びわ湖のホンモロコ個体群における変動要因の解析 : II. 冬期の個体群変動の要因解析の試み

抄録

In the previous paper (MAKI, 1966) it was stated that one of the life-cycle stages of this fish population which may be critically affected by certain factors for the yearly fluctuation is in winter, when the fish inhabit the bottom of the deepest part of the lake. Up to this season the fish are in the sub-adult or the adult stages. As the next step of this research the factors that regulate this fluctuation were investigated. Sample specimens of fry and juvenile were collected, using a small trap and a small beach seine from April to August in 1964,to examin their growth, condition factor and food. Sample specimens of the adults and sub-adults to examin the age, growth, condition factor, maturity and food were taken monthly from commercial landings between April 1964 and March 1966. Records of seasonal and vertical distributions of water temperature, pH, oxygen content and zooplankton from 1962 to 1964 by B.S.T. were used for the analysis of the factors of the population fluctuation. Records of the same environmental conditions measured by the Shiga Prefectural Fisheries Research Laboratory between 1949 and 1963 were also used for this analysis. Yearly changes of the environmental conditions such as water temperature, oxygen content and pH of the bottom layer of the lake in winter do not correlate to the abundance of the under-yearling population. The seasonal change of food organisms (planktonic crustaceans) in winter appear to have no correlation with that of the under-yearling population.