They do the exact same thing on land with almost every animal raised for food. Cattle, hogs, chickens and the list goes on. Why fish-farming has never become bigger than it already is, is what really suprises me the most.
If something isn't done now, what will happen in twenty or thirty years. Ultimately, this may be the only thing that saves the wild salmon and steelhead.
History tells us that when food or other demands are high enough, the animals in demand will be the ones to suffer. Look at the American Bison ("Buffalo") the Eastern Elk, Ducks and other animals that have been commercially hunted. Until the commercial hunting was stopped they were doomed. Luckily, many of them have since recovered.