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.

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A Pale Thrush (Turdus pallidus, ♀, 82.0 grams) was netted, in the Heiwa Park, Nagoya City, on Jan. 3,1969. She was immediately released with a transmitter and then tracked for four days. The location was determined every 30 minutes from sunrise to sunset during the period, and the home range and the activity related to light intensity were studied as determined by telemetry. The results obtained were : 1. The area of the home range was about 100m×100m. 2. The location of the roost changed every night but it was always in clamps of a broadleaved tree. 3. The light intensities for her first movement in the early morning as well as for the last movement in the evening were almost the same every day. The former was lower than the latter. The transmitter used was 17.5×16.0×20.0 (mm) in size and 8.5 grams in weight. The frequency of the carrier wave was 50.2 MHz. The battery life was about 10 days. The receiver used was designed especially for this study.

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The immigration number from the sea in spring, and population density, body-length distribution and social behaviour during the settling season (summer) of the Ayu-fish (Plecoglossus altivelis) were investigated from 1955 to 1969 in the River Ukawa. The population density varied between 0.03 and 5.5 indiv./m^2,but the natural mortality from spring to summer was stable being about a half to onethird. It was confirmed that the social behaviour was changed by its population density and that the growth was not limited directly by algal production but mediated by its own social structure. When overall population density was about four times to that the all fish had their own territories, territorial structure was established only in certain types of river-bed. The difference resulted from the relation between the value of the feeding or resting site and its closedness against the invasion of non-territorial ones. Territorial structure of Ayu had probably evolved as a self-regulatory process but was not so distinct as at the present time.

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1) Vertical distribution of heterotrophic bacteria in the water body of Lake Yuno-ko showed a clear seasonal variation; during the summer stagnation period, the number of heterotrophic bacteria was larger in the upper layer than in the lower layer, while during the circulation period, it was distributed uniformly in vertical direction. 2) Total plankton respiration measured with concentrated plankton gave the highest value (3.11g O_2/m^2/day) in August and the lowest (0.47g) in February. During the circulation period, the total plankton respiration was uniform vertically, while during the stagnation period, it was higher in the upper layer. 3) Respiration rate of the bacterial population in the lake water was calculated on the basis of the data on the number of bacteria and the respiratory activity of one bacterial cell, which was determined with separated and concentrated bacterial population. Percentage contribution of the bacterial respiration to the total lankton respiration was estimated as only 3 to 7. 4) From the above result, it was suggested that bacteria might play a minor role in the mineralization process occurring in the water body. The major agents participating in the mineralization in the water boyy were suggested to be the phyto-and/or zooplankton. 5) Organic matter loss from the lake water by the precipitation of particulate matter appeared to be smal ; the loss was largely attributed to plankton respiration in the warmer season, and to the outflowing river in the coleer season.

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