Pick Our Brain - February 2012

How do salmon survive going from fresh water to salt water?

Most fish live in either saltwater or freshwater, but not both. Home aquarium owners don’t put freshwater fish in a saltwater aquarium or saltwater fish in a freshwater aquarium. Some fish, however, like salmon, sturgeon, striped bass, and American shad can move from freshwater to saltwater and back without a problem—but how do they live in both conditions?

The process of osmosis showing a cell exposed to salty water
The process of osmosis showing a cell exposed to salty water
(Image source courtesy of http://www.kitsforkids.com)

The key to the answer is osmosis, a biological process that takes place in cells, the building blocks of life. Cells are responsible for the vital functions of all living things. We’re made of them, plants are made of them, and so are fish. The natural tendency is for the concentration of chemicals on the inside of a cell to be the same as on the outside of the cell. For example, if a cell that was previously surrounded by a fairly diluted solution (like fresh water) is suddenly exposed to salty water, the cell membrane will allow most of the water inside the cell to leave. This makes the cell shrink, and increases the concentration of salts so that the concentration inside the cell becomes equal to the concentration outside the cell.

Fish need a certain amount of salt in their bodies and cells to stay healthy. Since a fish’s natural habitat doesn’t always have that correct salt concentration, it has to be able to adapt by controlling how much salt and water is present in its body. The salt concentration inside a freshwater fish is higher than that of the surrounding water so it tends to absorb water. Freshwater fish get rid of this extra water through well developed kidneys that produce copious amounts of dilute urine. They also have to hang on to whatever salt they get from their food because they live in an environment where salt is scarce. A saltwater fish has the opposite problem; its environment has a much higher salt concentration than its body so it tends to lose water and take in too much salt. Saltwater fish constantly drink seawater, excrete small quantities of very salty urine and get rid of extra salt through their gills. Salmon and other fish that move from fresh to salt water have to somehow be able to operate in either mode. So, it takes specialized kidneys (and gills) for a salmon to be able to thrive in both saltwater and freshwater.

A young salmon looks very different as it makes its way from where it hatched in freshwater upstream and grows, matures and moves through the estuary to the ocean. Part of this transition is the ability to change its bodily functions from freshwater mode to saltwater mode. This change in the way that a salmon’s body manages salt takes some time (days or weeks) so the time spent moving through the estuary, from fresh to salt water is critical. The outside of the salmon also changes appearance, going from a brownish juvenile with camouflaging parr marks (wide, vertical marks on the sides of fish until they mature) to a silvery smolt. It’s a whole new world inside and out for fish moving from the relative confines of a freshwater stream to the wide open salty environment of the ocean.