Tanzania is considering legal action after 22 European trawlers were spotted illegally fishing in its waters.
By Vicky Ntetema
BBC correspondent in Dar es Salaam
The ships were spotted by European Union surveillance planes and the information was passed to the Tanzanian authorities, EU sources say.
The ships were operating within the 12-mile zone, which is reserved for Tanzanian fishing boats.
EU sources estimate that some 70 ships are operating illegally, targeting tuna, kingfish, lobsters and prawns.
Tanzania is losing a fortune to illegal fishing every year, they say.
One of the ships reportedly does not even have a licence, despite the newly introduced regulations by the ministry of natural resources and tourism aimed at protecting Tanzania's dwindling fish stocks.
Legal foreign trawlers catch between 4-5,000 tons of tuna per day.
A licence costs some $18,000 a year, which some experts say is very low, when a medium-sized tuna sells for about $5.
However, the Minister of Natural Resources and Tourism Zakhia Meghji told the BBC that the government had to start somewhere, since many fishing companies used to pay nothing at all.
A joint project between Tanzania and the EU is expected to increase patrols on its mostly unguarded coastal areas and its 200km Economic Exclusive Zone.
The tuna fishing season is between June and July when the fish migrate between the waters of Kenya, Tanzania and Mozambique.
French naval forces operating in the southern Indian Ocean Zone told the government of Tanzania last year that they would help to arrest illegal fishing vessels along Tanzania's coastline.