Nearly half of all fish species live in fresh water, which means they swim in the rivers, lakes, and wetlands that make up less than 3 percent of Earth’s water supply. There are more than 800 known freshwater fish species in North America alone. Worldwide, the number is over 10,000 species.

Some species of freshwater fish, such as salmon and trout, are called anadromous. They hatch in fresh water, head out to sea, where they live until they return to fresh water to reproduce. Then there are catadromous species, or those that do it in reverse, such as freshwater eels. These animals hatch at sea, live most of their lives in fresh water, then return to the ocean to reproduce.

Freshwater fish face threats from human activity, such as overfishing, marine pollution, habitat loss, dam building, and the introduction of invasive species. Around 40 percent of North American freshwater fish species have become imperiled in recent decades. And since 1900, at least 57 species of freshwater fish in North America have gone extinct.