When I read the title of your post, I was surprised to learn that they were salmons in the Rio Grande as I thought you were referring to the river that is the natural border between the US and Mexico, where the most common catch should consist in wetbacks! Thanks to your post, I have learnt that Rio Grande is also the name of a town at the very southern tip of Argentina.
But there has been no "grande" Rio Grande since the 1950 movie in which John Wayne starred!
Louis