"Salt marsh Harvest Mice have a very limited range—they live only in San Francisco Bay's tidal marshes and diked salt marshes—and they have very specific habitat requirements. They need dense cover. They are good swimmers, but if they are forced into the open they are picked off by predators such as owls, skunks, cats, rats, and foxes. They also need somewhere to go when the water rises at high tide. The Mice depend on pickleweed and other plants that grow in the salty soil of the marshes for both cover and safe places to climb above water level. Much of their habitat has been lost to development, and in some places, flow from sewage plants has changed the water quality, affecting which plants can grow. Although the Mice can have two or three litters a year, their survival in the wild is in question."

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account