Underwater 'forests' face bleak future
A few years ago, a local fisherman in Minami-Boso, Japan, found an unusual fish in his gill net: a yellow-brown rabbitfish around 20 to 30 centimeters (about 7 to 11 inches) long. Originally from tropical regions, the fish is eaten frequently in Okinawa, but never in the Kanto region.
“It is now spending the winter here. I toss them back if they get stuck in my net,” said Minoru Hirota, the 66-year-old fisherman who heads a branch of the Iwai-Tomiura fishing cooperative.
Three years ago in April, on the first day of the abalone free-diving season, the same fisherman was astonished when he dived into three- to nine-meter-deep (9 to 29 feet) water around 50 meters from shore. The arame seaweed that abalone and turban shells depend on for food had vanished.
“An underwater forest the size of a primary or middle school playground had completely disappeared. It was like a desert,” he said.
The Chiba Prefectural Fisheries Research Center began a full-scale investigation last month, focused on the possibility that new residents to the area, like rabbitfish, are eating the seaweed.
The disappearance of seaweed beds along coastlines nationwide has become more severe in recent years. According to a survey by the Environment Ministry, seaweed beds extended for 208,000 hectares (about 513,979 acres) in 1978, but a 2007 estimate by the Fisheries Agency suggests the area has declined by about 40 percent to 125,000 hectares. The Fisheries Agency cites rising water temperatures in recent years as the primary cause, in addition to environmental changes caused by seawall construction and other activities.
According to the Japan Meteorological Agency, average yearly water temperatures in the ocean around Japan have risen 1.07 C in the last 100 years. The Intergovernmental Panel on Climate Change (IPCC) predicts that the distribution of marine species will change around the world as global warming continues, leading to declines in seafood catches.
According to Hiroya Yamano, head of the Center for Environmental Biology and Ecosystem Studies at the National Institute for Environmental Studies, corals that form branches or table reefs are spreading northward around the Japanese islands. Types of coral once found only in tropical areas now regularly appear, and the speed of northward expansion is as fast as 14 kilometers (about 8 miles) per year. At the same time, ocean temperatures in southern regions have gotten so high that coral is disappearing.
There is also the growing threat of ocean acidification. As the amount of carbon dioxide in the atmosphere increases with global warming, the gas is absorbed into the ocean and causes acidification, making it difficult for coral, with its calcareous structure, to exist.
If carbon dioxide continues to be emitted at the current pace, coral will no longer be able to live in the waters around Japan by the 2070s, according to Yamano’s research in a bleak view of the future.
Competition also arises between seaweed and coral when the water temperature rises.
“We are seeing cases of seaweed largely disappearing and coral increasing in areas on the Pacific coast of the Kyushu and Shikoku regions,” said Daisuke Fujita, an associate professor at Tokyo University of Marine Science and Technology.
It is not clear how this trend relates to the size of catches, but species that rely on seaweed for food are declining, as seen in the near-zero catch of abalone in Kochi Prefecture since 2000. Fishermen have other reasons to dislike the growing presence of coral, since their nets often are torn after getting caught on it.
There are also signs of competition near the northern limit of reef-building coral in Shizuoka and Chiba prefectures. Researchers are investigating the hypothesis that rising water temperatures are making it difficult for seaweed to grow and that coral is filling the space left behind.
Some have also begun new businesses taking advantage of changes in the presence of coral and fish species. The Shizuura fishing cooperative in Numazu, Shizuoka Prefecture, opened the Shizuura Diving Center in April, entrusting the management to a private-sector entity. In the seven-meter-deep waters, blue neon damselfish with yellow tails can be seen swimming above a green hime-eda midori-ishi (Acropora pruinosa) coral reef.
Masanori Yamamoto, 60, who runs the center and was previously a professional diver who conducted inspections of aquaculture nets, said he dreams of “spreading an industry of showing fish.”
Seaweed beds and coral reefs are often called forests of the sea. They absorb carbon dioxide and produce oxygen, and provide nutrition crucial to many creatures. It is necessary to implement drastic measures to reduce greenhouse gases, while still protecting the fishing industry around us.