Yes, there is an excellent bus service from Barriloche to Mendoza (16 hours travel) (
www.andesmar.com.ar) They feature bed and semi-bed services at a cost of 47 and 57 dollars one way.
But if you want exciting fishing, you will not find it in Mendoza. There are small rainbow trout in the rivers (average 2 pounds) but you will have to fish downstream from the dams, where water is clearer. (Atuel river at El Nihuil or Valle Grande, or Mendoza river at Potrerillos).
Dorado fishing is far more exciting, they belong to the salmon family and you will find them in the Parana river, or up in Salta (Cafayate). In this province you can fish huge dorados in the Bermejo river. The national record for dorados (34 kilos or 75 pounds) was caught in the Bermejo river,near Embarcación.
Have a look at these two pages for fishing alternatives in Argentina.
www.tumejorpesca.com www.guarapo.com.
I know the owners of the first place, and they are reliable people. I do not know the people of the second place, but they seem to handle their business in a professional manner. The Esquina option seems very good: 990 pesos (330 dollars), including two days fishing, lodging, round trip from Buenos Aires in bed bus service and all meals... Here are the details:
http://www.tumejorpesca.com/ficha.asp?codigoProducto=arg008 By the way, Cafayate (Salta) produces excellent white wines, maybe the best in all Argentina (Michel Torino, Vasija Secreta...)
Best regards.