What are fish?

Fish generally describes finned vertebrates living in water; bony and cartilaginous fishes represent different evolutionary groups.

Breathing and diversity

Most fish extract dissolved oxygen from water with gills, although some can also use air. Fins support steering, balance and propulsion; a swim bladder helps regulate buoyancy in some bony fishes. Sharks and rays form a separate cartilaginous group. Species living in freshwater and seawater manage salt and water balance differently. A fishlike appearance alone does not explain kinship or diet.

Life cycles and threats

Many species lay eggs, but reproduction and parental care vary. Spawning sites, currents, temperature, oxygen and food affect persistence. Overfishing, pollution and broken river connections create different risks for different species. Protection must consider egg and juvenile habitats, not only the areas where adults are seen.