What are the reasons fresh water fish can not live in salt water and vise versa?

Question

why salmon and some sharks can live in both fresh and salt water? what othe fish can do that?

Answer 1

Freshwater - Contains low amounts of dissolved salts and solids (Hypotonic)

Saltwater - Contains high amounts of dissolved salts and solids (Hypertonic)

You see, each fish has a certain "balance" they need. The amount of "salt regulation"- if that's the term... in them...

Answer 2

This is not a simple question but I'll try:

1. Fish 'breathe' water.  This means that their blood

stream is exposed to the surrounding water across

a large area of permeable membrane.

2. Salt will tend to move across such a membrane from

high concentration to low. (Wiki 'Osmatic pressure`)

3. The blood of any organism must maintain a narrow

level of salt concentration for the organism to live.

In fresh water, salt must be retained.

In salt water, salt must be excluded.

Very few fish can do both.

Salmon, Eels, and some sharks can do this.

Most fish can't.

Answer 3

The reason for most fresh water fish can not live in salty water and vie versa is simply due to their evolution and dependency for their suitable habitat. The diadromous fishes ( which travel between salt and fresh water ) and amphidromous fishes ( thatmove between fresh and salt water during some part of life cycle, but not for breeding ) can live in both fresh and salty waters.

Good Luck !