Freshwater fish are fish that spend some or all of their live in water with a salinity level lower than .05%. Freshwater fish, due to the segmented nature of ponds, lakes, and rivers, are subject to speciation similar to species on islands. Even though only .3% of global water is freshwater, only about 15,000 different species of freshwater fish exist. Freshwater fish make popular pets, while other, larger species, such as salmon, trout, and carp are hunted for food. Freshwater fish have undergone adaptations to survive in freshwater; such as, gills that diffuse dissolved gasses and keep salts inside, scales that reduce water diffusion, and kidneys that reclaim salt from body fluids before excretion.
The largest freshwater fish in the world is the beluga sturgeon. The beluga sturgeon can reach a length of 24 feet (7 m) and a weight of 3,500 lbs (1,500 kg). This freshwater fish has been declared endangered since 2004, due to the demand for its eggs that are sold as caviar.
The diet of a freshwater fish depends on its habitat, species, as well as the ecosystem they live in. Freshwater fish can carry a carnivore, herbivore, or omnivore diet depending on their species. Carnivore freshwater fish consume other live fish, insects, larvae, and crustaceans. Herbivore freshwater fish eat plants, algae, as well as other vegetable matter.
Freshwater fish can’t live in saltwater because it is too salty for them. The water inside their bodies flows out and as a result they die from dehydration. Fish need to osmoregulate or maintain the right amount of water in the bodies. Freshwater fish can only survive in accordance to how much salinity their body can handle.