CHURCHVILLE, VA—In the 1980s, poor rice farmers in Asia and Latin America began digging out shrimp ponds to meet the soaring world demand for seafood. The environmental movement was, perhaps justifiably, aghast. The shrimp farmers had cut down lots of mangrove trees to make room for the ponds. Also, the effluent from the shrimp ponds was substantial, though we have to remember the ocean’s ability to dilute and disarm pollution quickly.