Largemouth bass, Micropterus salmoides, have inhabited Lake Biwa in central Japan for over three decades as top predators. Lake Biwa consists of two basins – a large, deep north basin and a small, shallow south basin. Since the mid 1990s, vegetation and bluegill sunfish, Lepomis macrochirus, have thrived in the lake – especially in the south basin. Because dense vegetation can mediate the predator-prey relationship between largemouth bass and bluegill, the largemouth bass in the south basin are assumed to be under less favorable conditions than those in the north basin. The length-weight relationship of the largemouth bass in the two basins differed; the body weights of largemouth bass in the north basin significantly exceeded those in the south basin. Moreover, largemouth bass in the south basin appeared to have significantly larger gapes than those in the north basin. Such fish in the south basin were likely selected during and after the ontogenetic diet shift stage, probably because they are capable of handling bluegill more efficiently than those with normal-sized gapes.