i'm not sure about steelhead in south america but i know there are sea-run browns there. they were introduced there,especially on the atlantic side in argentina. the rivers didn't seem to have a whole bunch of food to support them so a large chunk started to migrate to the ocean. from what i've heard and read it's turned into a killer fishery there now when they return to spawn,lots of fish and big ones too,twenty and thirty pound fish seem pretty common.if you've got the cash they have guided trips there and for fly fishers especially it's sounds like one of those once in a lifetime kinda experiences.no one around and great fishin'. i think they're in rivers all they way down to patagonia.