Fish excrement could solve a decades-old ocean mystery. After marine plankton die, their calcium carbonate exoskeletons dissolve, making seawater alkaline; however, past studies found that the surface waters are more alkaline than expected from plankton. Now scientists at the University of Exeter in England and their colleagues have determined that calcium carbonate “gut rocks,” first found in toadfish intestines about 20 years ago, could account for a dramatic percentage of marine carbonate. Their computer models estimate roughly 812 billion to two trillion kilograms of bony fish swim in the ocean, producing some 110 billion kilograms of calcium carbonate annually. The amount constitutes at least 3 to 15 percent of the total ocean carbonate production and possibly up to 45 percent. Increasing sea temperature and rising carbon dioxide this century could cause fish to produce even more calcium carbonate, the researchers suggest in the January 16 Science.