Haddock, which is closely related to (but smaller than) cod, is a salt water fish. It has a firm texture and mild flavor, which makes haddock good for most cooking methods like poaching, baking, roasting, sautéing and grilling.
You can substitute any other mild white fish, such as cod, sole, and flounder, though thickness of the fillets and flavor will vary somewhat.
The freshness of a haddock fillet can be determined by how well it holds together—a fresh one will be firm and translucent. Avoid older fillets, which will have turned a chalky hue. If you can, ask to inspect a fillet close up. It should smell fresh (not fishy), and it should look shiny, moist, and plump.
Like all fish, haddock should be stored in the coldest part of your refrigerator and used soon after buying. To slow down spoilage, try this: Put whole fish or fillets in a large strainer set over a bowl. Pile ice high on top of the fish and refrigerate. The ice keeps the fish close to 32°F, and as it melts, the water continually rinses off bacteria and drains it into the bowl. Or put the haddock in a plastic bag and set the bag on ice to maintain a temperature close to 33°F (spoilage occurs twice as fast at 40°F as it does at 32°).