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How are some fish able to breathe in both salt and fresh water why other fish can only live in one or the other? Do they change or are they always able to breathe in either place? Thank you.
Question Date: 2017-09-04
Answer 1:

In order to survive, a fish needs to keep its blood at a constant salinity or salt level, and the optimum level is less salty than seawater but a lot more salty than freshwater. Fish are “osmoregulators” which means they can actively control salt concentrations in their bodies using different strategies depending on the species and its environment.

Most fish are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. This is because the methods they use for maintaining the right level of salinity in their body only works in the environment they’ve evolved in. For example, tuna can only live in saltwater because their kidneys and gills excrete excess salt from the water they drink. On the other hand, a freshwater fish like a catfish excretes lots of water from its kidneys and gills while retaining salt.

However, some fish can osmoregulate across a broad range of salinities. Fish that can do this are called “euryhaline” and they either live in places where salinities change dramatically (like estuaries), or else they migrate between freshwater and marine environments (e.g., salmon).

Euryhaline species can switch back and forth between freshwater and marine water whenever they want, while others only do it once or twice during certain phases of their life cycle.

Answer 2:

They change - taking a salmon directly from the ocean and dropping it into a tank of fresh water will kill it, and vice-versa.

Answer 3:

I never thought about that before.

Fish get oxygen from the water through their gills, so that's the same in both fresh and salt water.

Here's an article that tells about how salmon can go from fresh water to saltwater and back; it was quite a mystery for a long time:
salmon traveling

The main problem seems to be for the young salmon to go from fresh water to saltwater. When the days get longer, the young salmons' brains start to change, and the young salmon produce a new enzyme to help them.

Young salmon also spend some time in brackish water, which is not as salty as ocean water. This helps them adjust to saltwater.

salmon life cycle

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