recently a portion of the migration route of sea turtles has been clarified by tracking using satellites, baby eel and eel eggs have been discovered in oceans far away from Japan, and accumulation of knowledge related to such animal movement has made remarkable progress (Figure 2-2-6).

Lastly, in contrast to the flow of nutrient from land to ocean, there are also flows that go from ocean to land through rivers. For example, nutrients from the ocean are carried up in rivers by salmon swimming upstream. The salmon that have come upstream are preyed on by brown bears and sea eagles and become wastes such as leftovers and excrement. Nutrient salts are supplied to land by sea birds, which catch fish in the ocean and carry them to their nests in land and return them to soil as waste. These organic molecules are used by the plants and animals living in the forest (Figure 2-2-7)

Figure 2-2-7 Connections based on the Food Chain between Land Areas and Sea Areas in Shirotoko

Source: Shirotoko National Park pamphlet (Ministry of the Environment)

# The History of Bird Dropping and Humans

From the time that agriculture began until the present, we humans had a serious problem concerning how to obtain the nutrient salts necessary for fertilization. The three major elements necessary for growing plants are nitrogen, potassium and phosphorous. Potassium is an element that is found in greater amounts in land areas than in the sea. Nitrogen is an element that is circulated through the air and soil by bacteria that can fix nitrogen from the air to the ground. However, phosphorous is abundant in the sea but scarce in land areas, and natural cycle systems for circulating the soil of ocean and land were limited.

Great Cormorants Building a Nest

Photograph provided by Kentaro Takagi of the NPO Japan Bird Research Association

Chile and Peru have large amounts of mineral ore derived from bird excrement called “guano” that sea birds such as Cormorant built up over the past several thousand years. Since the era of the Inca Empire, “guano” has been used as precious phosphorous and nitrogen fertilizer. It is believed that on Chincha Island, which is one of the islands that has guano, there were sediments derived from bird excrement that exceeded 30m. From the 19 $ ^{th} $ century to the 20 $ ^{th} $ century, countries mainly in Europe imported this mineral ore as a resource necessary for fertilizer. It contributed greatly to the increase of food production of in the European region at that time and was treated as important mineral resource for the region.

In Japan as well, it was a known wisdom from long ago to use the excrement of water birds as a means of solving problems related to a shortage of phosphorous. The Great Cormorant in the photograph on the left is one of such water birds.

The Great Cormorant is a fish-eating water bird that inhabits all of Japan. It dives into the water to catch fish across a wide range of water areas from ocean areas to brackish water areas in coastal regions and in freshwater areas in inland regions. It is also known that they cause enormous damage to inland fishery because of the greedy appetite. In order to raise their chicks, Great Cormorants build nests in the tree crowns of forests near waterfronts, and in recent years in Japan they have formed a giant colony of tens of thousands of birds on Chikubu Island in Shiga Prefecture.

Amounts of excrement of Great Cormorants is so large that it causes large impact on vegetation and trees around the colony wither and die. On the other hand, since long ago that excrement has been utilized as very valuable in the form of fertilizer for agriculture.