ANADROMOUS FISH

Annalita writes...

So Jimmy-Joe, what's the difference between saltwater and freshwater fish? Since all fish breathe through their gills, why can some only survive in saltwater, and others only survive in fresh water? Are there any fish that can survive in both? Please answer my question. I don't think I can continue without the answer.


This is what I love about Ask Jim. I get great questions, some of them I already know the answers, some not. Those that I don't know, I research and then I get to learn cool things I never knew before. I already knew the answer to the second part of the question but had no idea about the first part.

The reason fish are usually one way or the other has to do with the way kidneys function in order to keep the proper equilibrium of body fluid. Apparently, with saltwater fish, the liquid in its body is less salty than the water it swims in. So, the water inside the fish moves out through the fish's skin to the saltwater. Saltwater fish drink enormous amounts of water to keep from drying out.

The opposite happens to freshwater fish. Liquids inside freshwater fish are saltier than the water around them, so the fish absorb a lot of water. To prevent becoming waterlogged, freshwater fish excrete the extra water.

So.... saltwater fish are always thirsty (who'd have thunk that?), they drink a lot but pee very little. and freshwater fish never drink and always have to pee! How could I have gone this long without knowing that!

Sea creatures who live in tidal pools and estuaries, are constantly dealing with changes in the percentage of salt in the water, but they are basically salt water fish that can tolerate some variability. As far as fish living in both fresh and salt water, about two percent of fish do that. They are "diadromous". The term diadromy refers to fish that migrate between fresh and salt water. There are three types of diadromy: Anadromous (pronounced "ANNA" dro mus) fish, such as salmon, smelt and lamprey, spend most of their adult lives in salt water and then migrate to fresh water in order to reproduce. A few species, such as the American eel, are catadromous - that is, they spend most of their adult lives in fresh water and then migrate to salt water to reproduce. There are also amphidromous fish, such as the goby, that migrate between fresh and salt water, not for spawning but to feed.

Not all salmon have to get to the ocean to live. As a matter of fact, rainbow trout are land locked salmon. That is, they are steelhead salmon who live their entire life in fresh water.

All of these fish must make gradual changes from fresh to salt waters. If you were to take a salmon from a river, and throw it into the ocean, it would quickly die.