Your best choice is to eat at the places you know there’s fishing activity involved. For an example, say Mar del Plata (although it’s commercial fishing). But if you ever go north (from Zarate up to Corrientes or Misiones) you’ll see along the way a lot of people selling fishes like Surubi or Dorado (salt water fish) that are delicious to be grilled cooked or fried (particularly the surubi). These are fished by hand (or say, pole) from the river and are usually sold either as food, or as a consolation for those who actually went fishing and came back bare handed. These would actually count as free range fish (surubies for an example, are never “farmed”) though they may be a bit less delicious than lox