[paul.fr]

the title is the question.

[opus]

 Salt..........as there's more of it ??

[Vcoolspice]

I believe more in salt water, but there are times when it changes cause some fish can live in both!!!

Take a look: http://www.fishing-khaolak.com/fish_species/index.html

[DoctorBeaver]

I agree - salt. The diversity around a coral reef is amazing.

There probably aren't many unknown fresh water species, but there could be thousands of salt water species yet to be discovered.

[another_someone]

Salt..........as there's more of it ??

But is this not complicated by the the greater isolation of much of the fresh water.  The question is not really what is the volume of each environment, but how many separate environmental niches does each environment create.

Clearly, there still remains a large number of different niches in the oceans and seas, but (with a few exceptions) they are all connected together, so it is quite possible to find a species that is successful in one locale invading a similar locale that is geographically seperate, but linked, to the first locale.

In fresh water, one can image a fish able to live in one river, unless it was able to adapt to salt water life at some time in its life (as salmon is), being totally isolated from other rivers, and so not able to invade the other environment.

[sarahcp]

Well...the easy answer to this is that about 40% of fish are freshwater and 60% are saltwater, but the more interesting question is 'why?'...

The first vertebrates evolved in the sea around 470 million years ago and diversified into many forms and several groups that we know today, such as the sharks and rays (known as Elasmobranchs) and the 'living fossil', the Coelacanth. The bony fish went on to invade freshwater, and it was from here that our ancestors came out on to the land.

Most modern fish diversity is found in the teleosts (a group of ray finned fish, including most of the 'common' fish people know of such as herring, minnows, mudskippers, seahorses and catfish). In fact, only four groups of living fishes are not teleosts. This hugely diverse group is found in all aquatic environments and it is their diversity that increases the proportion of fish found in freshwater.

The larger number of species in marine environments is a product of there being more available niches - we are unlikely to find more fish species in deep lakes, as there is no oxygen in the depths and fish could not survive, but there is a high possiblity of finding as yet unknown fish species in the deepest oceans as water mixing means there is plenty of oxygen (if not always plenty of food) all the way down. Combine this with coastal environments, coral reefs (the most diverse aquatic habitats) and polar areas and the possibility for fish diversity is huge.

[DoctorBeaver]

Sarah - That's very interesting. So are there no cartilagenous fresh-water fish?

[fishwhiz]

It is as likely to find well oxygenated water in deep fresh water as it is in saltwater.  The physics of mixing are pretty much the same. 

Oceans certainly have the volume advantage, yet fresh water is divided by many barriers such as geological and thermal.  Many anadromous fish (those spawning in freshwater and rearing in saltwater) have mixed populations in the oceans but remain seperate species, subspecies and populations because they are sequestered during spawning.  It is an interesting question!