We have a number of fish hatcheries in Michigan's Eastern U.P., and my dad and I used to visit them after a day of walking the streams. They all had concrete tanks about 12 foot long, by 4 foot wide, with water from a nearby local spring that fell into the head tanks of each row, and a small divider that separated the tanks and that allowed water to spill over the top, into the next tanks. So there was always a current, and freshly flowing water in the tanks.
The tanks sat end to end, in rows of 5, with four rows set 3 feet apart, There were metal grates that acted as bridges over the tanks, so you could walk right above the fish, as well as walking along th tank sides.. The fish were separated by size, with fingerlings, in one tank, year old fries in another, 10 to 12 inches in another, and so on up to 2 foot long lake trout. They were also separated by species, with brook trout, rainbows, and lake trout. These fish were all raised to be planted in rivers , lakes and streams. After we visited a hatchery ,there was a store that served hard ice cream at 20 cents a scoop. I loved that ice cream. My favorites were the same that my dad liked, butter pecan, and maple-nut.
I have studied up on fish farms, especially in the Netherlands and China. These are areas of ocean, or lakes where mesh holding pens are huge, and contain thousands of fish. They are overrowded, and water quality is very bad as there is no movement of fresh water to naturaly replace and refresh the environment. The fish develop disease, and parasites, and are filled with PCB's, dioxin. I believe the fish farmed in South America is cleaner, and safer. Fish farmed in the U.Sl must meet conditions set by the appropriate governing agency.
So, farmed fish generally describes fish raised to sell-able size, in a captured environment, while hatchery fish are fish that have been grown from eggs to a size where they can be released into wild rivers, lakes, and streams.
I won't purchase tilapia, swai, or farm-raised salmon or trout as I know how they've been raised. Again, the inexpensive, low quality, even dangerous produce is sold to the less fortunate, while good fish is priced out of reach of most people.
Seeeeeeya; Chief Longwind of the North