As is shown by Fig. 1 in the text, Lake Naga-numa is found near Lake Oh-numa in the Kawadoi district of Nishikawa-machi, Yamagata Prefecture. These two lakes have never been investigated from the limnological point of view. In view of such circumstances, the present morphological and geochemical studies were carried out by the author in Lake Naga-numa in 1953.
The morphometrical data of this lake are summarized as follows :
Surface area 69, 400 m², Mean depth 0.345 m
Length of shoreline 1, 980 m, Volume 23, 900 m³
Maximum depth 2.21 m, Shore development 2.14
Judging from the temperature distribution of the surface water, it becomes clear that the water of the southern part of the lake moves from SW to NE and, different from this, the water of the northern part is at rest.
With regard to the water quality, pH and the concentration of chemical ingredients such as SO₄^2-, SiO₂ and Fe²⁺+Fe³⁺seem to increase respectively in summer.

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The pH value of the bottom mud measured in some Japanese lakes showed a close relation with the lake types. In general, the muds of eutrophic and mesotrophic lakes showed strong acidity, while those of deep oligotrophic lakes were neutral. The muds of dystrophic lakes reacted as weak acidity. An anaerobic layer established in the hypolimnion during the stagnation period seems to be the most important factor to determine the pH value of mud.
The pH value was closely related to the absorption of oxygen by the bottom deposits. The stronger absorption of oxygen was, the more was acidity of the mud. Black reduced mud showed particularly strong acidities as were seen in the lakes of the Nikko Volcanic Group.
The Japanese lakes were classified into five types according to the vertical difference in the pH values of the bottom muds.
The activity of bacteria in the bottom mud showed the relation with the pH value of mud. A remarkable multiplication of bacteria was observed in the oligotrophic and the mesotrophic lakes, the muds of which were neutral, while no bacterial multiplication was seen in the mud of the eutrophic and the mesobrophic lakes.

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Of the trophic levels in a gold-fish pond, the producer is the phytoplankton, the consumer the gold-fish, and the decomposer is bacteria. Benthic animals as well as water plants, both of which are found generally in the other inland waters, are not found in the gold-fish ponds throughout the year. The zooplankton is found only in winter, but its population is so small that we may disregard it, when we discuss the food-cycle of the ponds bioeconomically. Thus the structure of food cycle is very simple as shown in Fig. 1. We can, therefore, consider that the gold-fishes are cultured in the ponds practically by phytoplankton only.
In the ponds studied, there were cultured three forms of gold-fishes, i. e. Fringetail gold-fish (Ryûkin), Protruding-eyed gold-fish (Demekin), and Japanese gold-fish (Wakin). So as to determine the production of gold-fish, I examined only that of the Fringetail gold-fish, because this form was cultured in most ponds.
The young fishes of the Fringetail were planted in the ponds after their hatching. The mean weight of the young fishes was 0.024 g±0.001 g within the confidence limits of 95 per cent. Forty thousands to one hundred and fifty thousands of young fishes were brought to each of the ponds, and afterwards the artificial selection was made three times till the end of July in all the ponds. After the last selection, ten or twenty thousands fishes remained. Referring to the individual numbers in March of the next year (Table 2), it is easy to say that a half of the fishes died during the period from July to March next.
The production of Fringetail gold-fish per 1 m³ of water in each pond is calculated on the basis of the capacity of the ponds (Table 1), the yield of fishes in weight, and the mean individual weight (Table 2).
In the ponds of type A and type B, the floral components as well as the population of phytoplankton are different from each other [6] [8] and goldfishes in them are cultured only by phytoplankton. It is an important matter to discuss whether or not the production of gold-fish is different between both the types of ponds.
Analysing the variance in the stratification (Table 3, 4, 5), it was found that the difference was significant (0.05 > p > 0.01). As the result, the production in the first year was larger in the ponds of type A than in the ponds of type B.

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