This is a *big* question.
It depends a lot on the fish. Some species can and will adapt to freshwater perfectly well. The European flounder for example can become established in freshwater lagoons. The only thing it cannot do is reproduce, implying that in this instance, salt water is required for breeding, perhaps for the eggs to function properly. Sometimes the brackish water fish makes a perfect conversion to freshwater life and is able to breed. There are populations of archerfish Toxotes chatareus in one or two places that have done this, despite being brackish water fish elsewhere in their range.
However, in most cases brackish water fish forced into freshwater permanently operate "less efficiently" that comparable freshwater fish. A nice study of Takifugu spp. puffers illustrated this by measuring levels of chemicals in the blood serum. Over time, most of the brackish water species (e.g. Takifugu rubripes) gradually lost the normal balance of salts in the serum, in other words, became increasingly less healthy, and eventually more prone to sicknesses. Only one of the species studied (Takifugu obscurus) was truly euryhaline and able to maintain an approximately steady balance of salts in the serum, implying a more or less perfect adaptation to freshwater conditions.
A fundamental rule in biology is that you can't be good at everything. Evolution tends to work either by making animals very good at one particular thing (specialists) or acceptably good at lots of things (generalists). Take humans: we can eat meat and plants, but we're not especially good at either. We lack the claws and teeth to catch and eat raw meat effectively, and our digestive system is too poor to extract nutrients from most plant material (we can't eat grass, for example). A cow, on the other hand, is specialised to eat plants and can't handle meat at all, but the plant material it eats it extract lots more energy from than we can, specifically by ruminating and fermenting the plant material.
In the case of fish, they can either perfectly adapted to fresh *or* salt water, or else tolerably good at living in either. The fact that brackish water fishes are relatively uncommon implies that most of the time it is better to be either a freshwater fish or a saltwater fish. Brackish water fish have to carry around two sets of hardware if you like, one for freshwater and one for seawater, requiring twice as much energy to grow and maintain. Compared with true freshwater or marine fish, they are probably also less effective at living in either freshwater or the sea since neither set of equipment is optimised. Think Jack of all trades, master of none.
The instinct for most brackish water fish is to stay in brackish water, because ONLY there do they have the advantage. Neither freshwater fish nor marine fish can propser in an estuary, meaning there's lots of food and fewer predators. That's a characteristic of brackish water habitats: incredibly productive, but very low diversity of animals.
That's why the scat in freshwater is so nervous... it has a drive to swim towards the estuary. In the wild, scats migrate up and down rivers, and can be found dozens of miles inland in pure freshwater even as adults. But they don't stay there, and eventually they head back to the estuary. Similarly, while they do go into the sea, they rarely stray far from the coast.
So there you have it. In freshwater, brackish water fish may be unable to breed, they gradually lose physiological health, and their migratory instincts are frustrated.
Cheers, Neale
what actually happens to a brackish fish if it's kept in freshwater? like how does it effect it? inside and out?
i found someone keeping a scat in a freshwater community 55g, claiming it had been aclimitised to freshwater over a long period of time and was fine...but is it really? especially as he says that it's "the most scared fish he's ever seen" and it has "massive anxiety attacks every three minutes"...
what does the lack of salt do to a brackish fish?
