It is of utmost importance to buy fresh fish.
The time past form fishing it out of the sea and placing it on the market is crucial. Within this period the nutritive value and the taste are reduced. Just by looking at it you can see almost everything you need to know. If it is murky and soft, it is definitely old. In general, the thicker parts of the fish, from 2cm to 5cm, are the ones that are juicy and endure the cooking process without losing their density. Those parts are always close to the head and are the healthiest.
USFDA gives valuable tips when buying fish:
-Check if the fish is properly frozen – in ice or in a freezer
-Fresh fish has soft and easy smell; old one has pungent smell of ammonia or staleness
-When you press it with your finger it needs to be firm and should not leave a mark
-The meat should be always firm and shiny (whether it is uncut or in fillets)
-It must not have dark edges and brown-yellowish hue
-The eyes need to be shinny and bulging
-Only gills without slime and with dark red color are acceptable
-Do not buy frozen fish if the bag is damaged or open
-Do not buy pre prepared seafood such as shrimps, crabs or smoked fish placed next to a raw fish. This is a perfect example of easy contamination.
Fish live in cold water so to keep them fresh for a longer period of time you need low temperatures. The best temperature is from -2 to 0 °C. Nevertheless it should not be kept more than 2-3 days in a freezer because it deteriorates and its nutritive value diminishes.