Western Mosquitofish
Gambusia affinis
Family: Poeciliidae
Etymology: Gambusia comes from a Cuban work "Gambusino" which means "nothing", in the sense that to fish for Gambusia is to fish for nothing (Moyle 2002). affinis = "related"
Widely introduced throughout much of western North America, including California, for mosquito control. The intent was that mosquitofish, native to central North America, would eat the larvae of mosquitos and thus decrease the need for spraying to combat mosquitos. While it is true that mosquitofish will eat mosquito larvae, they also eat other things, possibly including native fauna.
They are a livebearing species and are highly prolific.
You can find these fishes in warm shallow backwaters along the edge of the Lower American River.