By Yi Whan-woo

North Korea has been banning its fishermen from going fishing in small boats amid the latest cases of North Koreans fleeing to the South by sea, according to sources, Thursday.

A fishing boat small enough to fit less than 20 passengers was recently used for a defection as it was not subject to military surveillance at sea.

Authorities have been issuing fishing permits only for those who plan to catch fish with larger boats, requiring an official of the State Security Department to be on board when in operation.

"This time of the year is good to catch octopus, which is a major source of income for most fishermen who work individually," a source said. "But they can't go out on the sea because they only have small boats and the authorities are refusing to issue them fishing permits."

The sources said the only way for those fishermen to work is to register their boats with the military.

The registered boats then must operate in groups of 10 under the surveillance of coast guard ships or torpedo boats.