Dietary Partitioning Mediates Species Coexistence in the High Seas Area of Northwest Pacific

Niche partitioning, manifesting along various axes of the ecological niche, is a fundamental mechanism mediating species coexistence in a system. However, the emergence of many species in marine ecosystems would face considerable constraints in achieving coexistence through spatiotemporal differentiation mechanisms under the framework of niche partitioning. Consequently, dietary specialization may be the key mechanism to mediate coexistence, but lacks empirical support, especially for the oceanic system exhibiting heterogeneity and phenological variability in resource availability. We herein used data from nine pelagic species (five fish and four squid) sampled from the high seas area of the northwest Pacific during the summer season, when they migrate there to optimize foraging efficiency and growth, and evaluated their niche partitioning in terms of dietary specialization. We measured the bulk carbon and nitrogen stable isotopes and essential fatty acids of the nine species to estimate the isotopic and nutritional niche spaces, and employed Hutchinson's n‐dimensional hypervolume framework to examine the degree of dietary specialization. Results revealed that pairwise overlap of isotopic niches was observed for all species, though some species occupied distinct isotopic niche spaces. In contrast, the nutritional niche space, estimated using essential fatty acids, was mostly separate for each species, coupled with small or zero overlap between paired species. Furthermore, the specialization index, estimated using either stable isotopes or essential fatty acids, was consistently large for the nine species, while nestedness was relatively low. These findings reveal that the nine species partition their nutritional niche spaces to mitigate the competition caused by dietary niche overlaps, coinciding with the mechanism of dietary specialization.