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October 25, 2001

Fresh and Saltwater Fish

Why some fish go from salt water to fresh water and back but others can't.

Q: Why can some fish go from salt water to fresh water and back but others can't? - Jessica Macuso, Minneapolis, Minnesota

A: It depends on the way they're designed. The salinity or amount of dissolved elements in water varies greatly. Seawater varies from 0.5 to 35 parts per thousand of dissolved elements such as sodium, chlorine, potassium, calcium and magnesium. Tidal pools and lagoons with high evaporation can be still more saline. Water containing less than 0.5 parts per thousand of dissolved elements is defined as fresh.
Whether fish live in a fresh or salt environment, most maintain their internal salt levels at around 10 to 12 parts per thousand. The fish's skin is a membrane that allows water to pass through it by osmosis, from the area of lower salt concentration to the area of higher salt concentration. Water flows into freshwater fish, which actively regulate to retain salt and eliminate excess water as urine. Water flows out of fish that live in salt water, so they must drink lots of water to replace the water lost by osmosis, and actively eliminate the excess salt accumulated.
Maintaining this internal balance between salt and water is called osmoregulation. Therefore, fish that live in fresh water need opposite osmoregulatory abilities from those living in salt water.
Generally, only species that live in estuarine or intertidal zones, or migrate between fresh and salt water to spawn, are efficient osmoregulators and can tolerate broad ranges of salinity. Species that live their whole lives in either all-fresh or all-salt environments have limited abilities to balance salt and water and may die if the salinity of the water changes significantly.