Passage: "Mangrove forests grow along sheltered coasts where rivers meet the sea. Their tangled roots slow down waves and trap mud, which protects nearby villages from storm surges and erosion. Mangroves are also nurseries for fish and crabs, so fishing families depend on them. Yet many mangrove areas in the country were cleared for fishponds in past decades. Today, local governments and communities are replanting mangroves, but experts warn that seedlings must be planted only where mangroves naturally grow, or most of them will die." As used in the passage, the word "nurseries" means: