What would happen to the fish/animals living in the ocean, if the water became 50% less salty, within the space of three months? Would everything die, or would they evolve very quickly?
Phase 1: The Extinction
Most life in the sea would die out almost immediately. Saltwater fish, invertebrates, and most importantly plants and microbes are finely-tuned for a particular level of salinity and their cells would burst under the osmotic pressure.
Sea mammals and reptiles can tolerate the change better, but since these animals tend to be in the middle or top of the marine food chain they would quickly starve. Some of them, like sea lions which are capable of leaving the water might be able to survive for a short time by turning to eating what they find outside the water, but it is unlikely that these specialized fish-eaters could survive for long enough to evolve back into a fully-terrestrial lifestyle. The fact that marine mammals are pretty much all obligate carnivores doesn't help their case either.
The Deadly Shores
Some carnivorous whales and a few salt-tolerant shark species might migrate to the estuaries and beaches, where large prey still lives. They will be hungry and competition will be fierce, with the larger individuals inevitably turning to cannibalism out of desperation. Orcas and bull sharks will plague the shorelines of the world - stay out of the water for a few months if you want to live. Their population will drop off sharply, but a few lucky ones might survive.
Short of Breath
Don't think that living on land means you'll have it easy. Between 50 and 80 percent of the world's oxygen comes not from trees, but from marine algae - the same marine algae that is now dead. Atmospheric oxygen will take a nose-dive. All animal life will suffer as a result, and many will go extinct, starting with warm-blooded, high-energy animals like birds and mammals. A CO2 buildup will accelerate global warming to absurd degrees. If we're lucky, land plants will take advantage of the extra heat and CO2 to multiply rapidly and pick up the slack before we run out of air. Hopefully the next phase will begin before we all suffocate.
Phase 2: The Green Sea
After marine life has all but disappeared, river life would slowly begin to evolve in order to fill the abandoned ecological niches. This would naturally begin with photosynthetic microbes seeking out the extra space in the empty seas. After a relatively short time (perhaps no more than a few months or years, thanks to the extreme ability of microbes to evolve quickly) the seas would once again bloom with algae evolving from salt-tolerant estuary strains. Animal life can once again breathe easy. Perhaps too easy...
Remember how the vast majority of oxygen is made by photosynthetic algae? In our world, that algae is regularly grazed by all kinds of herbivorous animals and microbes. The carbon-overdosed world will suddenly experience a violent shift in the other direction as the new algal blooms pump the atmosphere full of oxygen. Animal life will make a comeback, but the oxygenated atmosphere might present a prime opportunity for massive firestorms to erupt. Also insects might once again gain the ability to grow to sizes not seen in millions of years. Along with the earlier mass extinction of birds and mammals, the world's ecosystem may once again resemble the Permian era.
Phase 3: The Return
Over time, river-dwelling animals would return to the oceans. Salt-tolerant estuary-dwellers and fish capable of altering their own salinity such as salmon would be first, with the handful of surviving species diversifying to fill the empty ecosystem.
What is particularly interesting about this new situation is that river-dwelling reptiles and mammals would adapt more quickly to retaking the seas than most fish would, since they are already salt tolerant (assuming they survived the low oxygen levels, that is). Manatees and turtles might be among the pioneer species, leaving the safety of the estuaries to graze the green pastures of the relatively predator-free seas, and crocodiles would likely follow them.
From here, it is virtually impossible to predict how evolution will proceed. What is certain is that the world will be greatly altered. This would likely be the biggest extinction event the world has ever known, but life would survive.
Life would not be able to evolve in this short amount of time. Organisms able to survive the lower salt concentrations (of which there would be some in the ocean) would survive, organisms not able to survive the lower salt concentrations (which would be most of the organisms in the ocean) would go extinct.
This disruption to the food chain would cause even more organisms to die out, such as predators dependent upon the animals which went extinct, and this would turn into a major mass extinction event.
There are plenty of fish species which can live in variable salinity. And the gradual change in salinity over 3 months would give some the ability to adapt. Some species of sharks, like the bull shark already have the special talent of being able to change from salt to fresh water. Make no mistake, the ecosystem of the oceans would completely change, and there would be massive, massive die-offs. But there are estuary fish, mammals, algae and more, that will have an evolutionary advantage and will be ready to take advantage of it. How much time that will take is questionable, because these estuary systems are dependant on a salty ocean, so many of those may die as well, even with the advantage. It may be decades, hundreds or thousands of years before the advantage is fully taken, or it could be longer. More likely, at first, one species, positioned just right, will have a field day. I would expect the ocean to be filled with a particular type of algae and a particular type of fish that thrives on it, that was limited before by predators (many of which are dead) and a lack of space in a brackish environ.
Expect huge changes in ocean currents (less salty water will change that, as well as temperature layering in the ocean) and in weather over-all for the entire world. The ocean determines our weather.
Since the biology part is well covered by other answers, I added a more physically based answer:
A sudden removal of all that salt would make sea level to decrease a bit beacause you removed mass. Of course, that depends on HOW the salt is removed (in example if it condensateds into sub-marine crystals, then the sea level would not change at all).
There will be a climate change (temperatures would change winds too etc.) but how the weather is going to change I cannot tell.
Also poles would instantly increase the ice surface (low salt concentration increase melting point of ice/water), thus allowing to reflect back more sun energy, helping to lower global warming (a bit at least).
All oceans currents would change.
Decomposition of certain polluting substances due to life activity would stop/reduce (at least for first years of decreased salt) so there will be an increase in pollution.
I also would assume there will be a great increase in Greenhouse gases because all animals/fishs that died into the ocean will decompose and start floating on sea surface emitting gases like methane.
So we have greenhouse gases that heat up the hearth, and more ice on poles that cool down, can't say if those 2 effects counter each one or not.
Not all life will vanish however, despite most fishes are very delicate and would die even by minimal temperature or salt concentration changes, I guess there are more resilient life forms (or maybe not, I'm not a biologist so I cannot tell that for sure).
I would expect to see a complete decimation of the entire marine food pyramid. The smaller creatures are very carefully tuned for the amount of salt they deal with. A sudden decrease in salt (over months) would result in substantial death at the bottom of the food chain. At higher rungs of the food chain, which might be advanced enough to adapt to the salinity change, you will see mass starvation. The lack of food will also drastically affect their ability to adapt because they'd be spending their energy finding food.
Some creatures would survive, as is always the case with mass events like this. Perhaps they found a pocket of higher salinity, or perhaps they were used to living in brackish waters. However, which creatures survive is not always clear in such events.
Evolution (adaptation) works basically like this:
- Species can survive, but some individuals survive better because of trait X.
- Lots of individuals without trait X die, and lots of individuals with trait X flourish (and the children also have trait X, at least the ones that survive and flourish do).
- Eventually, the species is dominated by those flourishing because of trait X.
Based on this simple definition, you'd need at least some individuals among each species to survive change in salinity. (It's not that the individuals morph halfway through their lives. They were always capable of surviving in this environment). Then the fish weak to the new environment die off, and the ones happy with less salt flourish, become mainstream, and voila, your species is now adapted to the new ocean.
The only way to really speed up the process here is with organisms that have short lifespans, i.e. lots and lots of generations.
As is usual for questions of this kind, the major effect would be an intensive re-evaluation of everything we think we know about the universe, since this would be obvious evidence of the supernatural. There's no way it could happen naturally. The physical effects would be secondary to the societal and psychological effects.