Biologists have long marvelled at how anadromous salmonids - fish that pass from rivers into oceans and back - survive as they migrate between these two very different environments. Yet, relatively little is understood about what happens to salmonids in the estuaries where they make this transition from fresh to salt water. This book distills the current knowledge of how eighteen salmonid species around the world (including salmon, steelhead, char, and trout) are adapted to estuaries. It discusses why the waters where rivers and oceans meet are critical to salmonid survival and how these vital habitats can be preserved and recovered.