DANDONG, China (Reuters) - A thriving trade in seafood across the Yalu River that separates China from North Korea has dramatically slowed, traders said, although there is still nearly a month to go for a United Nations deadline to tighten sanctions on Pyongyang as punishment for its missile tests.

Customers select seafood at a wet market in Dandong, Liaoning province, China, August 8, 2017. REUTERS/Philip Wen

The U.N. Security Council unanimously passed a resolution on Saturday banning North Korean exports of coal, iron, iron ore, lead, lead ore and seafood, intending to press the Asian state to renounce its nuclear weapons and ballistic missile programs.

Countries have 30 days to enforce the tougher measures, which aim to choke off a third of the North’s $3 billion annual export revenue, after the isolated country persisted with two intercontinental ballistic missile tests in July.

But a Reuters reporter who visited Dandong, through which about three-quarters of China’s trade with North Korea flows, was told by traders and fishermen that authorities tightened enforcement on seafood coming from North Korea on Saturday itself.

Trade in other goods across the border however seemed to be unaffected, with long lines of trucks queuing on the Friendship Bridge across the Yalu.

“It has pretty much all slowed,” said one worker at Dandong’s small Yicuomao port, adding that of the 10 or so major operators in the seafood trade only a few still continued to operate, risking fines.

Like many of those interviewed, the worker declined to be identified.

The port is usually a hive of activity, with a steady stream of fishing vessels returning from North Korea with their hulls full of fresh seafood. On a Reuters visit in April, activity in the market was frenetic.

It is not clear how much of the trade has official permission or whether any customs duties are imposed.

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But during a visit on Tuesday by a Reuters reporter, activity at the port was subdued, with idle workers saying Chinese authorities had ordered a halt since Saturday, in line with new United Nations sanctions.

Chinese trawlers and smaller vessels bring in catch from North Korean fishermen after trips into North Korean waters in the Yellow Sea, buying crabs, puffer fish and mackerel in exchange for alcohol, cooking gas and vegetables, traders in Dandong say.

Some seafood traders said spot checks from Chinese customs patrols had increased in recent months.

Still, they expressed confidence they would remain unaffected by the sanctions.

“Can China really enforce them?” said one owner of five small fishing vessels.

“Thousands of mouths depend on this trade for food.”

“COVERT WAY”

At a fresh produce market in the city center, stalls were filled with fresh crabs, fish and other seafood that vendors said originated from North Korea.

“More or less there will be some impact from the sanctions,” one stall owner, Wang Li, said.

“But they (the traders) will find a covert way around the ban.”

It was not clear who might have told the fishermen to begin stopping their contacts with North Koreans a month early.

Dandong government officials reached by telephone declined to comment.

Requests for comment on the seafood trade halt to China’s foreign, trade and agricultural ministries, as well as the quality watchdog that overseas food imports, were not immediately answered.

While China says it strictly enforces all U.N. resolutions against North Korea, local officials sometimes take it upon themselves to interpret - or ignore - such instructions.

A U.N. diplomat said North Korea had been expected to earn an estimated $295 million from seafood in 2017, as well as $251 million from iron and iron ore, and $113 million from lead and lead ore.

Chinese customs figures show it imported $91 million worth of seafood and $86 million worth of iron ore from North Korea in the first half of the year.

In Dandong, cross-border trade companies are already reeling from China’s ban on North Korean coal in February, although tourism and trade in everyday goods, including clothes and food products, appears to continue as normal.

The new sanctions also prohibit countries from increasing numbers of North Koreans working abroad, and ban new joint ventures with North Korea and any new investment in current joint ventures.