魚類の摂食量と食物選択性の関係について : I.ハス稚魚の食物選択

抄録

The food selectivity of the juvenile Hasu (Opsariichthys uncirostris) was investigated in the summer of 1964,1965 and 1966 at Omatsuzaki situsted on the west coast of Lake Biwa, Shiga Prefecture. The fish specimens examined were caught with a small seine made of fine nylon mesh (2mm mesh), and simultaneously a series of quantitative sampling of zooplankton was done. The examination of the gut contents of the fish showed that they relied solely on zooplankton, especially Daphnia longispina, Diaphanosoma brachyurum and Mesocyclops leuckarti. Eodiaptomus japonicus, Leptodora kindti and Alona sp. played only a minor role, actually less than 5 per cent in total and thus they were neglected. Although the biomass of D. longispina was always smaller than that of other plankters in the water the fish fed on D. longispina exclusively. D. brachyurum was fed on in a large quantity only in the case where a high biomass level was associated with an extremely low one of D. longispina. M.leuckarti was fed on only when both of the former species were scarce. The selectivity index E_i (LVLBV, 1955) was employed to determine the grade of selectivity of each food organism, and the E_i value of each food organism fluctuates with the changes in the biomass level of all other plankters coexisting in the water. For example, the E_i of D. longispina usually took a high positive value, but the E_i of D. brachyurum took a positive value only when the former was very scarce.