Turnover of hydrogen isotopes in lake sturgeon blood: implications for tracking movements of wild populations

Comparative stable isotope analyses of green sturgeon (Acipenser medirostris) and white sturgeon (A. transmontanus) in the San Francisco estuary

Green sturgeon (Acipenser medirostris) and white sturgeon (A. transmontanus) are closely related, sympatric species that inhabit the San Francisco estuary. Green sturgeon have a more marine life history but both species spawn in the Sacramento River and reside for some duration in San Francisco Bay. These sturgeons are of conservation concern, yet little is known about their dietary competition when they overlap in space and time. To examine evidence of dietary differentiation, we collected whole blood and blood plasma from 26 green sturgeon and 35 white sturgeon in San Francisco Bay. Using carbon and nitrogen stable isotope analyses, we compared their relative trophic levels and foraging locations along the freshwater to marine gradient. Sampling blood plasma and whole blood allowed comparison of dietary integration over shorter and longer time scales, respectively. Plasma and whole blood δ13 C values confirmed green sturgeon had more marine dietary sources than white sturgeon. Plasma δ15 N values revealed white sturgeon fed at lower trophic levels than green sturgeon recently, however, whole blood δ15 N values demonstrated the two species fed at the same trophic level over longer time scales. Larger individuals of both species had higher δ13 C values than smaller individuals, reflecting more marine food sources in adulthood. Length did not affect δ15 N values of either species. Isotope analyses supported the more marine life history of green than white sturgeon and potentially highlight a temporary trophic differentiation of diet between species during and preceding the overlapping life stage in San Francisco Bay.

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 δ¹⁵N and δ¹³C diet (pellet aquaculture feed) for 525 days, then switched to a high δ¹⁵N and δ¹³C 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 δ¹⁵N and δ¹³C, allowing for calculation of diet-tissue discrimination factors (DTDFs) from two isotopically different diets (low δ¹⁵N and δ¹³C: pellets; high δ¹⁵N and δ¹³C: fish/squid) and turnover rates of ¹⁵N and ¹³C. DTDFs were diet dependent: Δ¹⁵N = 5.1‰, Δ¹³C = 3.6‰ for pellets and Δ¹⁵N = 2.6‰, Δ¹³C = 1.3‰ for fish/squid. Half-life estimates from ¹⁵N and ¹³C turnover rates for pooled yellowtail were 181 days and 341 days, respectively, but varied considerably by individual (¹⁵N: 99–239 d; ¹³C: 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.