South Korean fishermen who work the flashpoint maritime border with North Korea tend to be a resilient bunch, but these days a larger, more powerful neighbour is making them lose sleep.

"North Korea is nothing compared to the Chinese fishing ships," said Choi Won-Jin, who has fished the crab-rich waters around his home island of Daecheong for decades.

Daecheong is one of five "frontline" islands whose proximity to the disputed border with North Korea means they are manned by thousands of South Korean soldiers and bristling with artillery units and bomb shelters.

But all that weaponry has failed to guard against what Choi sees as the biggest threat to the livelihood of the islands' fishing communities -- the "invasion" of Chinese trawlers.

According to official estimates, more than 1,000 Chinese fishing ships illegally accessed exclusive South Korean waters around Daecheong last year, with only four coastguard ships on hand to pose a deterrent.

The numbers have been growing every year as China's increasing affluence and appetite for seafood pushes more fishermen to venture beyond its overfished waters.

Smaller, wooden Chinese ships sneaking into South Korean waters were once tolerated in an area where the top priority has always been guarding against potential incursions from North Korea.

- 'Nothing left' -

But in recent years, the small boats have given way to larger steel trawlers who engage in bottom trawling -- dragging a large, weighted net across the sea floor -- and sweep up "everything in their path," Choi told AFP.

"By the time they are gone, we have nothing left. It's all gone, including our fishing pots," he said.

Around 2,200 Chinese vessels have been stopped and fined by South Korea for illegal fishing in the past four years, and the number of arrested fishermen jumped from two in 2010 to 66 in 2013.

There were only five arrests in 2014, but coastguard officials said that was largely due to all resources being diverted to the lengthy rescue and recovery operation that followed the Sewol ferry disaster in April that year.

Chinese captains are well-organised, said coastguard commando Lee Kyung-Hak, and frequently chain their ships together "like a big floating city" in the event of a confrontation.

Crew members often arm themselves with steel pipes and knives, and have been known to throw burning gas canisters at officers trying to board their ships.

- Overwhelming numbers -

"We are trying our best to drive them off our territory... but the sheer number of them sometimes feels overwhelming," Lee told AFP.

A recent study estimated that 675,000 tonnes of fisheries products were illegally taken from South Korean waters in 2012 by China -- with a value of around 1.3 trillion won ($1.2 billion).

"If anything, the situation has worsened since then," said Lee Kwang-Nam, head of the Fisheries Policy Institute in Seoul who authored the 2014 study.

According to Lee, the undermanned coastguard only manages to seize or arrest less than one percent of Chinese poachers.

"Our fisheries resources are relatively well-preserved thanks to strict regulations... but may face serious shortages if this pace keeps up," he told AFP.

Under growing domestic pressure to crack down harder on the Chinese fishing vessels, South Korean officials have signalled a tougher line with the start of this year's fishing season in April.

"We were greatly outnumbered and overwhelmed by them last year... but we've had enough," said Yun Byoung-Doo, the chief of the Incheon coastguard which guards the Yellow Sea border islands.

Yun said the coastguard would use firearms, including handguns and onboard cannon more actively "if deemed necessary."

- China says be 'reasonable' -

Beijing's foreign ministry, when contacted by AFP, did not directly comment on Seoul's toughened stance against illegal fishing, but urged it to "enforce the law in a reasonable way, and ensure the safety and lawful rights and interests" of Chinese fishermen.

"China will continue to strengthen the education and guidance for its fishermen," it said in a statement faxed to AFP.

Two Chinese fishermen have been killed in violent clashes with the South Korean coastguard since 2012, prompting angry protests from Beijing.

Seoul insists the violence is initiated by the Chinese crews and point to the stabbing death of a South Korean coastguard member in 2011 by a Chinese fisherman.

South Korean fishing vessels have not been blameless themselves when it comes to illegal fishing in waters as far away as the seas off West Africa.

But the government has moved to eradicate the practice and South Korea was taken off the US list of countries engaged in illegal, unreported and unregulated fishing in February, and then from the EU list last month.

The fishermen of Daecheong island hope the government can be equally effective in curbing the illegal activities of the Chinese trawler fleets.

"This is one of the biggest crises I've ever seen on this island," said Kim Neung-Ho, whose father and grandfather also made their living in the waters off Daecheong.

"At this point we're not really counting on them all going away, because that's just impossible," Kim told AFP.

"We just hope that there will be fewer of them. Just a little fewer," he said.