Lake Biwa

Lake Biwa is the largest freshwater lake in Japan.

It is located entirely within the Shiga Prefecture on the island of Honshu. This region sits in west-central Japan, northeast of the former capital city of Kyoto. The lake is a tectonic lake, meaning it was formed by movements in the Earth's crust. Scientists estimate that it is over 4 million years old. This makes it the 13th oldest lake in the entire world. Because it is so close to Kyoto, the lake appears often in Japanese literature and historical poetry.

The name Biwa-ko was established during the Edo period. Many people believe the lake is named for its shape. It resembles a biwa, which is a traditional Japanese stringed instrument. A 14th-century monk named Kōsō provided a famous explanation for this name. He wrote that the lake is the Pure Land of the goddess Benzaiten. He believed she lives on Chikubu Island. He also noted that the lake's shape looks like the biwa, which is her favorite instrument. In the past, the lake was also known as Ōmi.

Water moves through the lake via a specific system of rivers. Small rivers from the surrounding mountains drain into the lake. The Seta River serves as the main outlet for the water. The Seta eventually becomes the Uji River. This river joins the Katsura and Kizu rivers to form the Yodo River. The Yodo River eventually flows into Osaka Bay at the Seto Inland Sea. This water system is a vital resource for the Kansai region. It provides drinking water for approximately 15 million people. It also supports cities like Kyoto and Ōtsu, and helps local textile industries.

The lake is a massive center of biological diversity. Because it has existed for millions of years without interruption, many unique species have evolved there. Naturalists have documented more than 1,000 species and subspecies in the lake. About sixty of these are endemic, meaning they are found nowhere else on Earth. This includes 46 native fish species and subspecies.

One notable predator is the giant Lake Biwa catfish. The lake is also a major breeding ground for trout and for the pearl culture industry. Additionally, about 5,000 waterbirds visit the lake every year.

Some of the endemic fish species are very specific to these waters. These include five types of cyprinids and two types of gobies. There are also two types of silurid catfish and a cottid. The Biwa trout is another famous endemic species, though some scientists classify it as a subspecies of the masu trout. The lake also supports many molluscs. There are 38 species of freshwater snails and 16 types of bivalves. Nineteen of the snails and nine of the bivalves are endemic to the lake.

However, the ecosystem faces challenges from invasive species. Foreign fish like black bass and bluegill have impacted the local biodiversity. Black bass were first introduced in 1925 to be used as game fish. One largemouth bass caught in 2009 weighed 10.12 kg. This catch tied the world record held by George Perry for 77 years. Bluegills were also introduced after being presented as gifts to Prince Akihito in 1966. These non-native species compete with the ancient, native residents of the lake.

Humans have interacted with Lake Biwa for thousands of years. Archaeologists have discovered the Awazu site near the southern end of the lake. This is a submerged shell midden from the Jōmon period. It dates back to approximately 9300 BCE. The site shows how Jōmon people used local plants and animals for food. At the Awazu Midden 3 site, researchers found many horse chestnuts. These findings suggest that people had mastered ways to remove harmful tannic acid from the nuts. This made them safe to eat.

Today, there are many laws to protect the lake's environment. In 1979, the Eutrophication Control Ordinance was enacted to stop industrial pollution. This followed a successful public campaign called the Soap Movement. This law sets standards for nitrogen and phosphorus in the water. Additionally, the lake is a designated UNESCO Ramsar Wetland. This protects its valuable wetland areas. There is also a specific ordinance to protect reed vegetation zones. These reeds are important because they purify the water and provide habitats for birds and fish.