Normally around 150 of them swarm the area from April to June when blue crab season starts. But their numbers suddenly started dwindling in early April, and by April 11 there were none. A handful returned on April 19 but since mid-May no more have been sighted.

According to the government authorities on Wednesday, no Chinese trawlers have been caught by the Navy radar in waters near the Northern Limit Line, the de facto maritime border, since May 15.

No Chinese trawlers have been seen fishing illegally in waters off Yeonpyeong Island in the West Sea since last month. They used to swarm these waters during blue crab season each year.

Coast guards attribute their disappearance to the launch on April 4 of a special commando to protect waters off Korea's five northwesternmost islands. It consists of three 1,000-ton ships, six 500-ton ships, three speedboats, and about 400 troops.

Only a week after its launch, it caught five illegal Chinese trawlers, and the rest fled, scared away by warnings that the commando vessels are armed and will use their machine guns.

The Chinese government is also trying to crack down on the incursions.

New artificial barrier reefs aimed to prevent illegal fishing are also starting to become effective. The government spent W10 billion building about 700 artificial barrier reefs in waters off Yeonpyeong, Baeknyeong, and Daecheong since April last year (US$1=W1,124).

Many of them have hooks that can rip or snag fishing nets. Chinese trawlers use dragnets along the seabed that trap even young fish and have a devastating effect of fish populations.

With the Chinese trawlers gone, fishermen on Yeonpyeong Island caught 113,513 kg of blue crabs in April and May, more than double last year's catch.

