Fish osmoregulate through their gills, kidneys and intestines. Fish that live in salty marine waters absorb most of the water they take in and expend energy to excrete the excess salt through their kidneys and gills. Freshwater fish excrete large amounts of water and retain most of the ions, as well as urea.
For those standing on the shore, the sea probably looks like one singular expanse of water, but ask those who call it home and you’ll find that not all water is the same, just like the plains are different from the mountains, even though they’re both made of the “same” earth. The waters on Earth offer two broad types of homes—freshwater and saltwater. Freshwater has a low salt content, specifically in terms of sodium chloride. Saltwater marine bodies, as the name suggests, is salty. Freshwater bodies have a salt content of less than 1%, while seawater (which is usually salty) has an average salt content of more than 3.5% of the seawater’s weight.
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How do fish deal with the saltiness or lack of water, and why does salt concentration matter?
First of all, not all fish can handle all levels of salinity. Those fish that cannot handle large changes in salt concentrations are called stenohaline fishes; they prefer the cozy little salt concentrations to which their bodies are physiologically adapted. Fish that can tolerate and adapt to fluctuation in salt levels are called Euryhaline fish. Your adorable goldfish is a stenohaline fish, preferring its freshwater habitat with very little salt. On the other hand, salmon and trout are euryhaline fish, living part of their lives in freshwater and then migrating to their marine saltwater habitats. Despite fish living in water, they can be at risk of becoming dehydrated (or more logically, over-hydrated). To prevent this, fish employ some very interesting tactics.
Dealing with Salt: Osmoregulation
All life is supported by water. As such, all the organic matter we are made of floats or interacts in some way with water. However, there is a critical balance of water and salts that life must maintain. Too much or too little of either and life isn’t happy (or alive). Living things balance their water needs through a process we call osmoregulation–the regulation of osmosis.
Osmosis is the process of water moving through a semipermeable membrane from a lower concentration solution to a more concentrated solution. To simplify this, osmosis is when water moves from a region with a lot of water to a region that has very little water, passing through a semi-permeable membrane—a membrane that will only allow water or similarly sized molecules to pass through it. Hopefully, this picture will make it easier to understand.

Diagrammatic representation of Osmosis (Photo Credit : Nasky/Shutterstock)
Tonicity
The reason freshwater fish cannot survive in saltwater and vice-versa has a lot to do with a property of any liquid called tonicity. In simple words, it is the ability of a solution to exert osmotic pressure upon a membrane.
Tonicity comes in three types: hypertonic, hypotonic and isotonic. For the scope of this article, we are only interested in the first two.
Hypotonic and hypertonic solutions
A hypotonic solution has a lower concentration of solutes (solutes are the substances dissolved in a solution, e.g., sugar is the solute in a sugar solution) inside the cell than outside of it. On the other hand, a hypertonic solution has a higher concentration of solutes outside of the cell than inside it.
Back to Osmoregulation: How freshwater and saltwater fish survive in their corresponding waters
Seawater is hypertonic to the fish living in it, which means that the salt content of the surrounding water is higher than the content inside the fish. As a result, they lose the water inside their body to the surrounding seawater due to osmosis. On the other hand, freshwater is hypotonic to the fish that live in it, i.e., the salt content in their body is higher than the salt content of the water surrounding them. Due to osmosis, therefore, water continuously flows into their body (the area of high solute concentration, salt being the solute, in this case).

The effect of hypertonic surrounding(above) and hypotonic (below) on the fish.

Fishes that live in freshwater excrete a lot of water to prevent getting over hydrated
How much do you know about freshwater and saltwater fish?
