Huge flocks of cormorants coming to the Han River during their annual mating season between April and June are wreaking havoc with the river's delicate ecosystem.
Bamseom islet on the river is littered with their droppings, causing trees to wither and perish. The waters around the islet are home to 32 different types of freshwater fish, but they are being decimated by the cormorants, whose numbers swelled from 427 in 2010 to 1,506 in 2015. Some 3,000 have been counted along the entire river.
Last Sunday, around 400 cormorants converged on the banks of the river. One 67-year-old who was fishing said, "I thought they were eagles. It was like a scene out of a scary movie."
The cormorants fill their stomach before flying on to Bamseom further up the river. "This is a unique phenomenon that began to appear just two to three years ago," said Eoh Kyung-yeon at the Seoul Grand Park Zoo.
Some 1,500 cormorants have gone to roost around the Paldang Dam in Hanam, south of Seoul and another 600 in Suwon. They fly to the Han River every day to fish. Some stay for several months on Bamseom.
The Seoul Metropolitan Government sends workers to the islet regularly to clean up the excrement at a staggering cost of W200 million per cleaning (US$1=W1,068). Last month, it deployed three cleaning barges, 10 support ships and 40 workers to clean the trees with water cannon.
"Just a few years ago, only the southern tip of the islet had to be cleaned and it took just a day, but now the entire islet is covered by bird dropping and it takes three or four days," a city official said.
The reason the birds are coming here is because their natural habitats in Russia and China have become overcrowded. Once only migrant visitors in the winter, they have now chosen to roost here too.
Cormorants are also considered delicacies in China and are hunted down. One bird is sold for W170,000. Fishermen in southern China famously tie ropes around their necks to use them for fishing. They catch the fish but are unable to swallow, which makes them the ideal instrument.
"Cormorants used to spend the winters in Korea and leave in the spring, but they recently began to roost here and propagate," Kim Hae-ni at Kyunghee University said.
Yoo Jeong-chil at Kyunghee University said, "There is an abundance of cornet fish and eels that cormorants eat. Cormorants are good at communicating with each other, so they will have told each other there are plenty of fish in the Han River."
But they are a threat to the fragile ecosystem. The birds can dive five to six meters below the surface and devour carp, cornet fish and eels and also intrude on the breeding grounds of other bird species.
Already herons have been pushed out of Jungnang Stream and moved to the upper tributary of Cheonggye Stream downtown.
Ecologist Lee Jin-hee said, "In Japan, authorities opted to shoot freshwater cormorants when faced with a surging population of the birds. We should allow that too, since chasing them away would only cause them to move to another area."
Cormorants roost in the trees on Bamseom in the Han River in Seoul on Monday.