^ Where exactly are they found in HK? Since i don't really know of any Freshwater River Systems in HK, except for the a few man made ponds for storing freshwater...
-EDIT-
So after some research, it seems there are a couple of freshwater streams in some of the country parks out in the New Territories... but i wouldn't say river system, as for the fish you asked about here, it doesn't seem to be a Native HK fish, more a fish found commonly in places like Vietname, Laos, and other parts of East Asia... so not sure if you will find them in a HK stream some where...
what are the native freshwater species in HK? i've been there a couple of times and it's just a really urbanized place. Any natives species left?