JOAL, Senegal — Once upon a time, the seas teemed with mackerel, squid and sardines, and life was good. But now, on opposite sides of the globe, sun-creased fishermen lament as they reel in their nearly empty nets.

“Your net would be so full of fish, you could barely heave it onto the boat,” said Mamadou So, 52, a fisherman in Senegal, gesturing to the meager assortment of tiny fish flapping in his wooden canoe.

A world away in eastern China, Zhu Delong, 75, also shook his head as his net dredged up a disappointing array of pinkie-size shrimp and fledgling yellow croakers. “When I was a kid, you could cast a line out your back door and hook huge…