By Andrew Harding
BBC News, Mindanao
By early morning, the last tuna of the day - a giant, glistening yellowfin as big as the fishermen who had hauled it ashore - was already gutted and cleaned, and ready for John Heitz to pass judgement.
Fish can travel from port to plate within 25 hours
Mr Heitz, a lanky, amiable American who has been in the tuna business for 20 years, took out a two-foot-long, hollow needle, and pushed it into the fish's side.
"Good," he declared, inspecting a thin worm-like sample of fresh sashimi. "Ice it and pack it."
Within 30 minutes, the tuna would be joining 50 others at the local airport.
Within 24 hours it would probably be raw and pink on someone's plate at a sushi restaurant in San Francisco.
Every day of the year, some 200 tons of fresh tuna are brought ashore at the quayside in this crowded port city at the southern edge of Mindanao island.
But the size of the catch is shrinking ominously, and today almost everyone involved in the industry here acknowledges that it is heading into deep trouble.
"It's getting worse," said John Heitz. "There are just too many people chasing too few fish. It seems like the industry is in denial. Taiwanese boats, mainline Chinese, South Korean, Japanese, European - everybody is fishing."
Low local catches mean many farmers seek fish in distant waters
Without radical change, he warned, "the industry will not survive".
Tuna is one of the world's favourite fish - from cans and steaks to the luxury sashimi market.
But in the past few decades, as demand has soared, the fleets chasing these migratory animals around the world's oceans have grown dramatically in size and sophistication.
Factory ships now stay at sea for years at a time, using giant "purse-seine" nets to catch entire shoals.
It has become a murky, ferociously competitive business, with a significant percentage of tuna, between 15-50% according to various estimates, "poached" by unlicensed operators who deliberately obscure the catch's origins.
Standing on the quayside, watching the controlled frenzy of a dozen boats being unloaded at once, Roger Lim - one of General Santos' most prominent fishermen with a fleet of more than 100 boats - sounded pessimistic.
"In 15 years there will be no more tuna," he said.
Mr Lim's boats all use traditional baited fishing lines to catch individual yellowfin tuna, primarily for the US and European steak and sashimi markets.
This method is, he says, sustainable. "We don't catch the small, immature fish. If all boats used handlines, there would be no overfishing."
He turned and pointed accusingly along the quayside, to where the purse-seine boats were unloading their own catch of smaller, skipjack tuna, destined for the canneries on the edge of town.
"They will destroy our waters," he said. "We need the government to intervene."
Marfenio Tan, 61, has been chasing tuna since he was 12.
Today, he owns 10 purse-seine boats in General Santos and - since there are few tuna left in the waters around the Philippines - a licence to fish off the coast of Papua New Guinea.
"We want a long-term industry," he said.
"Everybody must do conservation," said Mr Tan, pointing to new regulations halving the number of days he is now allowed to fish in the waters of Papua New Guinea.
Policing the seas
"We are just starting to implement a programme to save the tuna.
"For us to have a future, everyone must obey the law," he said.
But he acknowledged that many bigger ships were simply ignoring the new rules, and that the Philippines' navy, like many others in the region, was unable to police the seas effectively.
"And now China is starting to buy and fish for tuna too," he said. "If they develop a taste for it, then they will take everything."
For millions of subsistence fishermen, living in coastal communities around the Pacific, the decline in tuna stocks is already having a profound effect.
Half an hour's drive from General Santos, on the far side of the bay, the impoverished village of Kawas is struggling.
"A few years back, we would catch the tuna just off-shore - maybe one day's sail from here," said Raul Meijia, 38.
Now it takes five-to-seven days to reach the nearest fishing grounds, which are located, inconveniently, in Indonesian waters.
Mr Meijia's neighbour has just come home after three months in an Indonesian prison, having been caught poaching.
"This business is getting dangerous and risky," said Mr Meijia. "And life is getting harder."