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.