Investigations to niche evolution have received controversial results in recent times, leading to some confusions in ecological studies: competitive exclusion principle (CEP) predicts that closely related species should be adequately divergent; while niche conservatism (NC) predicts that closely related species should be ecologically most similar. In the present study, by using stable isotope to signify trophic niche, we analyzed niche divergences of 57 fishes from a lake (Poyang Lake, central China) at the phylogenetic scale, as well as niche divergences and overlaps of 7 fishes (same subfamliy) at 5 sites of this lake, and comprehensively investigated the concepts of NC and competitive exclusion principle (CEP). The results showed that, NC exists on a large scale (trophic differences showing positive relationships with genetic distances), but not at the local scale (sister species showing larger differences than closely related species, and almost no trophic overlap), where the CEP works better. Besides, trophic convergence was also found between some very distantly related species, leading to the possible limited divergence. We believe these findings will contribute to future theoretical and empirical niche explorations.