Is it fresh or is it not? How can you tell if the fish you see on ice in the supermarket was still wiggling its fins less than a day ago? Knowing what to look for when buying fish can keep you from getting sick, and help you select a better product. Fresh water and ocean fish share the same characteristics.
A fresh fish should have little or no smell. The digestive tract of living fish contains enzymes. When a fish dies, these enzymes help bacteria penetrate the stomach wall. This causes fish flesh to start spoiling. An ammonia-like smell or an overpowering fishy odor indicates the fish is no longer fresh. It is old or has spoiled from improper refrigeration.
Scales and Skin
The scales of a fresh fish are tight on the skin, not loose or falling off or dried out. They are also bright, reflecting light easily, and have a metallic look. A very thin and clear mucus should cover the skin. Unscrupulous fish markets sometimes import fish dipped in a preservative chemical called formalin to appear fresher. If you're unsure, ask if the fish is really fresh or has been rinsed in a preservative.
Flesh and Tail
A fresh fish has firm flesh that is not mushy or easily indented with your finger. It should not be separating from the bones. The tail should be very stiff with firm flesh as well. When buying a cleaned fish fillet, test the resiliency of the fillet's meat with your finger. Press down on the fillet. If the meat bounces back with its original shape, it is fresh. If the finger imprint remains, the fillet is about to spoil.
The eyes of a fresh fish should be bulging, clear and shiny, not cloudy or sunken into the head. If the eyes are cloudy, faded, milky or dull, the fish has aged a few days and is approaching a non-usable condition. The exception to this is for naturally cloudy-eyed fish types, such as the fresh water walleye pike and large catfish. Some ocean species of fish also have eyes that cloud shortly after death, such as snapper, grouper and flounder.
Gills and Fins
The gills must be red to pink, not gray or brown There should be no slime covering them. The fins should be intact, moist and and resistant to being pulled.