Online, your Local Fishmonger or at the Supermarket. What is the best option for buying Seafood?
At the Supermarket
They may have a fresh fish counter or a section in the chiller department, they will almost certainly have a freezer department.
You may or may not realize this, but all fish starts to spoil as soon as it's killed and the more the seafood is processed/handled the quicker it starts to deteriorate. When buying seafood, you want it as fresh as possible, preferably still flapping.

When buying fish, a good way to tell if it's fresh is to press it with you finger, the flesh should spring back, not leave your fingerprint in it. It should smell of the sea, slightly of ozone and most definitely not of FISH.
The reason that the seafood in supermarkets manages to have such a long shelf life is because those little sealed containers have pumped air in them - not air exactly as you and I breathe but air with the same elements calibrated to a different ratio and this is what stops the fish going off so quickly. As soon as that container is opened the fish will start to spoil faster (I might add that they do the same with bagged salad). Do you really want to be buying fish that's been artificially kept edible?
At the fish counter you will probably find not only whole fish and seafood but also fillets, steaks and cutlets. It's a good idea to ask the assistant when the fish was caught/brought in. They should be able to tell you. Be aware of any fish that's been covered in ice with just the heads sticking out, this is an old trick to disguise old product, get them to take it out so that you can see the fish properly. If there are fillets on sale, especially sole or plaice fillets check to see if there's any yellowing of the flesh; this is another indication of stale fish, and of course, ask to smell it.
If you're looking at a whole fish - take a good look at its eyes, they should be clear and bright, not sunken and cloudy. The gills should be a deep red color and the skin shiny and slippery. If you pick up the fish it should feel firm, not floppy like some old rag doll.
If you're after shellfish there's 2 very simple rules to follow. Before cooking, if it's open and doesn't close when tapped sharply don't buy it or cook it. After cooking if it's still shut, don't eat it.
When buying crab or lobster, pick it up, it should feel heavy for its size.
The third option in the Supermarket is the freezer department. This can be a very good alternative to fresh fish. Look for fish that has been 'flash frozen', this means that the fish has been caught and filleted very quickly - possibly at sea, then frozen very quickly thus retaining all the flavor and nutrients. This is often a far better option than fish lying around for days in the chiller cabinet.
If you are lucky enough to be close to a decent Fishmonger, get to know him and he'll look after you. He will invariably know where the seafood has come from and when it was caught. He can prepare it for you and give you tips and recipes. If you have a special occasion coming up, tell him beforehand and he will be only too happy to fulfil your order, handing it over ready to put in the oven or whatever.
Thankfully, now that the Internet has come into its own, buying fish and seafood online has never been easier. There are many small, specialized companies where you can buy not only fresh fish and seafood but delicacies such as Smoked Salmon and Caviar. They deliver excellent seafood either fresh in chilled boxes, frozen or vacuum packed.
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