Hokkaido is a treasure land for limnology. There are many lakes of different types ; large oligotrophic deep caldera lakes, mesotrophic lakes with intermediate depth, large shallow eutrophic lakes, large brackish water lakes on the shore and also many dystrophic bog waters. Natural lakes excluding small ponds in Hokkaido amount to 96. In addition, 55 artificial dams were erected for the purpose of getting electric power and agricultural irrigation (Hokkaido Fish Hatchery 1969).
Lake Toya approximately 11 km × 9 km (Figs. 1 and 2) is a large caldera lake located on the southwestern Hokkaido, with three islands in its center. Its maximum depth measures about 188 m. This lake is located in a comparatively mild climate area in Hokkaido with an annual variation of air temperature of 23.5 C to minus 3.2 °C (on decade average) (Fig. 3).The lake surface is never frozen except near the shore when winter is exceptionally cold. The temperature on the surface varies from 25 °C in August to about 4 °C in January-February. Thermocline develops from the middle of July, and is located at 5-20 m in August and disappears in late November. In recent years hydrogen ion concentration has been greatly increased due to the drainage from sulphur mining. This discharge has happened since 1939. The pH value was nearly neutral at the time before that discharge but it has gradually lowered year by year, in recent years the change was very rapid. It now measures pH 5-4 (1970) (Fig. 4 and Table 1). Along with this acidification of the water, dominant species of plankton altered (Table 2). Primary production during the period from August to November (1967-1968) was estimated as 40-150 mgC/m²/day.
In 1893 eggs of land-locked form of sockeye salmon, Oncorhynchus nerka (Himemasu in Japanese name), were transferred from Lake Akan into this lake ; and in 1926 eggs of rainbow trout, Salmo gairdnerii irideus (Nijimasu in Japanese name), were also transferred from the Chitose Hatchery. In addition, eggs of pond smelt, Hypomesus olidus (Wakasagi in Japanese name), were sown for 5 years since 1926 from Kasumigaura and other lakes. These fishes have well propagated in this lake and much supported commercial and sport fisheries. Fig. 6 shows the number of fry of land-locked sockeyes released from the hatchery on the shore and the catch of grown fish per fishing effort during the fishing season (December-August). It is indicated that the year class released in 1963 did not grow well because the catch of grown fish after 5-6 years was very poor. It directly or indirectly derived from the acidification of lake water. In addition, catch of the pond smelt and a commercially useful shrimp, Palaemon paucidens, were almost negligible after 1965 (Fig. 6).
Toya Limnological Station was established in 1937 taking over the Hatchery facilities operated by Fisheries Corporation (Figs. 9, 10 and 11). One of the writers, Wataru Otomo, Assistant Professor of Hokkaido University (formerly Hakodate College of Fisheries) has been a resident biologist for 34 years since 1937, and is retiring in March 1971 at the age of 63 (Fig. 7). Buildings of the station are old and the laboratory facilities are poor at present (Fig. 8). Every autumn, field operation and laboratory analysis in limnological observations and practice of artificial fertilization of land-locked sockeye salmon have been given to 40 students in biology course of the Faculty of Fisheries. Researches of visiting biologists were made at times. Limnological and productivity studies were conducted during 1966-1968 by a group participating in Japan International Biological Programme, Productivity of Fresh-water Communities.

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1) Aphanizomenon flos-aquae and Melosíra italica, and Keratella cochlearis and Trichocera dixon-nuttalli are the most abundant forms respectively of 26 species of phytoplanktons and of 19 species of zooplanktons found in the water of Lake Shirakaba during the summer of 1969. They occupy about 9095 % of total number of cells or individuals per unit volume of water. They live most densely in the surface water above 2 m. depth. And Microcystis aeruginosa and Anabaena sp. are present as well though the populations are by no means large.
2) Plankters peculier to cold water lakes seemed to predominate during the early years of the inundation. Holopedium gibberum was the most abundant. Recently species such as Vitreoscilla, Microcystis, Anabaena, Melosira, Asterionella, Golenkinia, Micractinium, Dictyosphaerium, Ciliates, Rotifers, and Bosmina have appeared in gradually increasing quantity.
3) The plankton chlorophyll is the largest in amount in the upper layer of 0.5-2m. depth maintaining 21 mg/m³ of water, from which the standing crop of the phytoplanktons is estimated about 1.8 g (dry wt.) /m³. The total quantity of seston is about 5 g/m³ in the same layer, the phytoplanktons constituting about 35% of the entire seston. The photosynthetic activity of the surface water amounts to 16.0 O₂mg/chl. a. mg/hr.
4) Lake Shirakaba is much poorer in the plankton crops than lake Suwa which lies not far from the former and is famous for its conspicuous eutrophication.
5) The standing crops of floating and Coli-group bacteria and of fungi are very large. The biomass of the floating bacteria is about 0.8-1.5g (wet wt.) /m³. The velosity of production about 0.6-1.6 g/m³·day, and the decomposition and production attain about to 1.42 mg O₂/1. day respectively. These figures are quite comparable with those of eutrophic lakes and fishponds of Nagano Prefecture where the present lake exists.
6) The bottom animals are poor in number, Chironomid larvae being about 300-700 and Tubificids less than 30.
7) Tthe benthic communities of the incoming streams of the lake are characterized by the inhabitants of the mountain streams such as Planaria, Asellus, Perissopeura, Rheotanyt-arsus, Uenoa, Simulium and Baetis, and algae such as Diatoma, Homoeothrix, and Navicula.
8) Originally riverine fish population composed of cold water endemic species such as Salvelinus pluvius, Oncorhynchus rhodurus and Cottus pollux disappeard soon after impoundment, and the fishes such as Cyprinus caprio, Carassius carassius, Pseudorasbora parva and Hypomesus olidus are now flourishing. But they can not live in the deeper water than 3 m. owing to the lack of oxygen.
9) It may be judged also from the standpoints of the biological phenomena that the lake is gradually becoming eutrophicated.

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