Short-term shifts of stable isotope (δ¹³C, δ¹⁵N) values in juvenile sharks within nursery areas suggest rapid shifts in energy pathways

We quantified temporal changes in blood plasma δ¹³C and δ¹⁵N values collected from recaptured juvenile blacktip reef sharks (Carcharhinus melanopterus, n=14) and sicklefin lemon sharks (Negaprion acutidens, n=4) at liberty in Moorea, French Polynesia for 10-50days, and juvenile bull sharks (Carcharhinus leucas, n=7) at liberty in the Florida Coastal Everglades for 34-127days to investigate shifts in assimilated biomass from energy reserves and consumed biomass. Blacktip reef and bull sharks exhibited significant changes in plasma δ¹³C as they grew, with a mean δ δ¹³C/cm total length±SD of 0.41‰/cm±0.72 and-0.82‰/cm±0.67. While low sample sizes precluded statistical analyses, sicklefin lemon exhibited a change of 0.49‰/cm±0.77. Blacktip reef sharks and bull sharks also exhibited significant shifts in δ¹⁵N values-mean δ δ¹⁵N/cm TL±SD=-0.23‰/cm±0.59, and -0.24‰/cm±0.20; shifts in δ¹⁵N values for sicklefin lemon sharks averaged 0.19‰/cm±0.52. When data were normalized across species (accounting for species-specific difference in δ¹³C and δ¹⁵N ranges), no significant difference were found in the rate of δ¹⁵N change between bull and blacktip reef sharks, but mean changes in δ¹³C/day among blacktip reef and sicklefin lemon sharks (~1%/day) were twice as fast as bull sharks (~0.5%/day). Comparisons between plasma and muscle isotope values in bull sharks yielded similar results to comparisons of plasma isotope values - rapid changes in δ¹³C and δ¹⁵N. The magnitude and direction of changes in δ¹³C and δ¹⁵N values, however, were not uniform among individuals within each species, suggesting intraspecific variation in trophic interactions within the shark nurseries studied. Further studies quantifying shifts in energy pathways may contribute to elucidating the factors that shape foraging development in sharks and variation in trophic interactions within shark nurseries.