To be able to recognize freshness in a fish is a skill well rewarded. If you are able to identify a fresh fish you will never again be stuck with that "funny-tasting piece of fish" or the pervasive and lingering odor caused by cooking old fish. A reasonable sense of smell and a willingness to look at and, if need be, touch fish are the only skills necessary.
Fresh fish should taste and smell that way: fresh, briny and sweet, not musty, yeasty, bitter or fishy. The texture should be resilient, firm and smooth. Older fish that has become mealy or mushy tastes bad and smells worse.
Fillets, especially the edges, should appear moist, with no brown or yellow discoloration on the white or inner side of the fillet. The darker side, closer to the skin, should remain a bright pink with silver highlights. Any brown or green color indicates age. Fillets should smell like the sea.
The signs of freshness in a whole fish are much easier to recognize than that of fillets. The signs of freshness in both round-bodied fish such as rockfish or salmon and flatfish such as halibut or flounder are the same.
--The skin should be moist, the colors bright and the scales firmly attached.
--The flesh should be resilient, bouncing back when pressed with the finger.
--Even whole fish should smell briny clean, not fishy.
--Gills are pink or red when the fish is fresh, turning brown, then gray, with age.
--The eyes should protrude and be clear, not cloudy.
Of course, all these are guidelines and there are exceptions that may be the result of unusual natural conditions or particular fishing methods. A fish or fillet that possesses half or more of these characteristics is very likely to be fresh.
Locally caught Pacific fish such as pan-size rex sole and sand dabs, flounder, petrale sole, rockfish, ling cod, or salmon in season are usually the freshest whole fish on the market.
Be flexible. Go to the market with a cooking method in mind rather than a particular fish that may or may not be in prime condition depending on the season, etc.
Eat what is in season. This will ensure that you are getting the best seafood available as well as the best value.
Buy the fish the same day you plan to cook it. Refrigerate it as soon as you get home from the market. Take the fish out of the refrigerator just before you cook it.