When purchasing fresh seafood, look, touch and smell. The eyes of the fish should be bright, bulging and clear. If dull and cloudy, beware. Fish should feel cold. The temperature must be at 32F and the skin should be elastic and spring back when touched. If old, the skin will begin to slip away from the flesh and it will be soft. Firm and elastic flesh that springs back when pressed gently with the finger. With time, the flesh becomes soft and will slip away from the bone.
Fillets and Steaks:
Look for bruises and reddened areas from blood, and if these are present, do not buy. There should be no browning around the edges and no separating of skin and flesh. If these are pre-packaged there should be only a little water.
Depending on availability, may be sold live. Shellfish should not smell fishy but have a breezy seaweed smell. Oysters, clams, and mussels should have locked shells with no gaps. Do not buy if they are opening, and no cracks should be on the shells. The shells should be moist. Fresh shucked clams, oysters, or mussel meats will, if fresh be moist and plump and covered with their liquor that is clear or only slightly milky or light gray. No sand or grit should be near them.
Lobsters and Crabs:
Lobsters and crabs will be slow in moving if chilled but will show some movement when touched. Their tails should curl tightly underneath the body and not hang down when they are picked up. Soft shelled crabs will have a soft shell but lobsters and crabs will have hard shells. Cooked crabs and lobsters will be red and will have no odor. The meat of lobster will be white with red tints, and white crab meat will be white with few red or brown tints.
Raw shrimp shells should not have blackened edges or black spots. The meat should be firm and have a mild odor.
Store all seafood in the coldest part of the refrigerator, the temperature should be near 32F.It should be used quickly, on the first or second day of purchase if at all possible. Absolutely there should be no fishy smell since this tells the buyer the fish may not be as fresh as is desired. The above information is from a University of Delaware online site, Delaware Sea Grant.
Canned fish sometimes is a better choice than fresh fish I have also learned while researching online. Salmon that is grown commercially is far less valuable as a fresh fish source and this is mainly what comprises most of the fresh salmon that is offered at markets. Salmon that is canned is mostly caught salmon from their ocean homes and is not from ponds grown especially for sale.