Interannual and ontogenetic stage differences on trophic ecology of neon flying squid Ommastrephes bartramii inferred from stable isotope analyses in beaks

To reveal the interannual and ontogenetic stage variation of trophic ecology, different subsections of Ommastrephes bartramii beaks from different years were used for stable isotope analysis. The δ13C and δ15N values of beaks within different subsections showed significant differences between different years and different ontogenesis stages (P < 0.05). Different ecological overlap rates were found between different years and the overlap rate between 2012 and 2016 was larger than that of 2012 versus 2015 and 2015 versus 2016. The δ13C and δ15N values in 2015 were lower than those in 2012 and 2016 and represented a narrower isotope niche, whereas the ellipse area of isotope niche in 2015 was the widest and indicated that the food organisms preyed on by O. bartramii were in the lower trophic level, but the species of food organisms were more abundant in 2015. The stable isotope analyses in different growth stages showed that the niches in the larvae stage and paralarvae stage showed no significant difference between different years, indicating that the species of food organisms in the larvae stage and paralarvae stage were similar. However, with the growth of individual size, the isotopic niche gradually showed differences and developed towards a wider isotope niche in 2012 and 2016. During the El Niño event in 2015, the marine environment was not conducive to the growth of prey organisms and the poor environment might affect the feeding behavior of O. bartramii, resulting in the narrower isotope niche. During the normal years and the La Niña event, the environment was relatively suitable for O. bartramii and resulted a wider isotope niche.