Benjamin Franklin said, "Guests, like fish, begin to smell after three days." Ol' Ben was a smart man and evidently knew the inherent value in the freshness aspect of purchasing fish. The flesh of fish is delicate, and according to marine biologists, it begins to deteriorate the second the fish leaves the water. Not only does this breakdown have a negative impact on the taste and texture of the fish, it also leads to an invitation for bacteria.
Unless you catch your own fish, you have no way of knowing exactly how long fish in the market have been out of their watery domain. When you consider processing time, shipping time, and marketing time, the fish you prepare for dinner could have been dead for days. There are several ways, however, to determine whether or not a fish is actually fresh. Use your senses!
When you step into a market to purchase fish, look at the whole fish. It's easy to disquise old fillets, so if you prefer boneless, skinless fish, buy the whole fish and have it filleted or cut into steaks.
If the fish are buried in ice with only the head protruding, the fishmonger might be trying to hide tell-tale signs of decomposition. Demand to see the entire fish. The eyes should be glossy. Cloudy eyes are a dead giveaway that the fish is not at its freshest. The gills should be bright red and not a sickly shade of gray. Look at the tail and dorsal fins. They should not be curled or dried out. Check the fish's body for dark spots or blemishes.
Smell the fish. Genuinely fresh fish will smell like the ocean or lake from which they came. Ironically, fresh fish do not smell "fishy." If the fish has a strong amonia smell, you don't want it. Put it back and continue your search.
Feel the fish. It should not feel slimy or "mushy." Press the flesh with your forefinger. The fish should feel firm and should "bounce back" quickly after being pressed. If the indention of your finger stays in the fish, it's not fresh.
Once you've located your fresh fish, place it in a small cooler and cover it with ice for its trip home. Even if your home is a short distance from the fish market, the flesh will break down quickly once it leaves its ice bed. It might not spoil, but the taste and texture could still be affected.