PUCUSANA, PERU—Aboard the Ocean Warrior in the eastern Pacific Ocean, José López proudly remembers his first catch: he was 13, and a local skipper, pitying his ragamuffin look, hired him as an extra hand. When he returned home, his pockets stuffed with a day’s wages, his mother protested. “She thought I had stolen the money,” López recalls between boisterous greetings to younger comrades who know him simply as “Pépe.” “I had to take her to the fisherman so she would believe me.” Since then, fishing has been a way of life for López and dozens of other artisanal fishermen in Pucusana, a port carved from the barren, desert-like hills south of Peru’s capital. For years the fleet thrived, earning López enough to buy a few boats and send his kids to college. But a decade ago the tuna that he once effortlessly fished started to vanish. So, the fishermen turned …