Recently, the Greenpeace ship Esperanza docked in Bangkok after patrolling in the Gulf of Thailand to witness the state of the sea. Over a fortnight, its crew documented more than a hundred examples of illegal and destructive fishing.

The ship encountered scores of bottom-trawling fishing vessels, which pull fine nets that scrape the bottom of the ocean, picking up all sea life - big and small.

The environmental campaign group says the marine ecosystem in Thai waters is being destroyed as a result of the controversial method. The nets also capture so-called "trash" fish, juveniles that are picked up unintentionally, leading to a massive reduction in fish stocks.

Fishermen controversially use fine nets to scour the bottom of the sea indiscriminately

"Let's say that, out of 100 kilogram weight of a trawler's catch, 60 kilograms will be trash fish," said Sirassa Kantaratanakul from Greenpeace Southeast Asia.

"Eighteen to 32 percent of the catch could be left to grow to be the next generation of the ecosystem or be our food in the future."

However, as margins get squeezed and fisheries look to maximize profits, almost everything the nets collect is sold. Firms don't earn much revenue from the trash fish, despite its uses - often being processed into cheap feed for farm animals or fertilizer.

"Trash fish can only be sold for just 4 baht (12 US cents) per kilo. But they still take out these juvenile fish so they can offset some of the cost of the oil to power their vessels. It's irresponsible," Kantaratanakul told DW.

Scale of problem worsens

Greenpeace's figures suggest that fish catches have declined continuously over the past 50 years. What was 300 kilograms (660 pounds) per hour in 1961 has plummeted to just 25 kilos per hour in 2011, according to a measurement known as Catch Per Unit Effort (CPUE).

But the Thai government says the figures are misleading and that the decline has been arrested in recent years.

"The CPUE from the last five or six years has been quite stable because of government action," said Dr. Wimol Jantrarotai, Director-General of the Fisheries Department.

"We've put artificial reef as a barrier along the coast line and from next year we're going to close the Gulf of Thailand for two months at a certain time of year to all fishing."

Small fish should be left in the ocean to breed and allow stock to recover, say Greenpeace

Currently, a few areas are closed off but the project's extension will see a much wider exclusion zone.

Dr Jantrarotai says in those areas where trawling has been restricted, the scheme has been successful at improving fish stocks.

Sailing too close

New fishing legislation is currently being picked over by members of Thailand's National Assembly. Officials say when it comes into effect next year, there will be even stricter penalties for illegal and destructive fishing.

But some commentators think that larger fisheries ignore current regulations and widespread corruption doesn't help.

"The commercial fishing vessels make up 20 percent of the total fishing vessels compared to 80 percent for artisan fisherman. But the commercial trawlers take 90 percent of the fish from Thai waters, so that is where the problem is," said Kantaratanakul.

The crew of Esperanza witnessed several instances where trawlers had entered protected waters. Thailand has a three-kilometer (1.86-mile) exclusion zone in place along its coast.

Greenpeace wants the law changed to expand the no-fishing zone to five or even twelve kilometers.

Fishing for trouble?

During the NGO’s monitoring activity, there were also incursions into Marine National Parks. There are six such parks around the Gulf of Thailand, which are also supposedly off limits.

The lack of police patrols means illegal fishing is rampant, according to Greenpeace.

Margins on fish sales are squeezed, despite there being plenty of demand

Kantaratanakul, who is Oceans Campaigner for Southeast Asia, said they saw many boats whose crews told them not to film as they knew they were fishing too close to the shore.

"The next day we went out again and no-one was there. So it shows that if the government had stronger enforcement, no one would break the law."

Officials cite a lack of resources for not stepping up patrols.

The Department of Fisheries says it has recruited the fishing community as its eyes and ears in the sea, which some experts think is flawed - a system of policing by those who need to be monitored.

Threat to livelihoods

"Last year there were more than 500 cases of illegal fishing and we apprehended a lot of big fishing vessels. In these cases, the big penalty is (can be) the confiscation of their vessel," said Dr Jantrarotai.

Greenpeace warns that Thailand's marine ecosystem is on the verge of collapse, threatening the livelihoods of millions of traditional fishermen as well as the larger fisheries.

It has demanded that officials investigate instances of unreported and unregulated fishing and says if the government fails to act, it will have a major political crisis as well as an environmental disaster on its hands.