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Stomach Contents of Introduced Bluegill in Lake Shojiko, Yamanashi Prefecture

Hiroshi TSUNODA, Yoshito MITSUO, Mitsuru OHIRA and Yutaro SENGA （Tokyo University of Agriculture and Technology）

Abstract:
The stomach contents of introduced bluegill (Lepomis macrochirus) were investigated during 2007 to 2009 in Lake Shojiko in Yamanashi Prefecture, central Japan. Fry and juvenile fishes of the species mainly foraged zooplankton and aquatic insects, whereas adults foraged aquatic plants and algae. Food items were also changed seasonaly and the species mainly foraged zooplankton in spring and autumn and aquatic insects and snails in summer. Seasonal changes of stomach contents might be related to growth of bluegill and the seasonal changes of abundance of prey in the lake. The results of this study was compared with the previous one conducted in Lake Shojiko about ten years ago. Stomach contents of bluegill were largely different between the two studies. The main food items were fish eggs and chironomidae larvae in the previous study, but aquatic snails and trichoptera larvae frequently occurred in this study. Food niche breadth was broader in this study than the previous one, so the species foraged various food items in recent years. The results of this study showed that fish eggs were less foraged by bluegill than the results of the previous study, but it was not clear whether economic damage of bluegill to fisheries was decreased or not.

Key Word:
bluegill, exotic alien species, feeding habit, fisheries damage, highland lake

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