Stomach content and stable isotopes reveal an ontogenetic dietary shift of young-of-the-year scalloped hammerhead sharks (Sphyrna lewini) inhabiting coastal nursery areas

Abstract

Sphyrna lewini is a placental viviparous shark that uses coastal nursery areas in Jalisco, Mexico, where pups stay 4–12 months. Changes in size or swimming speed may be reflected in diet composition. The main objectives of this study were as follows: (1) analyze the differences in trophic ecology of juvenile S. lewini from coastal nursery areas of Jalisco by sex and size, through stomach content and stable isotope analyses; and (2) analyze changes in muscle and liver δ15N and δ13C values with shark size. Samples were collected from the artisanal fishery from September 2013 to December 2016. Three size classes were compared: neonates, stretched total length (STL) ≤ 75 cm, and STL > 75 cm (75–100 cm). Bony fishes were the most important group in all size classes, and the importance of shrimps decreased with STL. Significant differences in diet composition were found between neonates and STL > 75 cm, which showed the lowest niche overlap (0.32). STL ≤ 75 cm showed the largest niche width (0.75). The δ15N muscle and liver values declined with total length, reflecting the maternal isotopic signal. No differences in liver δ13C values were observed among size classes, but larger size class showed higher muscle δ13C values. Both isotope and stomach content analyses classified all sharks as tertiary consumers, but the trophic position (TP) estimated using δ15N was higher in neonates. No significant differences in the diet and TP were observed between sexes. The liver C:N ratio decreased sharply with STL up to 55 cm, from which increased smoothly, reflecting the lipid reserves consumed during their neonatal stage.