They are attracted to the freshwater marshes and riparian habitat along the New and Alamo rivers. The greatest number of species are present from November to May. Visibility varies greatly from species to species due to the nocturnal habits of some and seasonal hibernation of others. Most rodent species exist in terrestrial habitats where they provide important food resources for raptors and other predators. During winter months, rodents provide food for heron and egret species as well. Muskrats are present in freshwater drains and ponds where their feeding and burrowing activities help maintain marsh habitats for various other wildlife species. Due to environmental factors, amphibians are not found in large numbers or diversity at the Sonny Bono Salton Sea NWR. Species occurring on the refuge include bullfrogs and lowland leopard frogs. Lowland leopard frogs respond well to shallow, permanent wetland habitat created for the Yuma clapper rail. They are not present elsewhere on the Refuge due to competition from exotic bullfrogs. Woodhouse's toad and red-spotted toads are also found on the refuge. Many different species of reptiles occur on the refuge. Common species include the gopher snake, western diamondback snake, coachwhip, common kingsnake, whiptail lizard, desert spiny lizard and side-blotched lizard. The spiny soft-shell turtle and the desert tortoise are also found on the refuge. Spiny soft-shell turtles are found in freshwater drains and ponds, while the desert tortoise, although rarely seen, can be found in the upland desert areas. Fish populations thrive in the Salton Sea. The aquatic ecosystem is extremely productive because of the large amounts of nutrients it receives. The nutrients stimulate growth of phytoplankton and algae, which in turn, support zooplankton and worms. All of this provides a continuing supply of food for fish. Periodically, decomposition of large algal blooms diminishes the dissolved oxygen in the water. This decomposition has been tied to occasional fish die-offs that occur throughout the year. Tilapia is the most common fish found in the Salton Sea. Tilapia is the most populous fish in the Sea due to its ability to adapt to highly saline conditions and the fact that it is a prolific breeder. Tilapia are an important food source for birds and other fish, along with being a popular game fish. They can weigh more than . The refuge has two walking trails: the Rock Hill Trail and the Michael Hardenberger Trail.