Raul Gomez is an old man who fishes with five crew on a clipper in the seas known as the coral triangle, and he has spent two months now without taking enough to feed his family.

Riding out storms and searing heat in western Pacific waters, the burly, sun-inked Filipino uses a pole and line to reel in yellowfin tuna the size of an adult human.

This has been his trade for 40 years, but it is becoming tougher as fisheries in this region – one of the planet’s most important centres of tuna production – face the prospect of total collapse.

Gomez did not hear the dire prediction this year by the world’s leading body of biodiversity scientists, who warned that exploitable fish reserves in Asia-Pacific waters were on course to crash to zero by 2048.

All he knows is that with every year that passes, he has to spend ever longer at sea to catch ever smaller tuna, at greater personal risk and lower reward.

Gomez says that when he started out at the age of 14, he and his father would sail from General Santos port to the Sarangani Bay on a small bangka (outrigger boat). One night was enough to return the next day with 10 tuna, often weighing in at more than 100kg.

But the global surge in demand for sushi, sashimi and tinned tuna has brought more competition and massive overfishing.

Today, the bay is largely fished out, as are most of the surrounding waters in the Mindanao Sea, so he voyages further in clippers that can spend months away from port. But as the boats get bigger, the tuna are getting smaller. These days, Gomez considers himself fortunate to take a bigeye tuna of 70kg.

On his latest trip, his share of the take is just 4,000 pesos – equivalent to £1 for each of the 60 days he spent at sea.

“It is harder now to make a living and feed my family,” he says.

As soon as his boat docks, Gomez unloads his catch and then immediately prepares to set out again to make up for lost income. Once the hold is refilled with ice, he will be gone.

The fisherman sees his wife and children just two or three times a year even though they live less than half an hour from the port. His eldest son comes to spend a few hours with him on the foredeck, knowing his father could be away not just for months, but for years.

‘It is total exploitation’

Gomez has been arrested and jailed three times for illegally fishing in Indonesia, where the tuna are more abundant. His Filipino boss – the owner of the boat – orders him across the maritime border, but takes no responsibility when Gomez gets caught. Instead, the fisherman has to find his own way home.

“It’s a kind of slavery,” says Gomez (who asked that his name be changed due to concerns about retribution). “At times, I’ve gone years at sea for no pay.”

This is far from unusual. More than 600 Filipino fishermen have been jailed by Indonesia. Almost all were subcontracted by boat owners who gave no help in repatriating them.

“It is total exploitation. The fishers have no choice. They know that if they don’t cross the border they won’t have their contracts extended,” says Benjamin Sumog-oy of the Sentro labour organisation. “The relationship is feudal. The workers are not recognised as employees. They cannot join unions. They have no rights, no salaries. It is semi-slavery.”

Consumers in rich nations increasingly get their protein from seafood, but the rush to cash in has put pressure on fishing crews, driven down the quality of catches and eroded the sustainability of fisheries.

Several tuna species are in peril. Bluefin are critically endangered with just 2% of their 1950 biomass left, bigeye recently fell below the 20% level necessary for replacement, and yellowfin are also down more than 70%.

Major fishing nations such as Japan and South Korea have tightened restrictions in their coastal waters. But big companies increasingly use foreign-flagged ships and less-regulated overseas ports, particularly in the western and central Pacific.

World’s ‘tuna capital’

General Santos, which proclaims itself the “tuna capital”, is at the heart of this region, which is where one in every two of the world’s tuna is caught.

Tuna landings have steadily increased and there are plans to expand the port further by 2021. But many grumble that too much of the business is geared towards big – often foreign – vessels using purse-seine nets that land unsustainable catches of juvenile fish that would not be allowed in many other countries.

This leads to a rivalry inside the port, which cranks into life from 4am, when fishermen bring their catch to shore.

At Market One, pole-and-line fishermen like Gomez haul hefty adult fish over their shoulders along wooden planks to weighing machines, where they are hooked, measured and then carried to a bloodied table for butchering. At the highest end, traders email photographs of samples to buyers in Japan, South Korea, the US and Europe. The carcass is then packed with dry ice and sent to the nearby airport for freight via wholesalers to restaurants, sushi shops and supermarkets. Everything is done for a global market to global standards.

The story is very different at Market Two, which theoretically supplies domestic buyers. Bigger purse-seine ships dock here and discharge tonnes of smaller net-caught tuna on to a conveyer belt for sorting. Taking small, young fish would be prohibited in much of the world, but it is legal in the Philippines. While this may be necessary to feed a local population that has been largely priced out of the market for bigger fish, the bulk of this catch goes to canneries, which then sell to multinationals. A trader at Market One describes this as a form of tuna laundering.

“They are taking out a generation of tuna.”

A port regulator said warnings of fisheries disappearing before 2050 are realistic and terrifying.

“There are more fishers and less fish. That’s the problem,” said the official. “There will have to be restrictions in the future.”

A Filipino fish port worker pours water on small tunas in General Santos, southern Philippines, which is known as the country’s tuna capital. Photograph: Ritchie B Tongo/EPA

Four years ago, the Philippines was given a yellow card over its fisheries by the European Union, prompting the government to pass a stricter fishing law. But there is still no harvesting strategy and illegal and unregulated fishing remains commonplace.

During the Guardian’s visit, young yellowfin and big-eye – none more than 50cm long – were seen on the conveyor belt from ships into General Santos market.

“There is a lot of illegal fishing in General Santos,” says Vince Cinches, Philippines oceans campaigner for Greenpeace south-east Asia. “They are taking out a generation of tuna.”

Harvest limits, better monitoring and traceability – not to mention unionisation – could also help fishers such as Gomez, who feels the pressure of declining stocks and rising demand more than anyone.

For now though, what the old salt catches he cannot afford to eat; what he earns is no longer enough for his family. His wife is now the main breadwinner and the main dish on the dinner table is ever more likely to be chicken than tuna.