Nobody in his hut, including his seven children, sleeps under a net at night. Instead, Mr. Ndefi has taken his family’s supply of anti-malaria nets and sewn them together into a gigantic sieve that he uses to drag the bottom of the swamp ponds, sweeping up all sorts of life: baby catfish, banded tilapia, tiny mouthbrooders, orange fish eggs, water bugs and the occasional green frog. “I know it’s not right,” Mr. Ndefi said, “but without these nets, we wouldn’t eat.” Across Africa, from the mud flats of Nigeria to the coral reefs off Mozambique, mosquito-net fishing is a growing...