The Sunday Times newspaper quoting John Church, an international fisheries and maritime security specialist as saying that an estimated 600 illegal, unreported and unregulated (IUU) vessels were operating in Sri Lankan waters at any given time.
Most of the media attention in recent times has been on several hundred Indian fishing vessels entering Sri Lankan waters under cover of darkness.
But the Sunday Times said Church told a forum at Kadirgmar Institute in Sri Lanka's capital Colombo that unlicensed Japanese tuna fishing vessels alone may be causing losses of 30 to 50 million dollars in foregone license fees.
Church is involved in a fisheries management protection program for Sri Lanka, the report said.
Church had said Sri Lanka may have to declare about 20 to 25 percent of the coastal zone as marine reserves to prevent depletion of fish stocks. When left undisturbed, fish stocks tend to bounce back quickly, he had said.
Church has said that according to one repo