Diet-tissue discrimination factors and turnover rates of carbon and nitrogen stable isotopes in the mottled skate Beringraja pulchra based on diet-switching experiments

Stable isotope analysis is robust tool to elucidate the diet, trophic position, and migration of typically large, highly mobile marine predators such as elasmobranchs. However, it is necessary to understand the diet-tissue discrimination factors and isotopic turnover rates of target animals. In this study, diet-switching experiments were conducted on the mottled skate ( Beringraja pulchra (Liu, 1932)) , caught off Sarufutsu, Hokkaido, Japan, to elucidate the diet-tissue discrimination factors and isotopic turnover rates for carbon and nitrogen stable isotopes. Ninety juvenile skates were fed Euphausia superba (C:N ratio = 3.9 +/- 0.06, delta 15 N = 3.7 +/- 0.26%o and delta 13 C = - 25.9 +/- 0.15%o) to satiation at intervals of 2-3 days, and their muscle and liver tissues were serially sampled during 400 experimental days and used for stable isotope analysis. Both carbon and nitrogen stable isotopes reached steady state within the experimental days, and the estimated diet-tissue discrimination factors based on the fitting models were Delta 13 C = 1.5%o and Delta 15 N = 2.3%o for liver tissues and Delta 13 C = 3.4%o and Delta 15 N = 3.5%o for muscle tissues. The estimated turnover rates were 0.034 day -1 for delta 13 C and 0.039 day -1 for delta 15 N for liver tissues and were higher than those for muscle tissues (0.021 day -1 for delta 13 C and 0.019 day -1 for delta 15 N). In addition, the growth-based isotopic turnover rate model comprising juvenile skates revealed that the contribution of growth rate to the isotopic turnover rate was low.