Faced with declines in coral reefs from pollution, overuse and climate change, reef managers have begun implementing “marine protected areas”—no-fishing zones. One of the first studies of the ecological effect of such zones shows how they might help. A group led by researchers at the University of Exeter in England studied a sea park in the Bahamas that has not been fished since 1986. An open question was whether the resurgence of predatory grouper fish would kill off helpful parrotfish, which foster reef growth by grazing seaweed. The team found that net grazing actually doubled, coinciding with a fourfold reduction in seaweed proliferation in the protected area compared with fishable areas. The reason: larger parrotfish became more plentiful after fishing ceased. The grouper have trouble feeding on the bigger fish, which in turn graze seaweed more extensively than smaller ones. Dive into the January 6 Science for more.