I've trout fished a few streams down there that have both salmon and steelhead in them. I was there outside of the typical run timing for each though.

The Rio Del Las Vueltas near town of Chalten and the Fitzroy-Cerro Torre gets a pretty good run of Chinook. Most of the locals I spoke with and the pictures I saw were of pretty dark fish, which made sense seeing as how they are pretty far from the salt by the time they get that far into the system.

The rio Santa Cruz is the only river with a run of steelhead that uses the Atlantic ocean in the world, or so I've read and been told. I spoke with a few shops in Calafate (near the Santa Cruz) that said the fishing can be awesome at times. It's big water and fairly nondescript and my general impression was that it would be a tough nut to crack. Seems like most of the guides worked out of boats from bars mid-river. Most of the gear, flies, and methods I saw being employed I saw being used were very low tech. Beautiful country. Very windy.

The picture Doc posted shows the Torres Del Paine in the background and there are several glacier fed systems which come out of the park outside of Puerto Natales. Not sure which one that fish might have come from but it should be easy to figure out.

The sea run browns were a bigger attraction for me and I never got to see one in the handful of days I had to fish. I was on the early end of the season for them. I'll give it another try one day for sure.