South Korea has seized four Chinese fishing boats on charges of illegal fishing in its exclusive economic zone in the West Sea, the oceans ministry said Thursday.

The West Sea Fisheries Management Service seized the four vessels, including two 99-ton fishing boats, between Saturday and Wednesday.

The four boats had permits to enter South Korea's western exclusive economic zone for fishing, but they are suspected to have used gillnets with a mesh 41 millimeters in diameter on average.

The ministry said gillnets with openings less than 50 millimeters in diameter are banned to protect marine resources.

The West Sea Fisheries Management Service -- controlled by the Ministry of Oceans and Fisheries -- said it later released all four boats after the fishermen paid a combined 290 million won ($242,000) in fines to the South Korean authorities.

Chinese fishing boats are often caught poaching in South Korean waters, and Chinese fishermen have used violence during raids by the South Korean Coast Guard.

In 2011, a South Korean coastguard officer was killed by a Chinese skipper during a raid. (Yonhap)