Abstract: Marine ranching is an effective way to protect and restore coastal fishery resources. In order to assess the impacts of marine ranching construction on the trophic niches and interspecific competition of Sebastiscus marmoratus and Larimichthys polyactis, we analyzed the relationship between the changes of intraspecies trophic niche and the interspecific competition, based on the carbon and nitrogen stable isotope data of these two species collected from 2013 to 2016. The results showed that the intraspecies trophic niches of both species increased significantly in marine ranching area from 2013 to 2016. The total area of the convex polygon (TA) niche increased from 1.89 to 12.43 for S. marmoratus, and increased from 0.34 and 8.86 for L. polyactis, with the interspecific niche overlap area of these two species increasing from 0.13 to 7.69. The standard elliptic trophic niche increased from 1.11 to 4.94 for S. marmoratus, and from 0.14 to 4.06 for L. polyactis, with the interspecific niche overlap area increasing from 0 to 0.26. Values of the δ¹³C and δ¹⁵N of both species showed a tendency of decreasing with time, with significant changes of δ¹³C, but non-signi-ficant for δ¹⁵N. Dietary overlap between those two species increased from 68.9% to 90.6%.