When the lake water moves periodically, stationary oscillations of the water level are observed.They are known as seiches. It is to be expected that islands have some effects on the seiche.
To investigate these effects, oscillations of the lake surface and velocities have been calculated numerically using two-dimensional models. The results of these calculations show that the island makes the period of the seiche longer. The experimental equations for this period prolongation are presented.

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Mercury in fifteen rivers and two lakes was measured to determine a background level of mercury in fresh waters. Mercury in unpolluted fresh waters contained concentrations between 1.0-5.1 ng/l, which approximate sea water values. Since mercury concentration in precipitation is ca. 1 ng/l, mercury levels in the world's rivers would be expected to be similar to those in Japanese rivers. Mercury was one order of magnitude higher in rivers receiving wastes from mercury mines than in unpolluted waters.

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The course of morphological development of feeding organs of the phytoplankton feeders, Kawachi-buna, which is a domesticated race of Gengorobuna (Carassius auratus cuvieri) and Silver carp (Hypophthalmichthys molitrix), were compared with those of the omnivorous fishes, Nigorobuna (C. auratus grandoculis) and Big head (Aristichthys nobilis). In addition, a comparison of the functions of the feeding organs in Kawachibuna, Nigorobuna and Silver carp was attempted by measuring the ingestion rates of two different sizes of ¹⁴C labeled green algae (Closterium moniliferum, 300μ in length and Selenastrum sp., 10μ) at various growth stages of these fishes.
In Kawachibuna and Nigorobuna of 13 mm in total length, the number of gillrakers are almost equal, but the feature of micro-spines on each gillraker appears considerably different (See Fig. 3). In Kawachibuna of more than 20 mm length, the number of gillrakers exceeds that of Nigorobuna of the same length, and the micro-spines on gillrakers are more complicated. In Silver carp less than 20 mm in total length, numerous filaments on gillrakers are not fused as in the adult Big head's. At 26 mm, one or two fused parts are found in a middle part of the gillrakers, and the number of the fusions increases with growth (see Fig. 4). Among the four cyprinid fishes with the same length, no marked difference in the maximum length of gillrakers on the first gillarch is observed. However, the maximum length of gillrakers on the second gillarch becomes longer in the order of Nigorobuna, Kawachibuna, Big head and Silver carp.
Relationship between the ingestion rates (I, mgC/hr) in water containing a high density of Selenastrum and body weight (W, mg) were expressed by the following formula; I=3.28W^0.79 in Kawachibuna and I=2.25W^0.73 in Silver carp. The ingestion rates for Closterium relating to body weight of the both fishes were also expressed as I=5.14W^0.72 in Kawachibuna and I =3.18W^0.89 in Silver carp. The both ingestion rates for Closterium and Selenastrum took a close value in Kawachibuna and Nigorobuna less than 11 mm in total length. However, in Nigorobuna of more than 11 mm, the ingestion rates for the both species of algae became lower than those of Kawachibuna with the same length. It is suggested that the difference in the ingestion rates for the both species of algae in Kawachibuna and Nigorobuna of more than 11 mm in length is attributable to the morphological difference in their feeding organs as mentioned above and probably to the difference in their preference for algae.
In young Silver carp (T. L. 42-45mm), relationships between the ingestion rates and the food density of the both species of algae were well fit to the following formula; r=5.54(1-e^-0.160p) for Clasterium and r=3.61(1-e^-0.072p) for Selenastrum, where r is the ingestion rate (mgC/W^0.81/hr) and p is the food density (mgC/L). The formula for Clasterium was also applicable to young Kawachibuna (T. L. 40-43mm). However, the ingestion rate of Kawachibuna relating to various food densities of Selenastrum was much lower than that of young Silver carp and was rather fit to a linear equation ; r=0.108p-0.225. It is suggested that the difference in the ingestion rates for Selenastrum between the two is mainly caused by the morphological difference in their feeding organs.

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