Feeding Habits of Cyclothone in the North Pacific Subtropical Gyre Inferred From Stable Isotopic Compositions

Abstract

Micronekton communities are important in the ocean’s food web and are an important prey resource for larger predatory species, thus they are a gateway for biomass transfer through the mesopelagic food web. Understanding the feeding habits of Cyclothone, the most abundant micronektonic fish in the world, is important for determining the significance of food webs fueled by sinking particulate organic matter (15N ~3%o bulk) or suspended particles (15N 8%o bulk). The 15N values of animals are commonly used to estimate trophic position and recognize trophic connectivity. Bulk tissue and amino acid compound specific stable isotope analyses were performed on Cyclothone specimens collected from two cruises at Station ALOHA (22.45°N, 158°W). Species differences were observed. C. pallida, C. atraria, and C. microdon average bulk tissue 15N values were similar to that of large predatory species (10.77 + 2.45%o, 12.75 + 0.21%, and 12.85 + 1.88%o, respectively), whereas C. alba and C. pseudopallida average bulk tissue 15N values were similar to that of other meso- and bathypelagic micronekton species (7.08 + 0.51%o and 7.78 + 0.70%0, respectively). 15N values of individual amino acids were used to investigate if the differences in the 15N value of bulk tissues of these two groups of Cyclothone were due to variations in nitrogenous nutrient sources or trophic position. C. pallida trophic position were greater by ~0.4 compared with C. alba and C. pseudopallida. Mass balance models using source AAs indicated suspended particles as a significant base of food webs contributing to C. pallida diet and which influence their bulk tissue 15N values.