Well, there is certainly some ambiguity surrounding the noun
fish and it's plural and is generally regarded as troublesome. When you
Google search for the answer you actually have a whole host of sites claiming one or the other that it either doesn't change or that it has the '-es' form after it and both are right.
As said, a good example of the confusion that surrounds some words/grammar rules/punctuation in the modern English language.
As with many other situations/examples, both are correct, and this often is a cause for confusion in non native students that are learning as they are used to the rigid rules of their own native language.
To be honest, in the UK whilst young and at school, I'd never even heard of the
fishes form and was taught that it didn't change.
To me the
fishes form just sounds alien, like it belongs in a under 5s cartoon book with something like "Look at all the fishes."
Just a personal perspective even if both are correct in certain situations.
Wikipedia seems to think that in general it doesn't change and that it is possible that using the plural
fish could imply many individual fish(es) of the same species and using the form of
fishes could imply many individual fish(es) of differing species.
http://en.wikipedia.org/wiki/English_plural
So, "Fishes of Mekong" could well be correct, or not, depending on the context and what the user means.
All shades of grey and hardly a clear-cut case to be honest. It appears that both are correct but both need to be used in the right way or at the right time.