The National Wildlife Federation

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Fish are water-dwelling vertebrates. These aquatic animals breathe by absorbing oxygen from the water using gills. Almost all fish are cold-blooded, and some have scales to protect their bodies. Most species lay eggs—in fact, certain species are able to lay millions of eggs at a time.

Some fish prefer saltwater or freshwater, while others have the ability to survive in both. Common threats to fish in and around the United States include overfishing, habitat degradation, and poor water conditions. Climate change is also a threat, particularly for coldwater fish, which are extremely sensitive to changes in water temperature. When streams get too warm, the fish can experience slower growth rates, lower oxygen levels in the water, and greater susceptibility to poisons, parasites, and disease.

Bull Shark

Warmwater Fish

Fish that thrive in higher temperatures

Bull Shark Flying Fish
Mangrove Rivulus Pupfish
Rainbow Trout Steelhead

Coldwater Fish

Fish that thrive in lower temperatures

Brook Trout Chinook Salmon
Mudminnows Rainbow Trout and Steelhead

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