General Santos, Philippines - This city on the southern Philippine island of Mindanao is known as the nation's "tuna capital", but unregulated overfishing by industrial players using unsustainable methods to catch the deep-sea fish is threatening a collapse of the industry.

Traditional handline fishermen on small boats say they now have to travel farther and stay longer at sea to catch bigeye and yellowfin tuna, whose numbers are on the decline, according to conservationists.

They blame a failure by regulatory agencies to manage tuna purse seine fishing, which uses fish aggregate devices (FADs) - buoys or rafts that drift or are anchored to attract fish, making them easier to find and catch tuna. These fleets, however, also catch a large number of young fish that are sold along with smaller skipjack tuna for tinning, rather than being thrown back to sea to spawn.

"I fear a collapse of the tuna industry if this goes on," Raul Gonzales, president of the Alliance of Tuna Handliners, told Al Jazeera at a recent "tuna festival" meant to raise awareness about sustainable fishing.

"The main problem is the overfishing of juvenile tuna. You look at the catch now and the average weight of one fish is about 30 percent less than what we used to catch," he said. "They are getting smaller - which means they are not spawning well."

According to statistics compiled by the Alliance of Tuna Handliners and Greenpeace, Philippine fishing fleets in the high seas deploy an estimated 3,000 anchored FADs and 7,200 drifting FADs in the western Pacific Ocean, home to rich tuna fishing grounds.

Within the country's exclusive 322km economic zone, statistics show there are at least 3,000 to 5,000 FADs, with the numbers steadily rising due to a lack of government regulation.

The situation has become so alarming that the European Union has warned the Philippines it would freeze tuna imports from the country unless illegal fishing methods were stopped. The EU accounts for about 40 percent of the Philippines' tinned tuna exports.

"Tuna is the backbone of our economy," General Santos City Mayor Ronnel Rivera said. "We are working with the industry to address these concerns."