Reef fish are an important commodity for both human populations and coral reef ecosystem processes. In developing island nations, fish from multiple trophic levels are exploited, potentially compromising ecosystem function. For example, in the Philippines, large-bodied planktivorous and herbivorous Naso and Siganus are valuable subsistence fishery species. This study uses stable isotope analysis of δ¹³C and δ¹⁵N from fish muscle tissue and primary source samples to investigate the plasticity in diet and trophic ecology of N. unicornis, N. lituratus, N. minor and S. virgatus, caught near inshore (mainland) and offshore islands. We hypothesize differences in primary production and terrestrial nutrient input between island types are contributing to variation in diet and trophic level. Thus, we expect individuals inhabiting platform reefs on small offshore islands will have different signatures than the same species on mainland island fringing reefs. Further, relative nutrient availability among island types is inferred by comparing carbon and nitrogen ratios from algal sources collected from multiple sites on these island types. With expanding human populations, coastal development and a changing climate modifying coral reef fish assemblages globally, understanding whether the diet of important reef fish is influenced by environmental conditions is question of significance.