Rapidly increasing numbers of great cormorants, forming colonies near lakes and rivers, have been causing serious damage to local fish stocks such as “ayu” (sweetfish) and koi, according to researchers.

The domestic population of the great cormorant once tumbled to about 3,000 during the 1970s due to environmental pollution, but has since grown back to about 70,000 due to preservation efforts and stricter controls on pesticides.

The bird is about 80 cm long and weighs about 2 kg, but can be as big as 130 cm with its wings spread.

It consumes as much as 500 grams of fish a day and migrates through the year, flying at about 60 kph. The breeding period continues from January through November, with two to three birds leaving one nest on average.

Overall damage caused by the increase is currently estimated to exceed ¥10 billion a year, according to the researchers.

Kiichi Hoshino, a 66-year-old koi farmer in Nagaoka, Niigata Prefecture, said he had his fish eaten by great cormorants for the first time last July.

One day he saw two great cormorants taking off from the ponds where he farms some 4,000 carp, and later found they had taken about 100 young fish.

“I had never imagined that great cormorants flew to such deep mountains and decided to put nets over the ponds,” Hoshino said. “I thought I would otherwise lose all my carp.”

Although Hoshino installed nets on 18 ponds, the great cormorants have torn them with their sharp bills and still fly into the ponds.

In its 2012 survey, the Niigata prefectural freshwater fisheries research facility discovered that 73 percent of 70 koi farmers in the prefecture suffered damage from the birds.

The facility also reported that overall damage totals roughly ¥20 million.

The ayu business on the Uono River in the city of Uonuma, Niigata Prefecture, has also suffered a serious blow from great cormorants as they fly to the area to feed on the roughly 1 ton of sweetfish that are released into the river every year.

Sales from recreational fishing have plunged over the years as anglers have stopped coming to the area.

Masayuki Suzuki, 63, a senior official for the Uonuma fisheries cooperative association, said, “As damage keeps expanding and our sales are dwindling, the association has been badly affected.”

Maki Yamamoto, an assistant professor at Nagaoka University of Technology, said that figuring out the actual extent of the damage is not easy because the kind of fish the great cormorants eat depends on where they live or on the season, and that administrations have failed to take any effective measures.

She said that to estimate the value of damage, researchers need to find out what kind of fish and what amount the birds have eaten by examining the tiny remnants of fish in the birds’ stomachs.

But some areas have succeeded in reining in the population in their own way. In 2009, the Shiga Prefectural Government hired professional hunters to catch cormorants and as a result the population dropped from 40,000 to 9,000 by 2012.

Yamanashi has created specific environments that are friendly to great cormorants so that their colonies do not spread across the prefecture.

But Yamamoto said that “eliminating the great cormorant means ignoring the local ecology.”

“It is natural that a proper number of great cormorants live and eat an amount of fish that local fishermen can accept,” she said.

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