Here in washington state, where we have 2 cent per kilowatt hour hydro power, and more bitcoin mines then we know what to do with... and all of our dams have fish ladders. it can be done, and done profitably.
also doesnt vienna have a nuclear power plant that has never been turned on?
The 2 cents are what you get when you run the power plant or what you pay including the fee for the network, taxes etc?
The Austrian nuclear power plant:
http://en.wikipedia.org/wiki/Zwentendorf_Nuclear_Power_Plant - never went live.
Regarding the fish ladders: I guess the dams in Washington state are all huge, the area where I live and Austria in general is full of small privately owned power plants, I guess most in the 10kW to 50kW capacity range. For them fish ladders are often very difficult to integrate. There is anyways little option because if you don't build the fish ladder you have dismantle you power plant and build back the river to it's shape before the plant, which is often even more expensive.