Nature of Lake Ashi

Lake Ashinoko is located in central Hakone, with Mt. Kurakakeyama to the south, and the outer rim of the crater, e.g. Mt.Mikuni to the west, and the central volcanic cone group, e.g. Mt. Kami to the east. On a clear day, Mt. Fuji is seen, wonderfully positioned in contrast to Lake Ashinoko, with its blue surface. Let’s look into the origin of  Lake Ashinoko, together with its flora and fauna, and animal life, existing in the surrounding environment.

The birth of Lake Ashinoko

About 3,000 years ago, there was a large-scale explosion of steam on the northwestern slope of Mt. Kamiyama and the stratovolcano located in central Hakone. The eruption triggered the sector collapse of a mountain causing an avalanche of rock debris that flowed down to the foothills of the Sengokuhara area. The Hayakawa River, which had previously flowed slowly in the caldera, then formed a dam near present-day Kojiri. This landslide in turn became a natural dam. The new Lake Ashinoko, a caldera lake, was created. The distance around the lake is 21 kilometers, and has a surface area of 7 square kilometers. There are many ancient trees of considerable size called ‘’Sakasa-sugi’’, buried in  Lake Ashinoko standing in an upright position.

Mizubachi (Agriotypinae Haliday), is a species of wasp—agriotypus that exists as a parasite on the pupa of ningyo-tobigera (Geoea japonica Bankes) named after the specimen collected in the vicinity of the Torii(gate) of Hakone Shrine. There are many other types of specimens found in Lake Ashinoko.



In Lake Ashinoko, there are many huge ancient trees termed ‘’Sakasa-sugi’’ that are buried in a standing position. When looking at the trees from above the lake, the topmost branches seem to be in the foreground, and the roots appear deep down at the bottom of the lake. Looking at the trees this way must have appeared to be looking at it from the top downwards, hence the name sakasa(upside down) sugi (cedar). In former times, the protruding branches,  together with the reflected image of Mt.Fuji over the serene surface of the lake, was one of the principal attractions of Lake Ashinoko.

This ‘’Sakasa-sugi’’ is firmly rooted at the bottom of the lake, therefore prior to its formation, cedar trees were thought to have covered its entire area of the present lake. However, when the ancient cedars were examined by taking underwater samples, their age was found to be 1,600 years old. Since the lake is said to have formed some 3,000 years ago, the theory marking the age of sugi forests to be 1,600 years old, contradicts itself. The solution to this anomaly could well be solved when one considers that the trees shifted in their standing position on account of landslides, caused by the earthquake. And in the 3,000 years since the lake was created, there have been huge earthquakes on a number of occasions in the South Kanto and Izu region.

The term ‘’Sakasa-sugi ‘’refers to trees relocated by landslides on a steep slope caused by earthquakes, and submerged whilst standing in lake water. Indeed, the cycle of large earthquakes can be determined by measuring the age of the buried cedar trees. Evidence reveals that there were huge earthquakes some 1,100, 1,600, and 2,100 years ago.

Lake Ashinoko is located at an altitude of 723 meters, the water  temperature is in winter, 2.5〜4.5°C, and in summer, 25〜27°C at its highest. Fog tends to form easily around the lake and can become quite dense, so available sunlight is lessened. Although the altitude is under 800 meters, beech trees grow prominently, together with the growth of yamaboshi (kousa dogwood; Cornus kousa), himeshara (tall stewartia; Stewartia monadelpha), and togokumitsubatsutsuji (Rhododendron wadanum), all pleasing to the eye. And where the bed of the lake is shallow,  one finds sekishomo (Vallisneria asiatica), and kuromo (waterthymes; Hydrilla verticillata), ibaramo (Najas), and shajikumo(Chara braunii)  at a depth of 10〜15 meters.


As with other caldera lakes, Lake Ashinoko was thought to be originally an unfavorable habitat for ichthyofauna. Fish that had once swum the Hayakawa River upstream, and intruded the Lake Ashinoko are sparse in number, e.g. ugui (Japanese dace;Tribolodon hakonensis), unagi(Anguilla japonica), and yamame(Oncorhynchus masou), all bearing the name hakonensis.

However, in 1879, the program to breed fish first started. Most of the 23 species of fish found today were introduced by transportation, and then released into the lake.

Much later, in 1985, the breeding of numachichibu(Trdentiger brevispinis Katsuyama) and bluegill (Lepomis macrochirus), and in 2005, utsusemikajika(Cottus pollux sculpins) were confirmed. The most famous of these newly introduced fish was the black bass imported from California in 1925,  by Tetsuma Akaboshi. Black bass was found to be popular among fishing enthusiasts for Lure fishing. But when taken from the lake, it spread throughout  Japan, thereby causing damage to the ecosystem by feeding on the indigenous aquatic organisms. They were designated as Specific alien creatures. The permitted fish for fishing include; brown trout(Salmon trutta), rainbow trout (Oncorhynchus mykiss), sakuramasu masu salmon (Oncorhynchus masou), hime masu(Kokanee trout), and wakasagi (Hypomesus nipponensis). Today, there are 40 species including jibotaru, found in the habitat.



The aquatic animals in Lake Ashinoko

Aquatic animals recorded are; 12 species of shellfish including otanishi (Cipangopaludina japonica), dobugai (Sinanodonta woodiana), and 6 species of crustacean,e.g. sujiebi (lake prawn; Palaemon paucidens), tenagaebi (freshwater shrimps; Macrobrachium), sawagani ( Japanese freshwater crab; Geothelphusa dehaani) .

A further 40 species of Aquatic insects are also to be found; toyomonkagero (Ephemera orientalis), shirinaga-madara-kagero(Ephacerella longicaudata), aosanae (Gomphidae luciola cruciata), genjibotaru (Japanese firefly; Luciola cruciata). Among them, one finds the distinctive northern cold water type of nihonmizushitadami (freshwater shell; Valvatidae Cincinna japonica) which is the type specimen found locally, and with Lake Biwa’s komizu shitadami (valvatidae), marks the southern limit of its distribution. Although the vivalve-Hakoneshijira (Inversidens hakonensis. Another name; Yokohamashijira) has become extinct, however, Lake Ashinoko was the locality in which this particular type specimen was produced.

Mizubachi (Agriotypinae Haliday), is a species of wasp—agriotypus that exists as a parasite on the pupa of ningyo-tobigera(Geoea japonica Bankes) named after the specimen collected in the vicinity of the Torii(gate) of Hakone Shrine. There are many other types of specimens found in Lake Ashinoko.

Hakone sansho uo

Hakone sansho uo, the Japanese clawed salamander, so named, is based on a specimen collected in 1776, in Hakone mountain by the Swedish botanist Thunberg. It is closely associated with Hakone, as it was first introduced in a book bearing the name Hakone.

In 1962, it was designated as the natural monument of Hakone town, and it is still carefully protected. This same species is stated to have been distributed in the mountains of Aomori  to Yamaguchi Prefectures and Shikoku Prefecture. However, according to research carried out by Dr. Natsuhiko Yoshikawa of Kyoto University Graduate School, there were five other types of concealed species than that of the Hakone clawed salamander. This was previously thought to have been a single species.

The genuine Hakone sansho uo inhabits only the southwestern part of Honshu, the main island of Japan, including Hakone. The others are recorded as more recent species. Three types of species have already been listed.

Considering they are amphibians, it is remarkable to find as many as five new species at the same time, as these amphibians are not as common as fish. This reveals the importance of the regional-type specimen and shows the significance of irreplaceable local organisms.