Seaweed beds, the 'cradle of the sea,' vanishing

Fisherman Hideo Kawamura recalls how the Suruga Bay seabed full of kelp and other sea grasses gave him an eerie feeling as a novice abalone catcher.

"I went down to find grasses dancing as if they were the long hairs of a woman. It was scary," the 75-year-old says, but he could harvest "lots of large abalones."

That was more than 50 years ago.

The rich seaweed beds stretching 8,000 hectares and brimming with fish in the western coast of the bay off Shizuoka Prefecture have been transformed into a barren wasteland.

The sea grasses began to disappear around 1985; by 1994, almost all were gone. So were the fish and abalones.

"After the seaweed beds died off, horse mackerels and striped pigfish quickly swam away," says fisherman Mitsugi Tomita, 70.

The area's annual catch of abalones, which feed on sea grasses, has plummeted from 23.5 tons in 1992 to less than 1 ton.

At an area 10 minutes by boat from Sakai-Hirata port in Makinohara, the seabed 10 meters deep had nothing but rocks and stones covered by a thin layer of mud. Few signs of marine life were seen during a 20-minute dive.

Seaweed beds are called the "cradle of the sea" because they provide fish with oxygen, as well as places to hide and lay eggs.

The symbol of marine biodiversity, however, is fast disappearing from Japan's coastal regions in a phenomenon called isoyake, or denudation of rocky shores.

In 1991, an Environment Agency survey found 200,000 hectares of rich seaweed beds around the nation. The Marine Ecology Research Institute in Tokyo estimates about 20 percent had been lost by 2008.

The underwater deforestation is attributed to overgrazing by herbivorous fish, pollution and other factors, but the exact causes have not been determined.

In the Genkai Sea, a good fishing ground is showing visible damage from voracious feeding by the aigo mottled spinefoot off Matsushima island, in Saga Prefecture.

In the shallow waters 3 meters deep in the area, kurome kelp and other plants were growing thickly and glistening in the sunlight.

During a dive in the area last October, about 100 yellow ocher mottled spinefeet were seen swarming a kelp community. The young fish, about a few centimeters long, were pecking at the leaves and buds. Some plants were reduced to the stalks.

"This is damage from mottled spinefeet," said Shogo Arai, director of Aquascape Research Co., an environment consultant firm based in Shimane Prefecture, as he spread a leaf of wakame brown seaweed marred by teeth marks.

In the Matsushima area, the damage from mottled spinefeet was first confirmed in winter 2008.

The fish, which grows to about 30 cm, lives in warm seas of the Pacific. Arai says the feeding damage off Kyushu may be a result of a rise in sea water temperatures in recent years.

In winter, when sea grasses and algae grow, mottled spinefeet are in a hibernating state, rarely feeding themselves at 14 degrees or below.

But as sea water temperatures have risen, the fish remain active in winter, feeding on sea grasses in their season of growth.

According to the Saga Prefectural Genkai Fisheries Research and Development Center, the water temperature near Matsushima island in February has risen 1 to 1.5 degrees over the past 30 years.

A Fisheries Agency survey, meanwhile, has shown the effects of global warming on seaweed beds.

Its satellite survey measured sea surface temperatures around Kyushu between May and October from 2006 to 2008 to find more isoyake damage in higher-temperature areas.

Parrotfish, surgeonfish, northern sea urchin and other species also cause feeding damage on seaweed in Japanese waters.

Changes in sea currents and water quality due to inflows of fresh water from rivers and waste water are also blamed for the isoyake phenomenon.

A loss of balance in the local ecosystem exacerbates the problem.

"Feeding damage is troublesome as it drags on if left unaddressed, while other factors could improve over time," says Daisuke Fujita, an associate professor of coastal ecology at the Tokyo University of Marine Science and Technology.

When sea plants and grasses disappear, so do the fish, leading to a loss of biodiversity.

In western Suruga Bay, the sagarame kelp forests that scared young Kawamura are gone; sagarame harvest dropped from more than 40 tons in 1982 to zero in 1995.

At Sakai-Hirata port, the haul of abalones, flounders, sea breams and other fish totaled 700 million yen ($7.7 million) around 1990; it is about half that now.

A U.S. study in 1980 found 8,238 fish in a square meter of seaweed bed off Virginia, compared with 303 outside of such beds.

A 1996 study by Kumamoto University similarly found 274 to 440 marine creatures of 50 to 61 species per 346 square centimeters of seaweed bed off Iwate Prefecture, compared with 40 to 292 of 13 to 20 species outside of the bed.

Owase, Mie Prefecture, is one of the nation's first cities to take countermeasures after isoyake damage was reported in Owase Bay in the 1970s.

To grow sea plants, Owase's project sank about 900 concrete blocks planted with arame kelp over 17 years from 1992.

But the project that cost nearly 1 billion yen did not do much to restore the rich sea grass beds. In some areas, no sea grasses were found.

"It is really difficult to restore seaweed beds once lost," said Kazuharu Kurifuji, a former city official studying sea grasses.

Hiroshi Mukai, a professor of marine ecology at Kyoto University, explains seaweed beds are rich in oxygen that helps zooplankton grow and attract fish.

A loss of seaweed beds, in turn, leads to less oxygen and creates an environment too harsh for creatures other than bacteria.

"Unless the marine environment is restored to the original state, it would be hard to revive seaweed beds just by replanting grasses," he says.

The Environment Ministry's Biodiversity Center of Japan studied 129 seaweed beds across Japan over five years from fiscal 2002.

Its report said: "When (the environment has changed) beyond the limit for seaweed beds to regenerate themselves as part of the natural environment, any step (to stem the loss) would be too late."