FishProfiles.com Message Forums

I'm doing a research project an need to know the scientific reason that saltwater fish can't live in freshwater and visa versa. Any contribution would be much appreciated. I have a feeling it involves solute concentration gradients and diffusion but just want to be sure. Thanks

The main reason that I know of is the osmotic pressure difference between salt and fresh water. Freshwater fish have to deal with water constantly entering their bodies (because water flows from levels of higher concentration to lower, and the bodies of freshwater fish contain less water than their surrounding environment). Saltwater fish are used to having to fight to retain water (because the saltwater they live in is saltier than their bodies). The fish have evolved differently to adapt to these different environments.

Another problem could be the fact that saltwater fish are used to high solute levels, whereas many/most freshwater fish are not. Saltwater critters prefer a higher pH with harder water than is found in most freshwater areas.

It's all much more complicated than that, but I don't understand it all yet. That's just a basic summary of my take on it.

---

And when he gets to Heaven, to Saint Peter he will tell: "One more Marine reporting, Sir! I've served my time in Hell."

Thanks for the help... now i just have to write the paper...

Theres loads of factors, to do with renal function, the regulation of salts , chemicals like taurine and its effect on the heart etc.One of the major factors between salt and freshwater fish is the relationship between use of oxygen , aerobic and anerobic energy production , and the related function of urine. This might help you see the systematic differences between fresh and saltwater fish. Basically the renal system has to function completely differently when living in saltwater as oppose to fresh, one system getting rid of excess salt, and the other concentrating on preserving as many salts as possible, while dealing with the problems of uric acid expulsion....and then theres the fish that live in brackish water or mediate between fresh and saltwater during different stages of their lives. It might help to study fish such as bullsharks and salmon, and eels like congers,and see the unique biology , organ structure, blood chemistry and physionomical changes they go through to achieve a dual life.

It gets complicated. You could write a simply HUGE paper on this particular subject.

http://oxyedge-chum.com/oxygen_or_salt.htm

From what I heard they eventually drain all their fluids... See the book It's Disgusting and We Ate It! for more info on this topic...

It gets really interesting when you examine catadromous and amphidromous species. Some fish are able to adapt thier body chemistry to go from highly alkaline, hypersaline lagoon waters to highly acidic streams. For example, the white-cheecked moray (Echidna rhodochilus) from SE Asia, is found in atoll reefs and lagoons to small streams hundreds of miles inland. Most, like the afformentioned eel, take a long time to do this, but certain fish, like mollies and some sleepers, have been known to move between huge extremes in less than 24 hours.

Catadromous fish, on the other hand, usually don't change body chemistry all that much. They are just generally extremely tolerant.