54‰ higher, respectively. Across the four diets, there were also significant differences in Δ 13 C and Δ 15 N between a natural diet and diets based on formulated feeds. Δ 13 C and Δ 15 N of muscle were 1.51 to 2.76‰ and 3.13 to 5.44‰, respectively. Highest Δ for both isotopes was from a formulated feed based on plant material that resulted in lower dietary protein content and quality. Thus, diet and environment fundamentally affected the isotopic discrimination factors and these factors require consideration within trophic studies based on stable isotopes.