Stable isotope turnover rates and fractionation in captive California yellowtail (Seriola dorsalis): insights for application to field studies

Stable isotope analysis (SIA) measurements from long-term captivity studies provide required parameters for interpretation of consumer SIA data. We raised young-of-the-year (14-19 cm) California yellowtail (Seriola dorsalis) on a low delta N-15 and delta C-13 diet (pellet aquaculture feed) for 525 days, then switched to a high delta N-15 and delta C-13 diet (mackerel and squid) for 753 days. Yellowtail muscle was sequentially sampled from each individual after the diet switch (0 to 753 days) and analyzed for delta N-15 and delta C-13, allowing for calculation of diet-tissue discrimination factors (DTDFs) from two isotopically different diets (low delta N-15 and delta C-13: pellets; high delta N-15 and delta C-13: fish/squid) and turnover rates of N-15 and C-13. DTDFs were diet dependent: Delta N-15=5.1 parts per thousand, Delta C-13=3.6 parts per thousand for pellets and Delta N-15=2.6 parts per thousand, Delta C-13=1.3 parts per thousand for fish/squid. Half-life estimates from N-15 and C-13 turnover rates for pooled yellowtail were 181 days and 341 days, respectively, but varied considerably by individual (N-15: 99-239 d; C-13: 158-899 d). Quantifying DTDFs supports isotopic approaches to field data that assume isotopic steady-state conditions (e.g., mixing models for diet reconstruction). Characterizing and quantifying turnover rates allow for estimates of diet/habitat shifts and "isotopic clock" approaches, and observed inter-individual variability suggests the need for large datasets in field studies. We provide diet-dependent DTDFs and growth effects on turnover rates, and associated error around these parameters, for application to field-collected SIA data from other large teleosts.