Niche width expansion of coral reef fishes along a primary production gradient in the remote central Pacific

The trophic niche of species can vary spatially due to numerous natural and anthropogenic factors, yet separating these distinct drivers can be difficult. We examined the role of natural oceanographic variation in the trophic ecology and dietary niche breadth of 8 common coral reef fishes spanning multiple trophic guilds. These fishes were collected from the Southern Line Islands of Kiribati, a chain of 5 uninhabited islands spanning a strong gradient of oceanic primary production. A combination of stomach contents and stable isotope analyses (d15N, d13C) were used to elucidate spatial variation in diet composition, trophic niche width, and degree of individual dietary specialization. Across species, populations were generally characterized by larger dietary niche widths at the islands exposed to greater nearshore primary production, although patterns among species were variable. Estimates of niche width varied by fish guild as a function of methodology, with planktivores exhibiting stronger effects using metrics calculated from stomach contents, and carnivores and herbivores exhibiting stronger effects from metrics calculated with stable isotope data. At the island level, the trophic niche of the fish community expanded in isotopic space as a function of increasing nearshore production, reflecting increased multispecies dietary diversity at the most productive islands. These results highlight the importance of considering natural oceanographic variability when evaluating the trophic structure of coral reef ecosystems, and provide a foundation for future research on ecosystem functioning across oceanographic gradients.