Food habits of bluegillLepomis macrochiruspopulations in reservoirs in Kagawa, Japan

Abstract

Food habitats of bluegill Lepomis macrochirus populations in Hachiman and Saburo Reservoirs, Japan, were examined. In both populations, stomach content weights reduced relative to an increase in body weight, there being no gender-specific differences. However, relative stomach weights of males were significantly greater than those of females, a sexually dimorphic characteristic of bluegill. Feeding amounts of bluegills in Hachiman Reservoir tended to be low in winter–spring, and high in summer–autumn. Stomach contents, including insects, zooplankton, crustaceans, fishes, plants and inorganic debris, showed highly generalized feeding, although insects were frequently taken, particularly during spring–summer, with chironomids being a principle dietary item. In spring, numerous eggs of the crucian carp were ingested, suggesting a significant impact upon reproduction of the latter. In summer, many bluegill stomachs were filled with green algae. However, similar cramming of the intestinal tract suggested no digestive ability of bluegill for plant materials, including algae. In both habitats examined, the stomach contents of small bluegills (ca. 20–50 mm TL) comprised almost entirely zooplankton, diets subsequently diversifying gradually with growth. The almost complete absence of zooplankton in bluegills greater than ca. 110 mm in TL indicated a growth-related food habit shift from planktivory to omnivory.