1. The ecological niche is a fundamental concept for understanding the interaction among species that can change the function at the level of species and guilds to adapt ecological conditions. The stable isotope analysis has conventionally been employed to evaluate such adaptabilities frequently in the scenopoetic and bionomic views, as isotopic niche width.

2. We applied compound-specific isotope analysis (CSIA) of nitrogen within amino acids to understand trophic variability of the pike gudgeon Pseudogobio esocinus and the largemouth bass Micropterus salmoides as representative specialist and generalist fish species, respectively, in 16 ecologically-variable habitats on the four major rivers of Korea.

3. There is little variation (1σ) in the trophic position (TP) among the habitats for P. esocinus (±0.2), but a considerably large variation for M. salmoides (±0.6). Moreover, the TP of M. salmoides is negatively correlated with benthic invertebrate indices in the habitats, while the TP of P. esocinus shows no significant correlation with any indices. These two representative fish species thus have different trophic responses to ecological conditions, which is well consistent with known differences in the trophic niche between specialists (i.e., small niche width) and generalists (i.e., large niche width).

4. During the last four decades, the traditional stable isotope analysis for bulk carbon and nitrogen in samples has not been able to deconvolute ‘scenopoetic’ and ‘bionomic’ information. However, in the present study, we demonstrated that CSIA of amino acids can isolate trophic niche from traditional ecological niche that is accompanying both trophic and habitat information, and evaluated how biological and ecological indices affect to the trophic responses of specialists and generalists.