Feeding habits of largemouth bass in a non-native environment: the case of a small lake with bluegill in Japan

Little is known about the ecology of largemouth bass, Micropterus salmoides in non-native environments. The stomach contents of a total of 381 bass ranging from 21 to 283 mm TL in a small lake in Japan were examined monthly from April to November 1992 by the ‘points’ method. The proportion of bass with empty stomachs increased in larger fish. The principal foods of the bass regardless of size consisted of two numerically dominant gobiids, the trident goby, Tridentiger obscurus, and paradise goby, Rhinogobius giurinus. The young-of-the-year (YOY) bass fed exclusively and selectively on the abundant young of paradise goby. As they grew, the bass also consumed fewer but larger trident goby. Bass of all sizes preferred these gobies from spring to autumn when several successive gobiid cohorts recruited from offshore and settled inshore. In contrast, YOY bluegill, Lepomis macrochirus, of 10-20 mm TL were selected only during summer. Bass switched from zooplanktivory to piscivory at 30-40 mm TL, much smaller than previously reported in their native range. Presumably because of the high availability and vulnerability of the gobies, the bass became piscivorous earlier and grew quickly. The gobies also allowed the bluegill to largely escape predation risk from the bass.