Largemouth bass ‹i›Micropterus salmoides‹/i› and smallmouth bass ‹i›M. salmoides‹/i› were introduced to Japan from North America in 1925. Largemouth bass expanded the distribution earlier than did smallmouth bass. In recent years, however, smallmouth bass has also expanded the distribution quickly, resulting in the partially overlapped occurrence. To predict future ecological status of the two bass species, it is of critical importance to know to which degree the feeding habits of the two species differ, in particular under the coexistence.

We investigated the feeding habits of the two species by examining the stomach contents once a week during April to October 2010 in Inukami reservoir, in Shiga, Japan. The stomach contents were classified into 5 categories; natatorial fish, demersal fish, crustaceans, aquatic insects, and terrestrial insects, to calculate the index of relative importance (IRI) and it’s percentage (%IRI) for comparing feeding habits.

A total of 250 largemouth basses and 52 smallmouth basses were captured. The analyses on the basis of %IRI revealed some degree of food partitioning: The largemouth and smallmouth basses fed more crustaceans and more terrestrial insects, respectively. In contrast, no significant difference was detected for utilization of natatorial and demersal fish.