Salmon are anadromous, which means their fragile young are hatched in protected freshwater lakes and streams then migrate to saltwater. They spend their adult lives in the ocean before swimming back to where they began their lives. Like salmon, striped bass and sea lamprey also spawn in freshwater and mature in saltwater. Catadromous fish, like eels, do it the other way around; they spawn in the sea then migrate to freshwater. This migration is made possible by a biological process called osmoregulation.
What Is Osmoregulation?
Osmoregulation is the way salmon control bodily fluids as they move between freshwater and saltwater. The process begins during the smolt phase, when juvenile salmon prepare themselves to swim out to sea.
The first thing that happens is a smolt begins taking in a lot of water. Next, its kidneys stop producing as much urine. Finally, a molecular pump in the salmon’s gills switches direction. Instead of capturing sodium from the freshwater, it begins pumping sodium out. When a mature fish swims back to its freshwater spawning grounds, the changes reverse themselves as the fish spends a few days in intertidal zones between freshwater and saltwater.
How Do We Know About Osmoregulation?
In 1957, a Danish chemist named Jens C. Skou discovered that animal cells that control nerve impulses, muscle contractions, and digestion require more potassium inside the cell than outside it. Likewise, the cell needs more sodium ions outside the cell than inside it. Skou discovered an enzyme in our cells called Nz+/K+_ATPase — or NKA for short — which acts as a pump to transport these sodium ions in and out of cell walls. Many cells spend a fifth of its energy on these pumps; nerve cells spend 2/3rds of their energy moving around sodium ions. For his research, Skou won the Nobel Prize in Chemistry in 1997.
Salmon use NKA pumps to move sodium in the right direction. In freshwater, salmon pump sodium in, but once they enter the ocean, they begin pumping out sodium and chloride ions extracted from ocean salt that enters their bodies.
Can People and Animals Drink Seawater?
If salmon can survive in saltwater, why can’t human beings and other animals do it too? Most animals are only adapted to live either in freshwater or saltwater. Humans can’t produce urine that’s saltier than blood, and ocean water has three times more salt than our blood. Our kidneys can’t filter out the salt in seawater, and we become dehydrated over time.
Some marine animals, known as osmoconformers, don’t drink water the same way we do. Instead, invertebrates like shrimp and jellyfish take water in through their skin, filtering out the salt as they do. Like salmon, saltwater fish have efficient kidneys that can remove excess salt through urine, gills, and skin. Seabirds have efficient kidneys, as well as special glands to manage saltwater. The birds can dribble salty water out of their beaks. The kidneys of marine mammals, such as sea lions and whales, are also especially adapted for flushing out seawater. The urine of those animals can be up to 2.5 times more salty than seawater.