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 the 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 the 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 δ¹³C values confirmed Green Sturgeon had more marine dietary sources than White Sturgeon. Plasma δ¹⁵N values revealed White Sturgeon fed at lower trophic levels than Green Sturgeon recently, however, whole blood δ¹⁵N values demonstrated the two species fed at the same trophic level over longer time scales. Larger individuals of both species had higher δ¹³C values than smaller individuals, reflecting more marine food sources in adulthood. Length did not affect δ¹⁵N 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 the San Francisco Bay.