Ontogenetic Trophic Shifts by Ommastrephes bartramii in the North Pacific Ocean Based on Eye Lens Stable Isotopes

Abstract

The neon flying squid (Ommastrephes bartramii) plays an important ecological role in the North Pacific. An analysis of stable isotopes in eye lenses was conducted to investigate the inter- and intravariation of the trophic ecology of stocks at the eastern and western North Pacific throughout the life cycle. δ13C and δ15N values gradually increased with ontogenetic growth of the squid, which was associated with geographic migrations and increased the trophic level. For both stocks, from the paralarval to the juvenile stage, the trophic niche breadth increased, which might be the reason that the swimming and feeding ability improved as they entered the juvenile stage. Meanwhile, interactions between different ecosystems led to a greater diversity of food sources; thus, their feeding targets were no longer limited to plankton but shifted toward small fish and other cephalopods. Then, from the juvenile to the subadult–adult stage, the trophic niche breadth decreased, which can be explained by that O. bartramii had a selective preference for certain prey as ontogenetic growth proceeded, and they seemed to focus more on larger prey in the subadult–adult stage. Furthermore, the small amount of overlap between early and later life cycles suggested a significant trophic niche separation among different trophic ecologies and spatial ecologies. This study provides an understanding of diet shifts in neon flying squid in the North Pacific Ocean, primarily including diet shifts during their individual development and differences in trophic variation between the two stocks.