The surplus is created by the environment that grows the fish. Sunlight, plankton, little fish, big fish, plate. Then someone brings in a trawler that scoops u

The surplus is created by the environment that grows the fish. Sunlight, plankton, little fish, big fish, plate. Then someone brings in a trawler that scoops up all the fish, and the heretofore independent fishermen now need to do something to convince the seafood company to give them the fish sticks now pooped out by the ship. So the fishermen become tax accountants and baristas, to provide services for the fish company's clerks. Net, there is now more economic activity and more GDP. Whereas before, the country only produced as much fish as the fishermen needed to eat (and some surplus to sell at the local market), now it also produces tax services, and espresso services, and fishstick exports. This lasts for about a decade until the fish stocks are depleted. The fishermen, who used to live in bamboo huts they'd rebuild themselves every few years, now live in concrete apartment blocks, and they can no longer afford rent.