Biwako - Lake and Rivers

2. Measures at Biwako

2.5 Water level control for environment

2.5.1 Environment at Biwako

Biwako environment and habitats of the various organisms that live in the lake

While Biwako spans a large distance and has deep water, it also has a varied coastline. The surface of the open lake has an upper layer that receives sunlight and a deeper layer that remains at the same temperature throughout the year. Along the lakeshore reeds and other aquatic plants thrive, but besides sandy and gravelly shorelines, coastal features also include rocky shores and shores susceptible to rockslides. In some places drowned terraces make shallows that spread into the lake from the shore. The creatures living in the lake are active at different times of year and at different times of day and may exploit different ecological niches during their life cycle. Altogether, the flora and fauna comprise an ecological community that is special to the lake.

Fish that live in the lake and in the reed beds

Reeds flourish around the margins of the lake and aquatic plants grow on beds of the lake and inlets. This environment favors the gin-buna (a subspecies of crucian carp) and tamoroko (a type of carp), carp (farmed), eels and other fish that are distributed widely beyond Biwako.

Spawning grounds of fish that live in the lake and in the reed beds

The parts of the lake and inlets where reeds and aquatic plants grow abundantly are not only important to the fish that live there all the time. They also serve as spawning grounds for fish that spend their adult lives in deep water. Because plankton and other food organisms thrive in these waters, there is food for the fry that hatch and juvenile fish, so the margins of the lake are important in the life cycle of the open-water fish population.

Open water fish and species only found in Biwako

Fourteen species of fish are found in Biwako and nowhere else. Most of these endemic species live along rocky shores or in open water but, sharing common ancestors, some of these, such as the hon-moroko, nigoro-buna, and isaza are closely related to fish such as the tamoroko, gin-buna, and ukigori, which inhabit reed beds or inlets.

Fishing in lake and in the reed beds

Spring to early summer is the spawning season for the carp species. Adult fish that usually live in open water such as the hon-moroko, nigoro-buna, and gengoro-buna come inshore to lay eggs on the reeds and aquatic plants that thrive on the edge of the lake. After this, fish are caught by methods such as tatsube trap fishing or line and net fishing.

*Gengoro-buna, Carassius cuvieri; gin-buna, Carassius auratus langsdorfii; hon-moroko, Gnathopogon caerulescens; isaza, Gymnogobius isaza; nigoro-buna, Carassius auratus grandoculis; tamoroko, Gnathopogon elongatus; ukigori, Gymnogobius urotaenia.

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2.5.2 Water-level management at Seta River

If, after heavy rain, the lake water level is suddenly lowered to prevent flood damage around Biwako, it may adversely affect the spawning and growth of the fry of nigoro buna [Carassius auratus grandoculis], hon moroko [Gnathopogon caerulescens], and other fish living in the lake. Consequently, at the weir, trial operations to hold down abrupt lowering of the water level are being conducted.

The Seta River Weir is operated taking into consideration the many forms of life that live in the lake.

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