Trophic Ecology during the Ontogenetic Development of the Pelagic Thresher Shark Alopias pelagicus in Baja California Sur, Mexico

The trophic ecology of the Pelagic Thresher shark (Alopias pelagicus) was evaluated based on chemical ecology using stable isotope ratios of carbon (delta C-13) and nitrogen (delta N-15) in the vertebrae and muscles. Individuals were caught between August 2013 and October 2019 on both the coasts of Baja California Sur, Mexico. In Bah & iacute;a Tortugas, the mean vertebrae (n = 35) values were 12.72 +/- 1.06 parts per thousand (delta N-15) and -14.79 +/- 0.61 parts per thousand (delta C-13), while in muscles (n = 32) these values were 16.63 +/- 0.76 parts per thousand (delta N-15) and -17.18 +/- 0.39 parts per thousand (delta C-13). In Santa Rosal & iacute;a, the mean vertebrae (n = 125) isotopic values were 14.4 +/- 1.59 parts per thousand (delta N-15) and -14.18 +/- 0.51 parts per thousand (delta C-13), while in muscles (n = 43), these values were 18.08 +/- 0.96 parts per thousand (delta N-15) and -16.43 +/- 0.34 parts per thousand (delta C-13). These results show higher delta N-15 values in Santa Rosal & iacute;a as an effect of baseline isotopic differences between the two regions, whereas the delta C-13 values were lower in Bah & iacute;a Tortugas, suggesting offshore ecological behavior (p < 0.05). In Santa Rosal & iacute;a, there were significant differences by sex for delta N-15 in muscle, whereas the delta C-13 showed ontogenetic shifts, indicating that neonates feed in coastal areas more commonly than juveniles or adults (p < 0.05). Neither sex nor ontogenetic differences were observed in Bah & iacute;a Tortugas (p > 0.05), suggesting a high overlap between their isotopic niches. Therefore, Alopias pelagicus uses the same ecological niche throughout its life, and there is consistency between sexes. The mean trophic position for both tissues and regions was 4.5, which corresponds to a tertiary predator, without any differences between stages or sex. Due to their higher energetic needs, juveniles and females showed the greatest isotopic niche amplitude; thus, their ecological niche is the widest.